High-resolution time-and angle-resolved photoemission measurements were conducted on the topological insulator ZrTe_(5).With strong femtosecond photoexcitation,a possible ultrafast phase transition from a weak to a st...High-resolution time-and angle-resolved photoemission measurements were conducted on the topological insulator ZrTe_(5).With strong femtosecond photoexcitation,a possible ultrafast phase transition from a weak to a strong topological insulating phase was experimentally realized by recovering the energy gap inversion in a time scale that was shorter than 0.15 ps.This photoinduced transient strong topological phase can last longer than 2 ps at the highest excitation fluence studied,and it cannot be attributed to the photoinduced heating of electrons or modification of the conduction band filling.Additionally,the measured unoccupied electronic states are consistent with the first-principles calculation based on experimental crystal lattice constants,which favor a strong topological insulating phase.These findings provide new insights into the longstanding controversy about the strong and weak topological properties in ZrTe_(5),and they suggest that many-body effects including electron–electron interactions must be taken into account to understand the equilibrium weak topological insulating phase in ZrTe_(5).展开更多
Detection of local strain at the nanometer scale with high sensitivity remains challenging.Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phono...Detection of local strain at the nanometer scale with high sensitivity remains challenging.Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phonon frequency.As a non-polar crystal,intrinsic bilayer graphene possesses little infrared response at its transverse optical phonon frequency.The reported optical detection of local strain is enabled by applying a vertical electrical field that breaks the symmetry of the two graphene layers and introduces finite electrical dipole moment to graphene phonon.The activated phonon further interacts with continuum electronic transitions,and generates a strong Fano resonance.The resulted Fano resonance features a very sharp near-field infrared scattering peak,which leads to an extraordinary sensitivity of-0.002%for the strain detection.Our results demonstrate the first nano-scale near-field Fano resonance,provide a new way to probe local strains with high sensitivity in non-polar crystals,and open exciting possibilities for studying strain-induced rich phenomena.展开更多
We propose a new type of quantum spin Hall （QSH） insulator in chemically functionalized As （110） and Sb （110） film. According to first-principles calculations, we find that metallic As （110） and Sb （110） fil...We propose a new type of quantum spin Hall （QSH） insulator in chemically functionalized As （110） and Sb （110） film. According to first-principles calculations, we find that metallic As （110） and Sb （110） films become QSH insulators after being chemically functionalized by hydrogen （H） or halogen （C1 and Br） atoms. The energy gaps of the functionalized films range from 0.121 eV to 0.304 eV, which are sufficiently large for practical applications at room temperature. The energy gaps originate from the spin-orbit coupling （SOC）. The energy gap increases linearly with the increase of the SOC strength λ/λ0. The Z2 invariant and the penetration depth of the edge states are also calculated and studied for the functionalized films.展开更多
We reveal the electronic structure in Yb Cd_(2)Sb_(2),a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the...We reveal the electronic structure in Yb Cd_(2)Sb_(2),a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the Fermi level are evidenced near the Brillouin zone center,consistent with the density functional theory(DFT)calculation.It is interesting that the spin-unpolarized bulk bands respond unexpectedly to right-and left-handed circularly polarized probe.In addition,a hole band of surface states,which is not sensitive to the polarization of the probe beam and is not expected from the DFT calculation,is identified.We find that the non-equilibrium quasiparticle recovery rate is much smaller in the surface states than that of the bulk states.Our results demonstrate that the surface states can be distinguished from the bulk ones from a view of time scale in the nonequilibrium physics.展开更多
Lattice superlattices constructed with different materials such as ferromagnets and insulators at atomic scale provide an ideal platform for exploring many emergent physical phenomena.In the present work,a new type of...Lattice superlattices constructed with different materials such as ferromagnets and insulators at atomic scale provide an ideal platform for exploring many emergent physical phenomena.In the present work,a new type of superlattices composed of ferromagnetic half-metal CrO_(2),with a thickness of two atomic layers,together with insulating MgH_(2) are constructed.Systematic theoretical studies on the(CrO_(2))_(2)/(MgH_(2))_(n) (n=2,3,4,5,6)superlattices are carried out based on first-principles density-functional theory calculations.These superlattices are ferromagnetic semiconductors with similar intra-layer magnetic exchange couplings between Cr ions.As the thickness of the MgH_(2) layer increases,the magnetic exchange interaction between inter-layer Cr ions shows oscillating decaying behavior,while the energy band gaps show a small increase.The understanding of magnetic couplings in these superlattices provides a pathway for constructing new ferromagnetic semiconductors.展开更多
基金support from the National Key R&D Program of China(Grant Nos.2021YFA1400202 and 2021YFA1401800)the National Natural Science Foundation of China(Grant Nos.12141404 and 11974243)+3 种基金the Natural Science Foundation of Shanghai(Grant Nos.22ZR1479700 and 23XD1422200)support from the China Postdoctoral Science Foundation(Grant No.2022M722108)support from the National Key R&D Program of China(Grant Nos.2022YFA1402400 and 2021YFA1400100)the National Natural Science Foundation of China(Grant No.12074248)。
文摘High-resolution time-and angle-resolved photoemission measurements were conducted on the topological insulator ZrTe_(5).With strong femtosecond photoexcitation,a possible ultrafast phase transition from a weak to a strong topological insulating phase was experimentally realized by recovering the energy gap inversion in a time scale that was shorter than 0.15 ps.This photoinduced transient strong topological phase can last longer than 2 ps at the highest excitation fluence studied,and it cannot be attributed to the photoinduced heating of electrons or modification of the conduction band filling.Additionally,the measured unoccupied electronic states are consistent with the first-principles calculation based on experimental crystal lattice constants,which favor a strong topological insulating phase.These findings provide new insights into the longstanding controversy about the strong and weak topological properties in ZrTe_(5),and they suggest that many-body effects including electron–electron interactions must be taken into account to understand the equilibrium weak topological insulating phase in ZrTe_(5).
基金Supported by the National Key Research and Development Program of China (Grant No.2016YFA0302001)the National Natural Science Foundation of China (Grant Nos.11774224,12074244,11521404,and 61701394)+1 种基金support from the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learningadditional support from a Shanghai talent program。
文摘Detection of local strain at the nanometer scale with high sensitivity remains challenging.Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phonon frequency.As a non-polar crystal,intrinsic bilayer graphene possesses little infrared response at its transverse optical phonon frequency.The reported optical detection of local strain is enabled by applying a vertical electrical field that breaks the symmetry of the two graphene layers and introduces finite electrical dipole moment to graphene phonon.The activated phonon further interacts with continuum electronic transitions,and generates a strong Fano resonance.The resulted Fano resonance features a very sharp near-field infrared scattering peak,which leads to an extraordinary sensitivity of-0.002%for the strain detection.Our results demonstrate the first nano-scale near-field Fano resonance,provide a new way to probe local strains with high sensitivity in non-polar crystals,and open exciting possibilities for studying strain-induced rich phenomena.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11474197,U1632272,and 11521404)
文摘We propose a new type of quantum spin Hall （QSH） insulator in chemically functionalized As （110） and Sb （110） film. According to first-principles calculations, we find that metallic As （110） and Sb （110） films become QSH insulators after being chemically functionalized by hydrogen （H） or halogen （C1 and Br） atoms. The energy gaps of the functionalized films range from 0.121 eV to 0.304 eV, which are sufficiently large for practical applications at room temperature. The energy gaps originate from the spin-orbit coupling （SOC）. The energy gap increases linearly with the increase of the SOC strength λ/λ0. The Z2 invariant and the penetration depth of the edge states are also calculated and studied for the functionalized films.
基金support from the National Natural Science Foundation of China(Grant No.11974243)support from the National Natural Science Foundation of China(Grant No.11521404)+1 种基金additional support from a Shanghai talent programsupport by the Natural Science Foundation of Shanghai,China(Grant No.17ZR1443300)。
文摘We reveal the electronic structure in Yb Cd_(2)Sb_(2),a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the Fermi level are evidenced near the Brillouin zone center,consistent with the density functional theory(DFT)calculation.It is interesting that the spin-unpolarized bulk bands respond unexpectedly to right-and left-handed circularly polarized probe.In addition,a hole band of surface states,which is not sensitive to the polarization of the probe beam and is not expected from the DFT calculation,is identified.We find that the non-equilibrium quasiparticle recovery rate is much smaller in the surface states than that of the bulk states.Our results demonstrate that the surface states can be distinguished from the bulk ones from a view of time scale in the nonequilibrium physics.
基金supported by the National Natural Science Foundation of China (Grant No. 11521404)the Materials Genome Initiative Center of Shanghai Jiao Tong University (SJTU)。
文摘Lattice superlattices constructed with different materials such as ferromagnets and insulators at atomic scale provide an ideal platform for exploring many emergent physical phenomena.In the present work,a new type of superlattices composed of ferromagnetic half-metal CrO_(2),with a thickness of two atomic layers,together with insulating MgH_(2) are constructed.Systematic theoretical studies on the(CrO_(2))_(2)/(MgH_(2))_(n) (n=2,3,4,5,6)superlattices are carried out based on first-principles density-functional theory calculations.These superlattices are ferromagnetic semiconductors with similar intra-layer magnetic exchange couplings between Cr ions.As the thickness of the MgH_(2) layer increases,the magnetic exchange interaction between inter-layer Cr ions shows oscillating decaying behavior,while the energy band gaps show a small increase.The understanding of magnetic couplings in these superlattices provides a pathway for constructing new ferromagnetic semiconductors.
