Multiple functional metasurfaces with high information capacity have attracted considerable attention from researchers.This study proposes a 2-bit tunable spin-decoupled coded metasurface designed for the terahertz ba...Multiple functional metasurfaces with high information capacity have attracted considerable attention from researchers.This study proposes a 2-bit tunable spin-decoupled coded metasurface designed for the terahertz band,which utilizes the tunable properties of Dirac semimetals(DSM)to create a novel multilayer structure.By incorporating both geometric and propagating phases into the metasurface design,we can effectively control the electromagnetic wave.When the Fermi level(EF)of the DSM is set at 6 meV,the electromagnetic wave is manipulated by the gold patch embedded in the DSM film,operating at a frequency of 1.3 THz.When the EF of the DSM is set at 80 meV,the electromagnetic wave is manipulated by the DSM patch,operating at a frequency of 1.4 THz.Both modes enable independent control of beam splitting under left-rotating circularly polarized(LCP)and rightrotating circularly polarized(RCP)wave excitation,resulting in the generation of vortex beams with distinct orbital angular momentum(OAM)modes.The findings of this study hold significant potential for enhancing information capacity and polarization multiplexing techniques in wireless communications.展开更多
The angular dependence of magnetoresistance(MR)in antiferromagnetic half-Heusler HoAuSn single crystals has been systematically studied.Negative MR,as large as~99%,is observed at 9 T,is not restricted to the specific ...The angular dependence of magnetoresistance(MR)in antiferromagnetic half-Heusler HoAuSn single crystals has been systematically studied.Negative MR,as large as~99%,is observed at 9 T,is not restricted to the specific configuration of applied magnetics fields and current and can persist up to 20 K,much higher than the Ne'el temperature(T_(N)≈1.9 K).Experiments and first-principles calculations suggest that the observed large negative MR is derived from a magnetic field that reconstructs the band structure and induces a Weyl point,which changes the carrier concentration.展开更多
EuB_(6),a magnetic topological semimetal,has attracted considerable attention in recent years due to its rich intriguing physical properties,including a colossal negative magnetoresistance(CNMR)ratio exceeding-80%,a t...EuB_(6),a magnetic topological semimetal,has attracted considerable attention in recent years due to its rich intriguing physical properties,including a colossal negative magnetoresistance(CNMR)ratio exceeding-80%,a topological phase transition and a predicted quantum anomalous Hall effect(QAHE)approaching the two-dimensional(2D)limit.Yet,studies of the influence of the dimensionality approaching 2D on the electronic transport properties of EuB_(6) are still scarce.In this work,EuB_(6) thin sheets with thicknesses ranging from 35μm to 180μm were successfully fabricated through careful mechanical polishing of high-quality EuB_(6) single crystals.The reduced thickness,temperature and magnetic field have a strong influence on the electronic transport properties,including the CNMR and carrier concentration of EuB_(6) thin sheets.As the thickness of EuB_(6) thin sheets decreases from 180μm to 35μm,the magnetization transition temperature and the corresponding suppressing temperature of the Kondo effect decrease from 15.2 K to 10.9 K,while the CNMR ratio increases from-87.2%to-90.8%.Furthermore,the weak antilocalization effect transits to a weak localization effect and the carrier concentration increases by 9.4%at 30 K in a 35μm EuB_(6) thin sheet compared to the value reported for a 180μm thin sheet.Our findings demonstrate an obvious tunable effect of the reduced dimensionality on the transport properties of EuB_(6) along with the temperature and magnetic field,which could provide a route to exploring the QAHE near the 2D limit in EuB_(6) and other topological semimetals.展开更多
Flat electronic bands in condensed matter provide a rich avenue for exploring novel quantum phenomena. Here, we report an optical spectroscopy study of a topological hourglass semimetal Nb_(3)SiTe_(6) with the electri...Flat electronic bands in condensed matter provide a rich avenue for exploring novel quantum phenomena. Here, we report an optical spectroscopy study of a topological hourglass semimetal Nb_(3)SiTe_(6) with the electric field of the incident light parallel to its crystalline ab-plane. The ab-plane optical conductivity spectra of Nb_(3)SiTe_(6) single crystals exhibit a remarkable peak-like feature around 1.20 eV, which is mainly contributed by the direct optical transitions between the two ab-initio-calculation-derived flat bands along the momentum direction Z–U. Our results pave the way for investigating exotic quantum phenomena based on the flat bands in topological hourglass semimetals.展开更多
Half-semimetals,characterized by their spin-polarized electronic states,hold significant promise for spintronic applications but remain scarce due to stringent electronic and magnetic criteria.Through a combination of...Half-semimetals,characterized by their spin-polarized electronic states,hold significant promise for spintronic applications but remain scarce due to stringent electronic and magnetic criteria.Through a combination of transport measurements and optical spectroscopy,we investigated the intermetallic compound Mn_(4)Al_(11),which features an exceptionally low carrier concentration and undergoes a magnetic phase transition near 68 K.Transport measurements reveal anomalies that deviate from typical metallic behavior at low temperatures.Optical spectroscopy indicates a small,nearly frequency-independent optical conductivity in the far-infrared region,with spectral weight decreasing as the temperature drops from 300 K to 50 K.These behaviors suggest a temperaturedependent carrier density and significant scattering of charge carriers.Combining experimental findings with calculated electronic band structures,we propose that Mn_(4)Al_(11) is a novel half-semimetal candidate exhibiting a ferrimagnetic ground state.展开更多
Recently, the Dirac and Weyl semimetals have attracted extensive attention in condensed matter physics due to both the fundamental interest and the potential application of a new generation of electronic devices. Here...Recently, the Dirac and Weyl semimetals have attracted extensive attention in condensed matter physics due to both the fundamental interest and the potential application of a new generation of electronic devices. Here we review the exotic electrical transport phenomena in Dirac andWeyl semimetals. Section 1 is a brief introduction to the topological semimetals（TSMs）. In Section 2 and Section 3, the intriguing transport phenomena in Dirac semimetals（DSMs） andWeyl semimetals（WSMs） are reviewed, respectively. The most widely studied Cd_3A_（s2） and the TaAs family are selected as representatives to show the typical properties of DSMs and WSMs, respectively. Beyond these systems, the advances in other TSM materials,such as ZrTe_5 and the MoTe_2 family, are also introduced. In Section 4, we provide perspectives on the study of TSMs especially on the magnetotransport investigations.展开更多
Topological Dirac semimetals are a parent state from which other exotic topological phases of matter, such as Weyl semimetals and topological insulators, can emerge. In this study, we investigate a Dirac semimetal pos...Topological Dirac semimetals are a parent state from which other exotic topological phases of matter, such as Weyl semimetals and topological insulators, can emerge. In this study, we investigate a Dirac semimetal possessing sixfold rotational symmetry and hosting higher-order topological hinge Fermi arc states, which is irradiated by circularly polarized light. Our findings reveal that circularly polarized light splits each Dirac node into a pair of Weyl nodes due to the breaking of time-reversal symmetry, resulting in the realization of the Weyl semimetal phase. This Weyl semimetal phase exhibits rich boundary states, including two-dimensional surface Fermi arc states and hinge Fermi arc states confined to six hinges.Furthermore, by adjusting the incident direction of the circularly polarized light, we can control the degree of tilt of the resulting Weyl cones, enabling the realization of different types of Weyl semimetals.展开更多
We study the Kondo screening of a spin-1/2 magnetic impurity in the hybrid nodal line semimetals(NLSMs) and the type-Ⅱ NLSMs by using the variational method. We mainly study the binding energy and the spin–spin corr...We study the Kondo screening of a spin-1/2 magnetic impurity in the hybrid nodal line semimetals(NLSMs) and the type-Ⅱ NLSMs by using the variational method. We mainly study the binding energy and the spin–spin correlation between magnetic impurity and conduction electrons. We find that in both the hybrid and type-Ⅱ cases, the density of states(DOS) is always finite, so the impurity and the conduction electrons always form bound states, and the bound state is more easily formed when the DOS is large. Meanwhile, due to the unique dispersion relation and the spin–orbit couplings in the NLSMs, the spatial spin–spin correlation components show very interesting features. Most saliently, various components of the spatial spin–spin correlation function decay with 1/r^(2) in the hybrid NLSMs, while they follow 1/r^(3) decay in the type-Ⅱ NLSMs. This property is mainly caused by the special band structures in the NLSMs, and it can work as a fingerprint to distinguish the two types of NLSMs.展开更多
Helicity-dependent ultrafast spin current generated by circularly polarized photons in topological materials holds the crux to many technological improvements,such as quantum communications,on-chip communication proce...Helicity-dependent ultrafast spin current generated by circularly polarized photons in topological materials holds the crux to many technological improvements,such as quantum communications,on-chip communication processing and storage.Here,we present the manipulation of helicity-dependent terahertz emission generated in a nodal line semimetal candidate Mg3Bi2 by using photon polarization states.The terahertz emission is mainly ascribed to the helicity-dependent photocurrent that is originated from circular photogalvanic effects,and the helicity-independent photocurrent that is attributed to linear photogalvanic effect.Our work will inspire more explorations into novel nodal line semimetals and open up new opportunities for developing ultrafast optoelectronics in the topological system.展开更多
Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously ge...Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping（i.e.,variation of the phonon frequency）is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.展开更多
Recently, the concept of topological insulators has been generalized to topological semimetals, including three-dimensional （3D） Weyl semimetals, 3D Dirac semimetMs, and 3D node-line semimetals （NLSs）. In particul...Recently, the concept of topological insulators has been generalized to topological semimetals, including three-dimensional （3D） Weyl semimetals, 3D Dirac semimetMs, and 3D node-line semimetals （NLSs）. In particular, several compounds （e.g., certain 3D graphene networks, Cu3PdN, Ca3P2 ） were discovered to be 3D NLSs, in which the conduction and valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional （2D） Dirac semimetal （e.g., graphene）, 2D topological semimetals are much less investigated. Here we propose a new concept of a 2D NLS and suggest that this state could be realized in a new mixed lattice （named as HK lattice） composed by Kagome and honeycomb lattices. It is found that A3B2 （A is a group-liB cation and B is a group-VA anion） compounds （such as Hg3As2） with the HK lattice are 2D NLSs due to the band inversion between the cation Hg-s orbital and the anion As-pz orbital with respect to the mirror symmetry. Since the band inversion occurs between two bands with the same parity, this peculiar 2D NLS could be used as transparent conductors. In the presence of buckling or spin-orbit coupling, the 2D NLS state may turn into a 2D Dirac semimetal state or a 2D topological crystalline insulating state. Since the band gap opening due to buckling or spin-orbit coupling is small, Hg3As3 with the HK lattice can still be regarded as a 2D NLS at room temperature. Our work suggests a new route to design topological materials without involving states with opposite parities.展开更多
YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-Ⅱ Weyl semimetal.However, as a representation of the new category of topological matters, the scanning tunneling microcopy(STM) results on such im...YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-Ⅱ Weyl semimetal.However, as a representation of the new category of topological matters, the scanning tunneling microcopy(STM) results on such important material are still absent.Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi2 samples.A hill and valley type of topography is observed on the YbMnBi2 surface, which is consistent with the non-layer nature of its crystal structure.Analysis of STM images yields the information of the index of the vicinal surface.Our results here lay a playground of future atomic scale research on YbMnBi2.展开更多
Recently, the non-centrosymmetric WC-type materials(i.e., MoP, ZrTe, TaN, etc) have attracted extensive interest due to the discovery of their topological properties.By means of the first-principles calculations, here...Recently, the non-centrosymmetric WC-type materials(i.e., MoP, ZrTe, TaN, etc) have attracted extensive interest due to the discovery of their topological properties.By means of the first-principles calculations, here we have investigated the structural, thermodynamic, elastic, and electronic properties of the WC-type MX compounds(TiS, TiSe, TiTe, ZrS, ZrSe,ZrTe, HfS, HfSe, and HfTe).Among these nine compounds, five of them(TiS, ZrS, ZrSe0.9, ZrTe, and Hf0.92 Se) have been experimentally synthesized to crystallize in the WC-type structure and other four members have never been reported.Our calculations demonstrated that they are all structurally, thermodynamically, and dynamically stable, indicating that all of them should be possibly synthesized.We have also derived their elastic constants of single crystalline and their bulk and shear moduli in terms of the R.Hill approximations.Furthermore, in similarity to ZrTe, all these compounds have been theoretically derived to be topological semimetals.Whereas TiS is unique because of the coexistence of the Dirac nodal lines(DNLs) and sixfold degenerate nodal points(sixfold DNPs), the other eight members are revealed to exhibit coexisted Weyl nodes(WPs) and triply degenerate nodal points(TDNPs).Their electronic and topological properties have been further discussed.展开更多
We report the magnetoresistance（MR）, de Haas-van Alphen（dHvA） effect and Hall effect measurements on a single crystal of TiSi, which is predicted to be a nodal line semimetal. With application of a magnetic field,...We report the magnetoresistance（MR）, de Haas-van Alphen（dHvA） effect and Hall effect measurements on a single crystal of TiSi, which is predicted to be a nodal line semimetal. With application of a magnetic field, a metal-to-insulator-like transition in ρ（T） and a nonsaturating MR are observed at low temperatures. The dHvA oscillations reveal a small Fermi-surface pocket with a nontrivial Berry phase. The analysis of the nonlinear Hall resistivity shows that TiSi is a multiband system with low carrier densities and high mobilities. All these results unambiguously prove the existence of Dirac fermions in TiSi.展开更多
Thermoelectrics has long been considered as a promising way of power generation for the next decades. So far, extensive efforts have been devoted to the search of ideal thermoelectric materials, which require both hig...Thermoelectrics has long been considered as a promising way of power generation for the next decades. So far, extensive efforts have been devoted to the search of ideal thermoelectric materials, which require both high electrical conductivity and low thermal conductivity. Recently, the emerging Dirac semimetal Cd3As2, a three-dimensional analogue of graphene, has been reported to host ultra-high mobility and good electrical conductivity as metals. Here, we report the observation of unexpected low thermal conductivity in Cd3As2, one order of magnitude lower than the conventional metals or semimetals with a similar electrical conductivity, despite the semimetal band structure and high electron mobility. The power factor also reaches a large value of 1.58 mW.m 1 .K-2 at room temperature and remains non-saturated up to 400 K. Corroborating with the first-principles calculations, we find that the thermoelectric performance can be well-modulated by the carrier concentration in a wide range. This work demonstrates the Dirac semimetal Cd3As2 as a potential candidate of thermoelectric materials.展开更多
Topological materials have aroused great interest in recent years,especially when magnetism is involved.Pressure can effectively tune the topological states and possibly induce superconductivity.Here we report the hig...Topological materials have aroused great interest in recent years,especially when magnetism is involved.Pressure can effectively tune the topological states and possibly induce superconductivity.Here we report the high-pressure study of topological semimetals XCd_(2)Sb_(2)(X=Eu and Yb),which have the same crystal structure.In antiferromagnetic(AFM)Weyl semimetal EuCd_(2)Sb_(2),the Néel temperature(TN)increases from 7.4 K at ambient pressure to 50.9 K at 14.9 GPa.When pressure is above 14.9 GPa,the AFM peak of resistance disappears,indicating a non-magnetic state.In paramagnetic Dirac semimetal candidate YbCd_(2)Sb_(2),pressure-induced superconductivity appears at 1.94 GPa,then Tc reaches to a maximum of 1.67 K at 5.22 GPa and drops to zero at about 30 GPa,displaying a dome-shaped temperature–pressure phase diagram.High-pressure x-ray diffraction measurement demonstrates that a crystalline-to-amorphous phase transition occurs at about 16 GPa in YbCd_(2)Sb_(2),revealing the robustness of pressure-induced superconductivity against structural instability.Similar structural phase transition may also occur in EuCd_(2)Sb_(2),causing the disappearance of magnetism.Our results show that XCd_(2)Sb_(2)(X=Eu and Yb)is a novel platform for exploring the interplay among magnetism,topology,and superconductivity.展开更多
The tunable propagation properties of 3D Dirac semimetal(DSM)-supported dielectric-loaded surface plasmons structures have been investigated in the THz regime,including the influences of the Fermi level of 3D DSM laye...The tunable propagation properties of 3D Dirac semimetal(DSM)-supported dielectric-loaded surface plasmons structures have been investigated in the THz regime,including the influences of the Fermi level of 3D DSM layer,the fiber shape and operation frequencies.The results indicate that the shape of dielectric fiber affects the hybrid mode significantly,on the condition that if ax(the semi-minor axis length of the dielectric semi-ellipse)is relatively small,the fiber shows good mode confinement and low loss simultaneously,and the figure of merit reaches more than 200.The propagation property can be manipulated in a wide range by changing the Fermi level of 3D DSM,e.g.if the Fermi level varies in the range of 0.05 eV–0.15 eV,the propagation length changes in the range of 9.073×10^(3)–2.715×10^(4)μm,and the corresponding modulation depth is 66.5%.These results are very helpful to understand the tunable mechanisms of the 3D DSM plasmonic devices,such as switchers,modulators,and sensors.展开更多
Based on Dirac semimetal metamaterials,the tunable plasmon induced transparency(PIT)is investigated elaborately in this work.The designed unit cell consists of a strip and a square bracket,which is periodically aligne...Based on Dirac semimetal metamaterials,the tunable plasmon induced transparency(PIT)is investigated elaborately in this work.The designed unit cell consists of a strip and a square bracket,which is periodically aligned on the dielectric substrate.Our numerical results illustrate that a pronounced transparency window exists due to near field coupling between two bright modes,which can be dynamically tuned with Fermi energy.Namely,the transparency window demonstrates a distinct blue shift with a larger Fermi energy.Moreover,an on-to-off switch of the PIT transparency window is realized with different polarization angles.In addition,the accompanied slow light property is examined with the calculation of phase and group delay.Finally,a small variation of the refractive index of the substrate can induce a clear movement of the PIT transparency window which delivers a guidance in the application of optical sensing.Thus,this work provides us a new strategy to design compact and adjustable PIT devices and has potential applications in highly tunable optical switchers,sensors,and slow light devices.展开更多
Topological semimetals are a new type of matter with one-dimensional Fermi lines or zero-dimensional Weyl or Dirac points in momentum space. Here using first-principles calculations, we find that the non-centrosymmetr...Topological semimetals are a new type of matter with one-dimensional Fermi lines or zero-dimensional Weyl or Dirac points in momentum space. Here using first-principles calculations, we find that the non-centrosymmetric PbTaS2 is a topological nodal line semimetal. In the absence of spin-orbit coupling （SOC）, one band inversion happens around a high symmetrical H point, which leads to forming a nodal line. The nodal line is robust and protected against gap opening by mirror reflection symmetry even with the inclusion of strong SOC. In addition, it also hosts exotic drumhead surface states either inside or outside the projected nodal ring depending on surface termination. The robust bulk nodal lines and drumhead-like surface states with SOC in PbTaS2 make it a potential candidate material for exploring the freakish properties of the topological nodal line fermions in condensed matter systems.展开更多
Non-stoiehiometry effect on the extreme magnetoresistanee is systematically investigated for the Weyl semimetal WTe2. Magnetoresistance and Hall resistivity are measured for the as-grown samples with a slight differen...Non-stoiehiometry effect on the extreme magnetoresistanee is systematically investigated for the Weyl semimetal WTe2. Magnetoresistance and Hall resistivity are measured for the as-grown samples with a slight difference in Te vacancies and the annealed samples with increased Te vacancies. The fits to a two-band model show that the magnetoresistanee is strongly dependent on the residual resistivity ratio （i.e., the degree of non-stoichiometry）, which is eventually understood in terms of electron doping that not only breaks the balance between electron-type and hole-type carrier densities, but also reduces the average carrier mobility. Thus the compensation effect and ultrahigh mobility are probably the main driving force of the extreme magnetoresistance in WTe2.展开更多
文摘Multiple functional metasurfaces with high information capacity have attracted considerable attention from researchers.This study proposes a 2-bit tunable spin-decoupled coded metasurface designed for the terahertz band,which utilizes the tunable properties of Dirac semimetals(DSM)to create a novel multilayer structure.By incorporating both geometric and propagating phases into the metasurface design,we can effectively control the electromagnetic wave.When the Fermi level(EF)of the DSM is set at 6 meV,the electromagnetic wave is manipulated by the gold patch embedded in the DSM film,operating at a frequency of 1.3 THz.When the EF of the DSM is set at 80 meV,the electromagnetic wave is manipulated by the DSM patch,operating at a frequency of 1.4 THz.Both modes enable independent control of beam splitting under left-rotating circularly polarized(LCP)and rightrotating circularly polarized(RCP)wave excitation,resulting in the generation of vortex beams with distinct orbital angular momentum(OAM)modes.The findings of this study hold significant potential for enhancing information capacity and polarization multiplexing techniques in wireless communications.
基金financially supported by the National Key R&D Program of China(No.2022YFA1402600)the National Natural Science Foundation of China(Nos.12304150 and 52161135108)support by the National Science Centre(Poland)(No.2021/40/Q/ST5/00066)。
文摘The angular dependence of magnetoresistance(MR)in antiferromagnetic half-Heusler HoAuSn single crystals has been systematically studied.Negative MR,as large as~99%,is observed at 9 T,is not restricted to the specific configuration of applied magnetics fields and current and can persist up to 20 K,much higher than the Ne'el temperature(T_(N)≈1.9 K).Experiments and first-principles calculations suggest that the observed large negative MR is derived from a magnetic field that reconstructs the band structure and induces a Weyl point,which changes the carrier concentration.
基金Project supported by the National Key R&D Program of China(Grant No.2022YFA1204100)the National Natural Science Foundation of China(Grant No.62488201)+1 种基金the Chinese Academy of Sciences(Grant Nos.XDB33030000 and YSBR-053)Innovation Program of Quantum Science and Technology(Grant No.2021ZD0302700)。
文摘EuB_(6),a magnetic topological semimetal,has attracted considerable attention in recent years due to its rich intriguing physical properties,including a colossal negative magnetoresistance(CNMR)ratio exceeding-80%,a topological phase transition and a predicted quantum anomalous Hall effect(QAHE)approaching the two-dimensional(2D)limit.Yet,studies of the influence of the dimensionality approaching 2D on the electronic transport properties of EuB_(6) are still scarce.In this work,EuB_(6) thin sheets with thicknesses ranging from 35μm to 180μm were successfully fabricated through careful mechanical polishing of high-quality EuB_(6) single crystals.The reduced thickness,temperature and magnetic field have a strong influence on the electronic transport properties,including the CNMR and carrier concentration of EuB_(6) thin sheets.As the thickness of EuB_(6) thin sheets decreases from 180μm to 35μm,the magnetization transition temperature and the corresponding suppressing temperature of the Kondo effect decrease from 15.2 K to 10.9 K,while the CNMR ratio increases from-87.2%to-90.8%.Furthermore,the weak antilocalization effect transits to a weak localization effect and the carrier concentration increases by 9.4%at 30 K in a 35μm EuB_(6) thin sheet compared to the value reported for a 180μm thin sheet.Our findings demonstrate an obvious tunable effect of the reduced dimensionality on the transport properties of EuB_(6) along with the temperature and magnetic field,which could provide a route to exploring the QAHE near the 2D limit in EuB_(6) and other topological semimetals.
基金Project supported by the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2021B1515130007)the National Natural Science Foundation of China (Grant Nos. U21A20432 and 52273077)+1 种基金the National Key Research and Development Program of China (Grant No. 2022YFA1403800)the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB33000000)。
文摘Flat electronic bands in condensed matter provide a rich avenue for exploring novel quantum phenomena. Here, we report an optical spectroscopy study of a topological hourglass semimetal Nb_(3)SiTe_(6) with the electric field of the incident light parallel to its crystalline ab-plane. The ab-plane optical conductivity spectra of Nb_(3)SiTe_(6) single crystals exhibit a remarkable peak-like feature around 1.20 eV, which is mainly contributed by the direct optical transitions between the two ab-initio-calculation-derived flat bands along the momentum direction Z–U. Our results pave the way for investigating exotic quantum phenomena based on the flat bands in topological hourglass semimetals.
基金supported by the National Natural Science Foundation of China(Grant Nos.12274033 and 12488201)the National Key Research and Development Program of China(Grant Nos.2022YFA1403901 and 2024YFA1408700)。
文摘Half-semimetals,characterized by their spin-polarized electronic states,hold significant promise for spintronic applications but remain scarce due to stringent electronic and magnetic criteria.Through a combination of transport measurements and optical spectroscopy,we investigated the intermetallic compound Mn_(4)Al_(11),which features an exceptionally low carrier concentration and undergoes a magnetic phase transition near 68 K.Transport measurements reveal anomalies that deviate from typical metallic behavior at low temperatures.Optical spectroscopy indicates a small,nearly frequency-independent optical conductivity in the far-infrared region,with spectral weight decreasing as the temperature drops from 300 K to 50 K.These behaviors suggest a temperaturedependent carrier density and significant scattering of charge carriers.Combining experimental findings with calculated electronic band structures,we propose that Mn_(4)Al_(11) is a novel half-semimetal candidate exhibiting a ferrimagnetic ground state.
基金Project supported by the National Basic Research Program of China(Grant Nos.2018YFA0305604,2017YFA0303300,and 2013CB934600)the Research Fund for the Doctoral Program of Higher Education(RFDP)of China(Grant No.20130001110003)+2 种基金the Open Project Program of the Pulsed High Magnetic Field Facility(Grant No.PHMFF2015002) at the Huazhong University of Science and Technologythe Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics,Tsinghua University(Grant No.KF201703)the Key Research Program of the Chinese Academy of Sciences(Grant No.XDPB08-2)
文摘Recently, the Dirac and Weyl semimetals have attracted extensive attention in condensed matter physics due to both the fundamental interest and the potential application of a new generation of electronic devices. Here we review the exotic electrical transport phenomena in Dirac andWeyl semimetals. Section 1 is a brief introduction to the topological semimetals（TSMs）. In Section 2 and Section 3, the intriguing transport phenomena in Dirac semimetals（DSMs） andWeyl semimetals（WSMs） are reviewed, respectively. The most widely studied Cd_3A_（s2） and the TaAs family are selected as representatives to show the typical properties of DSMs and WSMs, respectively. Beyond these systems, the advances in other TSM materials,such as ZrTe_5 and the MoTe_2 family, are also introduced. In Section 4, we provide perspectives on the study of TSMs especially on the magnetotransport investigations.
基金Project supported by the National Key R&D Program of China (Grant No. 2022YFA1403700)the National Natural Science Foundation of China (Grant Nos. 12074108 and 12347101)+3 种基金the Chongqing Natural Science Foundation (Grant No. CSTB2022NSCQ-MSX0568)the Fundamental Research Funds for the Central Universities (Grant No. 2023CDJXY048)the Natural Science Foundation of Jiangsu Province(Grant No. BK20230066)the Jiangsu Shuang Chuang Project (Grant No. JSSCTD202209)。
文摘Topological Dirac semimetals are a parent state from which other exotic topological phases of matter, such as Weyl semimetals and topological insulators, can emerge. In this study, we investigate a Dirac semimetal possessing sixfold rotational symmetry and hosting higher-order topological hinge Fermi arc states, which is irradiated by circularly polarized light. Our findings reveal that circularly polarized light splits each Dirac node into a pair of Weyl nodes due to the breaking of time-reversal symmetry, resulting in the realization of the Weyl semimetal phase. This Weyl semimetal phase exhibits rich boundary states, including two-dimensional surface Fermi arc states and hinge Fermi arc states confined to six hinges.Furthermore, by adjusting the incident direction of the circularly polarized light, we can control the degree of tilt of the resulting Weyl cones, enabling the realization of different types of Weyl semimetals.
基金supported by the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY19A040003)。
文摘We study the Kondo screening of a spin-1/2 magnetic impurity in the hybrid nodal line semimetals(NLSMs) and the type-Ⅱ NLSMs by using the variational method. We mainly study the binding energy and the spin–spin correlation between magnetic impurity and conduction electrons. We find that in both the hybrid and type-Ⅱ cases, the density of states(DOS) is always finite, so the impurity and the conduction electrons always form bound states, and the bound state is more easily formed when the DOS is large. Meanwhile, due to the unique dispersion relation and the spin–orbit couplings in the NLSMs, the spatial spin–spin correlation components show very interesting features. Most saliently, various components of the spatial spin–spin correlation function decay with 1/r^(2) in the hybrid NLSMs, while they follow 1/r^(3) decay in the type-Ⅱ NLSMs. This property is mainly caused by the special band structures in the NLSMs, and it can work as a fingerprint to distinguish the two types of NLSMs.
基金We thank Prof.J.B.Qi for helpful discussions and are grateful for financial support from the National Natural Science Foundation of China(Grant Nos.11804387,11802339,11805276,11902358,61805282,and 61801498)the Scientific Researches Foundation of National University of Defense Technology(Grant Nos.ZK18-03-22,ZK18-01-03 and ZK18-03-36).
文摘Helicity-dependent ultrafast spin current generated by circularly polarized photons in topological materials holds the crux to many technological improvements,such as quantum communications,on-chip communication processing and storage.Here,we present the manipulation of helicity-dependent terahertz emission generated in a nodal line semimetal candidate Mg3Bi2 by using photon polarization states.The terahertz emission is mainly ascribed to the helicity-dependent photocurrent that is originated from circular photogalvanic effects,and the helicity-independent photocurrent that is attributed to linear photogalvanic effect.Our work will inspire more explorations into novel nodal line semimetals and open up new opportunities for developing ultrafast optoelectronics in the topological system.
基金Supported by the National Key Research and Development Program of China under Grant Nos 2017YFA0303603,2016YFA0300303,2017YFA0302901 and 2016YFA0300604the National Natural Science Foundation of China under Grant Nos11774408,11574383 and 11774399+1 种基金the External Cooperation Program of Chinese Academy of Sciences under Grant No GJHZ1826the Interdisciplinary Innovation Team of Chinese Academy of Sciences
文摘Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping（i.e.,variation of the phonon frequency）is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.
基金Supported by the National Natural Science Foundation of China under Grant No 11374056the Special Funds for Major State Basic Research under Grant No 2015CB921700+1 种基金the Program for Professor of Special Appointment(Eastern Scholar)the Qing Nian Ba Jian Program,and the Fok Ying Tung Education Foundation
文摘Recently, the concept of topological insulators has been generalized to topological semimetals, including three-dimensional （3D） Weyl semimetals, 3D Dirac semimetMs, and 3D node-line semimetals （NLSs）. In particular, several compounds （e.g., certain 3D graphene networks, Cu3PdN, Ca3P2 ） were discovered to be 3D NLSs, in which the conduction and valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional （2D） Dirac semimetal （e.g., graphene）, 2D topological semimetals are much less investigated. Here we propose a new concept of a 2D NLS and suggest that this state could be realized in a new mixed lattice （named as HK lattice） composed by Kagome and honeycomb lattices. It is found that A3B2 （A is a group-liB cation and B is a group-VA anion） compounds （such as Hg3As2） with the HK lattice are 2D NLSs due to the band inversion between the cation Hg-s orbital and the anion As-pz orbital with respect to the mirror symmetry. Since the band inversion occurs between two bands with the same parity, this peculiar 2D NLS could be used as transparent conductors. In the presence of buckling or spin-orbit coupling, the 2D NLS state may turn into a 2D Dirac semimetal state or a 2D topological crystalline insulating state. Since the band gap opening due to buckling or spin-orbit coupling is small, Hg3As3 with the HK lattice can still be regarded as a 2D NLS at room temperature. Our work suggests a new route to design topological materials without involving states with opposite parities.
基金Project supported by the “Shuguang Program” from Shanghai Education Development Foundation and Shanghai Municipal Education Commission,Ministry of Science and Technology of China(Grant Nos.2016YFA0301003 and 2016YFA0300403)the National Natural Science Foundation of China(Grant Nos.11521404,11634009,11874256,11574202,11874258,11790313,11674226,U1632102,11674222,and 11861161003)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB28000000)
文摘YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-Ⅱ Weyl semimetal.However, as a representation of the new category of topological matters, the scanning tunneling microcopy(STM) results on such important material are still absent.Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi2 samples.A hill and valley type of topography is observed on the YbMnBi2 surface, which is consistent with the non-layer nature of its crystal structure.Analysis of STM images yields the information of the index of the vicinal surface.Our results here lay a playground of future atomic scale research on YbMnBi2.
基金Project supported by the National Natural Science Foundation of China(Grant No.51671193)
文摘Recently, the non-centrosymmetric WC-type materials(i.e., MoP, ZrTe, TaN, etc) have attracted extensive interest due to the discovery of their topological properties.By means of the first-principles calculations, here we have investigated the structural, thermodynamic, elastic, and electronic properties of the WC-type MX compounds(TiS, TiSe, TiTe, ZrS, ZrSe,ZrTe, HfS, HfSe, and HfTe).Among these nine compounds, five of them(TiS, ZrS, ZrSe0.9, ZrTe, and Hf0.92 Se) have been experimentally synthesized to crystallize in the WC-type structure and other four members have never been reported.Our calculations demonstrated that they are all structurally, thermodynamically, and dynamically stable, indicating that all of them should be possibly synthesized.We have also derived their elastic constants of single crystalline and their bulk and shear moduli in terms of the R.Hill approximations.Furthermore, in similarity to ZrTe, all these compounds have been theoretically derived to be topological semimetals.Whereas TiS is unique because of the coexistence of the Dirac nodal lines(DNLs) and sixfold degenerate nodal points(sixfold DNPs), the other eight members are revealed to exhibit coexisted Weyl nodes(WPs) and triply degenerate nodal points(TDNPs).Their electronic and topological properties have been further discussed.
基金Supported by the National Key Research Program of China under Grant Nos 2016YFA0401000 and 2016YFA0300604the National Basic Research Program of China under Grant No 2015CB921303+1 种基金the Strategic Priority Research Program(B)of Chinese Academy of Sciences under Grant No XDB07020100the National Natural Science Foundation of China under Grant No11874417
文摘We report the magnetoresistance（MR）, de Haas-van Alphen（dHvA） effect and Hall effect measurements on a single crystal of TiSi, which is predicted to be a nodal line semimetal. With application of a magnetic field, a metal-to-insulator-like transition in ρ（T） and a nonsaturating MR are observed at low temperatures. The dHvA oscillations reveal a small Fermi-surface pocket with a nontrivial Berry phase. The analysis of the nonlinear Hall resistivity shows that TiSi is a multiband system with low carrier densities and high mobilities. All these results unambiguously prove the existence of Dirac fermions in TiSi.
基金supported by the National Young 1000 Talent Plan Chinathe Pujiang Talent Plan in Shanghai,China+2 种基金the National Natural Science Foundation of China(Grant Nos.61322407 and 11474058)the Fund for Fostering Talents in Basic Science of the National Natural Science Foundation of China(Grant No.J1103204)the National Basic Research Program of China(Grant No.2011CB921803)
文摘Thermoelectrics has long been considered as a promising way of power generation for the next decades. So far, extensive efforts have been devoted to the search of ideal thermoelectric materials, which require both high electrical conductivity and low thermal conductivity. Recently, the emerging Dirac semimetal Cd3As2, a three-dimensional analogue of graphene, has been reported to host ultra-high mobility and good electrical conductivity as metals. Here, we report the observation of unexpected low thermal conductivity in Cd3As2, one order of magnitude lower than the conventional metals or semimetals with a similar electrical conductivity, despite the semimetal band structure and high electron mobility. The power factor also reaches a large value of 1.58 mW.m 1 .K-2 at room temperature and remains non-saturated up to 400 K. Corroborating with the first-principles calculations, we find that the thermoelectric performance can be well-modulated by the carrier concentration in a wide range. This work demonstrates the Dirac semimetal Cd3As2 as a potential candidate of thermoelectric materials.
基金the National Natural Science Foundation of China(Grant No.12174064)the Shanghai Municipal Science and Technology Major Project(Grant No.2019SHZDZX01)the research fund from the State Key Laboratory of Surface Physics and Department of Physics,Fudan University(Grant No.KF202009)。
文摘Topological materials have aroused great interest in recent years,especially when magnetism is involved.Pressure can effectively tune the topological states and possibly induce superconductivity.Here we report the high-pressure study of topological semimetals XCd_(2)Sb_(2)(X=Eu and Yb),which have the same crystal structure.In antiferromagnetic(AFM)Weyl semimetal EuCd_(2)Sb_(2),the Néel temperature(TN)increases from 7.4 K at ambient pressure to 50.9 K at 14.9 GPa.When pressure is above 14.9 GPa,the AFM peak of resistance disappears,indicating a non-magnetic state.In paramagnetic Dirac semimetal candidate YbCd_(2)Sb_(2),pressure-induced superconductivity appears at 1.94 GPa,then Tc reaches to a maximum of 1.67 K at 5.22 GPa and drops to zero at about 30 GPa,displaying a dome-shaped temperature–pressure phase diagram.High-pressure x-ray diffraction measurement demonstrates that a crystalline-to-amorphous phase transition occurs at about 16 GPa in YbCd_(2)Sb_(2),revealing the robustness of pressure-induced superconductivity against structural instability.Similar structural phase transition may also occur in EuCd_(2)Sb_(2),causing the disappearance of magnetism.Our results show that XCd_(2)Sb_(2)(X=Eu and Yb)is a novel platform for exploring the interplay among magnetism,topology,and superconductivity.
基金the Natural Science Foundation of Shanghai under Grant Nos.21ZR1446500Shanghai Local College Capacity Building Project under Grant No.22010503300 and 21010503200+4 种基金the Research Funding of Shanghai Normal University under Grant No.SK202240National Natural Science Foundation of China under Grant Nos.61674106,12073018,U1931205the Funding of Shanghai Municipal Education Commission under Grant No 2019-01-07-00-02-E00032the Funding of Shanghai Municipality Science and Technology Commission under Grant No 19590746000,20070502400,YDZX20203100002498Student Research Project of Shanghai Normal University(undergraduate)under Grant No.22LKY022.
文摘The tunable propagation properties of 3D Dirac semimetal(DSM)-supported dielectric-loaded surface plasmons structures have been investigated in the THz regime,including the influences of the Fermi level of 3D DSM layer,the fiber shape and operation frequencies.The results indicate that the shape of dielectric fiber affects the hybrid mode significantly,on the condition that if ax(the semi-minor axis length of the dielectric semi-ellipse)is relatively small,the fiber shows good mode confinement and low loss simultaneously,and the figure of merit reaches more than 200.The propagation property can be manipulated in a wide range by changing the Fermi level of 3D DSM,e.g.if the Fermi level varies in the range of 0.05 eV–0.15 eV,the propagation length changes in the range of 9.073×10^(3)–2.715×10^(4)μm,and the corresponding modulation depth is 66.5%.These results are very helpful to understand the tunable mechanisms of the 3D DSM plasmonic devices,such as switchers,modulators,and sensors.
基金Project supported by the Natural Science Foundation of Henan Provincial Educational Committee,China(Grant No.21A140026).
文摘Based on Dirac semimetal metamaterials,the tunable plasmon induced transparency(PIT)is investigated elaborately in this work.The designed unit cell consists of a strip and a square bracket,which is periodically aligned on the dielectric substrate.Our numerical results illustrate that a pronounced transparency window exists due to near field coupling between two bright modes,which can be dynamically tuned with Fermi energy.Namely,the transparency window demonstrates a distinct blue shift with a larger Fermi energy.Moreover,an on-to-off switch of the PIT transparency window is realized with different polarization angles.In addition,the accompanied slow light property is examined with the calculation of phase and group delay.Finally,a small variation of the refractive index of the substrate can induce a clear movement of the PIT transparency window which delivers a guidance in the application of optical sensing.Thus,this work provides us a new strategy to design compact and adjustable PIT devices and has potential applications in highly tunable optical switchers,sensors,and slow light devices.
基金Supported by the National Natural Science Foundation of China under Grant No 11504366the National Basic Research Program of China under Grant Nos 2015CB921503 and 2016YFE0110000
文摘Topological semimetals are a new type of matter with one-dimensional Fermi lines or zero-dimensional Weyl or Dirac points in momentum space. Here using first-principles calculations, we find that the non-centrosymmetric PbTaS2 is a topological nodal line semimetal. In the absence of spin-orbit coupling （SOC）, one band inversion happens around a high symmetrical H point, which leads to forming a nodal line. The nodal line is robust and protected against gap opening by mirror reflection symmetry even with the inclusion of strong SOC. In addition, it also hosts exotic drumhead surface states either inside or outside the projected nodal ring depending on surface termination. The robust bulk nodal lines and drumhead-like surface states with SOC in PbTaS2 make it a potential candidate material for exploring the freakish properties of the topological nodal line fermions in condensed matter systems.
基金Supported by the National Key R&D Program of China under Grant Nos 2016YFA0300404 and 2017YFA0403600the National Natural Science Foundation of China under Grant Nos 51603207,U1532267,11574288 and 11674327the Natural Science Foundation of Anhui Province under Grant No 1708085MA08
文摘Non-stoiehiometry effect on the extreme magnetoresistanee is systematically investigated for the Weyl semimetal WTe2. Magnetoresistance and Hall resistivity are measured for the as-grown samples with a slight difference in Te vacancies and the annealed samples with increased Te vacancies. The fits to a two-band model show that the magnetoresistanee is strongly dependent on the residual resistivity ratio （i.e., the degree of non-stoichiometry）, which is eventually understood in terms of electron doping that not only breaks the balance between electron-type and hole-type carrier densities, but also reduces the average carrier mobility. Thus the compensation effect and ultrahigh mobility are probably the main driving force of the extreme magnetoresistance in WTe2.
