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Youguo Shi

Publications and source records attributed to Youguo Shi.

At least 73 records · Page 4Linked to original sources
arXiv2023

Insulator-to-metal Mott transition facilitated by lattice deformation in monolayer $α$-RuCl$_3$ on graphite

Creating heterostructures with graphene/graphite is a practical method for charge-doping $α$-RuCl$_3$, but not sufficient to cause the insulator-to-metal transition. In this study, detailed scanning tunneling microscopy/spectroscopy measurements on $α$-RuCl$_3$ with various lattice deformations reveal that both in-plane and out-of-plane lattice distortions may collapse the Mott-gap in the case of monolayer $α$-RuCl$_3$ in proximity to graphite, but have little impact on its bulk form alone. In the Mott-Hubbard framework, the transition is attributed to the lattice distortion-facilitated substantial modulation of the electron correlation parameter. Observation of the orbital textures on a highly compressed monolayer $α$-RuCl$_3$ flake on graphite provides valuable evidence that electrons are efficiently transferred from the heterointerface into Cl3$p$ orbitals under the lattice distortion. It is believed that the splitting of Ru $t_{2g}$ bands within the trigonal distortion of Ru-Cl-Ru octahedra bonds generated the electrons transfer pathways. The increase of the Cl3$p$ states enhance the hopping integral in the Mott-Hubbard bands, resulting in the Mott-transition. These findings suggest a new route for implementing the insulator-to-metal transition upon doping in $α$-RuCl$_3$ by deforming the lattice in addition to the formation of heterostructure.

cond-mat.str-el↗
arXiv2023

Discovery of a Single-Band Mott Insulator in a van der Waals Flat-Band Compound

The Mott insulator provides an excellent foundation for exploring a wide range of strongly correlated physical phenomena, such as high-temperature superconductivity, quantum spin liquid, and colossal magnetoresistance. A Mott insulator with the simplest degree of freedom is an ideal and highly desirable system for studying the fundamental physics of Mottness. In this study, we have unambiguously identified such an anticipated Mott insulator in a van der Waals layered compound Nb3Cl8. In the high-temperature phase, where interlayer coupling is negligible, density functional theory calculations for the monolayer of Nb3Cl8 suggest a half-filled flat band at the Fermi level, whereas angle-resolved photoemission spectroscopy experiments observe a large gap. This observation is perfectly reproduced by dynamical mean-field theory calculations considering strong electron correlations, indicating a correlation-driven Mott insulator state. Since this half-filled band derived from a single 2a1 orbital is isolated from all other bands, the monolayer of Nb3Cl8 is an ideal realization of the celebrated single-band Hubbard model. Upon decreasing the temperature, the bulk system undergoes a phase transition, where structural changes significantly enhance the interlayer coupling. This results in a bonding-antibonding splitting in the Hubbard bands, while the Mott gap remains dominant. Our discovery provides a simple and seminal model system for investigating Mott physics and other emerging correlated states.

cond-mat.str-el↗
arXiv2023

Temperature-Dependent Collective Excitations in a Three-Dimensional Dirac System ZrTe$_{5}$

Zirconium pentatelluride (ZrTe$_{5}$), a system with a Dirac linear band across the Fermi level and anomalous transport features, has attracted considerable research interest for it is predicted to be located at the boundary between strong and weak topological insulators separated by a topological semimetal phase. However, the experimental verification of the topological phase transition and the topological ground state in ZrTe$_{5}$ is full of controversies, mostly due to the difficulty of precisely capturing the small gap evolution with single-particle band structure measurements. Alternatively, the collective excitations of electric charges, known as plasmons, in Dirac systems exhibiting unique behavior, can well reflect the topological nature of the band structure. Here, using reflective high-resolution electron energy loss spectroscopy (HREELS), we investigate the temperature-dependent collective excitations of ZrTe$_{5}$, and discover that the plasmon energy in ZrTe$_{5}$ is proportional to the $1/3$ power of the carrier density $n$, which is a unique feature of plasmons in three-dimensional Dirac systems. Based on this conclusion, the origin of the resistivity anomaly of ZrTe$_{5}$ can be attributed to the temperature-dependent chemical potential shift in extrinsic Dirac semimetals.

cond-mat.str-el↗
arXiv2023

Observation of Emergent Superconductivity in the Quantum Spin Hall Insulator Ta2Pd3Te5 via Pressure Manipulation

Quantum Spin Hall (QSH) insulators possess distinct helical in-gap states, enabling their edge states to act as one-dimensional conducting channels when backscattering is prohibited by time-reversal symmetry. However, it remains challenging to achieve high-performance combinations of nontrivial topological QSH states with superconductivity for applications and requires understanding of the complicated underlying mechanisms. Here, our experimental observations for a novel superconducting phase in the pressurized QSH insulator Ta2Pd3Te5 is reported, and the high-pressure phase maintains its original ambient pressure lattice symmetry up to 45 GPa. Our in-situ high-pressure synchrotron X-ray diffraction, electrical transport, infrared reflectance, and Raman spectroscopy measurements, in combination with rigorous theoretical calculations, provide compelling evidence for the association between the superconducting behavior and the abnormal densified phase. The isostructural transition was found to modify the topology of the Fermi surface directly, accompanied by a fivefold amplification of the density of states at 20 GPa compared to ambient pressure, which synergistically fosters the emergence of robust superconductivity. A profound comprehension of the fascinating properties exhibited by the compressed Ta2Pd3Te5 phase is achieved, highlighting the extraordinary potential of van der Waals (vdW) QSH insulators for exploring and investigating high-performance electronic advanced devices under extreme conditions.

cond-mat.mtrl-sci↗
arXiv2023

Extrinsic nonlinear Kerr rotation in topological materials under a magnetic field

Topological properties in quantum materials are often governed by symmetry and tuned by crystal structure and external fields, and hence symmetry-sensitive nonlinear optical measurements in a magnetic field are a valuable probe. Here we report nonlinear magneto-optical second harmonic generation (SHG) studies of non-magnetic topological materials including bilayer WTe2, monolayer WSe2 and bulk TaAs. The polarization-resolved patterns of optical SHG under magnetic field show nonlinear Kerr rotation in these time-reversal symmetric materials. For materials with three-fold rotational symmetric lattice structure, the SHG polarization pattern rotates just slightly in a magnetic field, whereas in those with mirror or two-fold rotational symmetry the SHG polarization pattern rotates greatly and distorts. These different magneto-SHG characters can be understood by considering the superposition of the magnetic field-induced time-noninvariant nonlinear optical tensor and the crystal-structure-based time-invariant counterpart. The situation is further clarified by scrutinizing the Faraday rotation, whose subtle interplay with crystal symmetry accounts for the diverse behavior of the extrinsic nonlinear Kerr rotation in different materials. Our work illustrates the application of magneto-SHG techniques to directly probe nontrivial topological properties, and underlines the importance of minimizing extrinsic nonlinear Kerr rotation in polarization-resolved magneto-optical studies.

cond-mat.mes-hall↗
arXiv2023

Energy landscape and phase competition of CsV3Sb5-, CsV6Sb6-, and TbMn6Sn6-type Kagome materials

Finding viable Kagome lattices is vital for materializing novel phenomena in quantum materials. In this work, we performed element substitutions on CsV3Sb5 with space group P6/mmm, TbMn6Sn6 with space group P6/mmm, and CsV6Sb6 with space group R-3 m, respectively, as the parent compounds. A total of 4158 materials were obtained through element substitutions, and these materials were then calculated via density function theory in high-throughput mode. Afterward, 48 materials were identified with high thermodynamic stability (E_hull<5meV/atom). Furthermore, we compared the thermodynamic stability of three different phases with the same elemental composition and predicted some competing phases that may arise during material synthesis. Finally, by calculating the electronic structures of these materials, we attempted to identify patterns in the electronic structure variations as the elements change. This work provides guidance for discovering promising AM3X5/AM6X6 Kagome materials from a vast phase space.

cond-mat.supr-con↗
arXiv2023

Flat band-engineered spin-density wave and the emergent multi-$k$ magnetic state in the topological kagome metal Mn$_{3}$Sn

Magnetic kagome metals, in which topologically non-trivial band structures and electronic correlation are intertwined, have recently emerged as an exciting platform to explore exotic correlated topological phases, that are usually not found in weakly interacting materials described within the semi-classical picture of electrons. Here, via a comprehensive single-crystal neutron diffraction and first-principles density functional theory study of the archetypical topological kagome metal Mn$_3$Sn, which is also a magnetic Weyl fermion material and a promising chiral magnet for antiferromagnetic spintronics, we report the realisation of an emergent spin-density wave (SDW) order, a hallmark correlated many-body phenomenon, that is engineered by the Fermi surface nesting of topological flat bands. We further reveal that the phase transition, from the well-known high-temperature coplanar and non-collinear k = 0 inverse triangular antiferromagnetic order to a double-$k$ non-coplanar modulated incommensurate magnetic structure below $T_1$ = 280 K, is primarily driven by the SDW instability. The double-$k$ nature of this complex low-temperature magnetic order, which can be regarded as an intriguing superposition of a longitudinal SDW with a modulation wavevector k$_L$ and a transverse incommensurate helical magnetic order with a modulation wavevector k$_T$, is unambiguously confirmed by our observation of the inter-modulation high-order harmonics of the type of 2k$_L$+k$_T$. This discovery not only solves a long-standing puzzle concerning the nature of the phase transition at $T_1$, but also provides an extraordinary example on the intrinsic engineering of correlated many-body phenomena in topological matter. The identified multi-$k$ magnetic state can be further exploited for the engineering of the new modes of magnetization and chirality switching in antiferromagnetic spintronics.

cond-mat.str-el↗
arXiv2023

Tunneling spectroscopic signatures of charge doping and associated Mott transition in $α$-RuCl${_3}$ in proximity to graphite

The layered Mott insulator $α$-RuCl${_3}$ has been extensively studied as a potential Kitaev quantum spin liquid candidate. Here, by constructing heterostructures with graphite, we employed electron tunneling measurements on few-layer $α$-RuCl${_3}$ using a scanning tunneling microscopy/spectroscopy. Characteristic tunneling spectra were detected on $α$-RuCl${_3}$ layers in proximity to graphite. In the single-layer $α$-RuCl${_3}$ in direct contact with graphite, distinct states in the Mott-gap regime were observed. The in-gap states are demonstrated to be closely related to the electron orbitals in $α$-RuCl${_3}$ and graphite, and to be sensitive to interfacial coupling, where a hybridization at the heterointerface is hypothesized. The in-gap states are also thought of as a charge reservoir for weakly doping the $α$-RuCl${_3}$ upper-layers. It demonstrated that the weak doping effect causes a considerable decrease in the Mott-gap within the upper-layers, suggesting that an unconventional Mott-transition is occurring in these layers. The results show that the heterostructure comprised of $α$-RuCl${_3}$ and graphite is a good platform for investigating the doping physics in $α$-RuCl${_3}$. Therefore, tunneling into such a doped system is a useful probe for studying otherwise insulating spin-liquid candidates.

cond-mat.str-el↗
arXiv2023

Controllable dimensionality conversion between 1D and 2D CrCl3 magnetic nanostructures

The fabrication of one-dimensional (1D) magnetic systems on solid surfaces, although of high fundamental interest, has yet to be achieved for a crossover between two-dimensional (2D) magnetic layers and their associated 1D spin chain systems. In this study, we report the fabrication of 1D single-unit-cell-width CrCl3 atomic wires and their stacked few-wire arrays on the surface of a van der Waals (vdW) superconductor NbSe2. Scanning tunneling microscopy/spectroscopy and first-principles calculations jointly revealed that the single wire shows an antiferromagnetic large-bandgap semiconducting state in an unexplored structure different from the well-known 2D CrCl3 phase. Competition among the total energies and nanostructure-substrate interfacial interactions of these two phases result in the appearance of the 1D phase. This phase was transformable to the 2D phase either prior to or after the growth for in situ or ex situ manipulations, in which the electronic interactions at the vdW interface play a nontrivial role that could regulate the dimensionality conversion and structural transformation between the 1D-2D CrCl3 phases.

cond-mat.mtrl-sci↗
arXiv2023

Ferrimagnetic Regulation of Weyl Fermions in a Noncentrosymmetric Magnetic Weyl Semimetal

The study of interaction between electromagnetism and elementary particles is a long-standing topic in physics. Likewise, the connection between particle physics and emergent phenomena in condensed matter physics is a recurring theme and condensed matter physics has often provided a platform for investigating the interplay between particles and fields in cases that have not been observed in high-energy physics, so far. Here, using angle-resolved photoemission spectroscopy, we provide a new example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the regulation of Weyl fermions by ferrimagnetism. Our results establish NdAlSi as a magnetic Weyl semimetal and provide the first experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.

cond-mat.str-el↗
arXiv2023

Testing Electron-phonon Coupling for the Superconductivity in Kagome Metal $\rm{CsV_3Sb_5}$

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal $\rm{CsV_3Sb_5}$, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength,$λ$, supporting an unconventional pairing mechanism in $\rm{CsV_3Sb_5}$. However, experimental determination of $λ$ is still missing, hindering a microscopic understanding of the intertwined ground state of $\rm{CsV_3Sb_5}$. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate $λ$=0.45~0.6 at T=6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in $\rm{CsV_3Sb_5}$. Remarkably, the EPC on the V 3d-band enhances to $λ$~0.75 as the superconducting transition temperature elevated to 4.4 K in $\rm{Cs(V_{0.93}Nb_{0.07})_3Sb_5}$. Our results provide an important clue to understand the pairing mechanism in the Kagome superconductor $\rm{CsV_3Sb_5}$.

cond-mat.supr-con↗
arXiv2023

Spectroscopic Evidence for Dirac Nodal Surfaces and Nodal Rings in Superconductor NaAlSi

The discovery of the topological states has become a key topic in condensed matter physics with the focus evolving from the Dirac or Weyl points to high-dimension topological states of the nodal lines and nodal surfaces. For a topological material to manifest its quantum properties and become useful in applications, the topological states need to be genuine and clean so that they lie close to the Fermi level without other trivial bands existing at the Fermi level. While a number of high-dimension topological materials are predicted, only a few of them have been synthesized and confirmed and the genuine and clean ones are especially scarce. Here we report the realization of the genuine clean multiple high-dimension topological states in NaAlSi. By performing high-resolution angle-resolved photoemission measurements and band structure calculations, we have observed two sets of nodal surfaces and the formation of two homocentric nodal ring states in NaAlSi. The observed nodal rings are distinct in that the inner one is a type-{\uppercase\expandafter{\romannumeral1}} nodal ring while the outer one is a type-{\uppercase\expandafter{\romannumeral1}} nodal ring embedded with four type-{\uppercase\expandafter{\romannumeral3}} nodal points. All the bands involved in the nodal rings lie very close to the Fermi level with no other trivial bands coexisting at the Fermi level. These observations make NaAlSi a desirable topological material to explore for novel quantum states and exotic properties.

cond-mat.mtrl-sci↗
arXiv2023

Observation of spin-polarized surface states in a nodal line semimetal SnTaS$_{2}$

The superconductor SnTaS$_{2}$ is theoretically predicted to be an intriguing topological nodal line semimetal without consideration of spin-orbit coupling. By carrying out angle-resolved photoemission (ARPES) and spin-resolved ARPES measurements combined with band structure calculations, we have provided a complete picture of the electronic structure and spin polarization property for the prominent surface states of SnTaS$_{2}$. The low-energy electronic states are dominated by surface states; two of them are from the S-terminated surface, while four of them are from the Sn-terminated surface. These give rise to interesting Fermi surface topology of SnTaS$_{2}$: three pockets located at $\barΓ$, $\bar{M}$ and $\bar{K}$ for the S-terminated surface and two pockets surrounding $\barΓ$ and $\bar{K}$ for the Sn-terminated surface. We further reveal that two surface states that cross the Fermi level are spin-polarized. Since SnTaS$_{2}$ is also a superconductor, our observations indicate that it may provide a new platform to explore topological superconductivity and other exotic properties.

cond-mat.supr-con↗
arXiv2023

Anomalous Nernst effect in a ferrimagnetic nodal-line semiconductor Mn$_3$Si$_2$Te$_6$

In the ferrimagnetic nodal-line semiconductor Mn$_3$Si$_2$Te$_6$, colossal magnetoresistance (CMR) arises below $T_\mathrm{c}=78$ K due to the interplay of magnetism and topological nodal-line fermiology. The Berry curvature associated with the topological nodal-line is expected to produce an anomalous Nernst effect. Here, we present sizable anomalous Nernst signal in Mn$_3$Si$_2$Te$_6$ below $T_\mathrm{c}$. In the low-magnetic-field region where CMR is most apparent, the scaling ratio between the Nernst signal and magnetization is significantly enhanced compared to that in conventional magnetic materials. The enhanced Nernst effect and CMR likely share the same mechanisms, which are closely linked to the nodal-line topology.

cond-mat.str-el↗
arXiv2022

Exploration of growth conditions of TaAs Weyl semimetal thin film by pulsed laser deposition

TaAs, the first experimentally discovered Weyl semimetal material, has attracted a lot of attention due to its high carrier mobility, high anisotropy, nonmagnetic and strong interaction with light. These make it an ideal candidate for the study of Weyl fermions and the applications in quantum computation, thermoelectric devices, and photodetection. For further basic physics studies and potential applications, large-size and high-quality TaAs films are urgently needed. However, it is difficult to grow As-stoichiometry TaAs films due to the volatilization of As during the growth. To solve this problem, the TaAs films were attempted to grow on different substrates using targets with different As stoichiometric ratios by pulsed laser deposition (PLD). In this work, we have found that partial As ions of the GaAs substrate are likely to diffuse into the TaAs films during growth, which was preliminarily confirmed by the structural characterization, surface topography and composition analysis. As a result, the As content in the TaAs film is improved and the TaAs phase is achieved. Our work presents an effective method to fabricate the TaAs films by PLD, providing the possible use of the Weyl semimetal film for functional devices.

cond-mat.mtrl-sci↗
arXiv2022

Pressure-Tuning Superconductivity in Noncentrosymmetric Topological Materials ZrRuAs

Recently, the hexagonal phase of ternary transition metal pnictides TT'X (T = Zr, Hf; T'= Ru; X = P, As), which are well-known noncentrosymmetric superconductors, were predicted to host nontrivial bulk topology. In this work, we systematically investigate the electronic responses of ZrRuAs to external pressure. At ambient pressure, ZrRuAs show superconductivity with Tc ~ 7.74 K, while a large upper critical field ~ 13.03 T is obtained for ZrRuAs, which is comparable to the weak-coupling Pauli limit. The resistivity of ZrRuAs exhibits a non-monotonic evolution with increasing pressure. The superconducting transition temperature Tc increases with applied pressure and reaches a maximum value of 7.93 K at 2.1 GPa, followed by a decrease. The nontrivial topology is robust and persists up to the high-pressure regime. Considering both robust superconductivity and intriguing topology in this material, our results could contribute to studies of the interplay between topological electronic states and superconductivity.

cond-mat.supr-con↗
arXiv2022

Cascadable in-memory computing based on symmetric writing and read out

The building block of in-memory computing with spintronic devices is mainly based on the magnetic tunnel junction with perpendicular interfacial anisotropy (p-MTJ). The resulting asymmetric write and read-out operations impose challenges in downscaling and direct cascadability of p-MTJ devices. Here, we propose that a new symmetric write and read-out mechanism can be realized in perpendicular-anisotropy spin-orbit (PASO) quantum materials based on Fe3GeTe2 and WTe2. We demonstrate that field-free and deterministic reversal of the perpendicular magnetization can be achieved by employing unconventional charge to z-spin conversion. The resulting magnetic state can be readily probed with its intrinsic inverse process, i.e., z-spin to charge conversion. Using the PASO quantum material as a fundamental building block, we implement the functionally complete set of logic-in-memory operations and a more complex nonvolatile half-adder logic function. Our work highlights the potential of PASO quantum materials for the development of scalable energy-efficient and ultrafast spintronic computing.

cond-mat.mes-hall↗
arXiv2022

Dramatic Plasmon Response to the Charge-Density-Wave Gap Development in $1\textit{T}-{\mathrm{TiSe}}_{2}$

1T-TiSe2 is one of the most studied charge density wave (CDW) systems, not only because of its peculiar properties related to the CDW transition, but also due to its status as a promising candidate of exciton insulator signaled by the proposed plasmon softening at the CDW wave vector. Using high-resolution electron energy loss spectroscopy, we report a systematic study of the temperature-dependent plasmon behaviors of 1T-TiSe2. We unambiguously resolve the plasmon from phonon modes, revealing the existence of Landau damping to the plasmon at finite momentums, which does not support the plasmon softening picture for exciton condensation. Moreover, we discover that the plasmon lifetime at zero momentum responds dramatically to the bandgap evolution associated with the CDW transition. The interband transitions near the Fermi energy in the normal phase is demonstrated serving as a strong damping channel of plasmons, while such a channel in the CDW phase is suppressed due to the CDW gap opening, which results in the dramatic tunability of the plasmon in semimetals or small-gap semiconductors.

cond-mat.str-el↗