arXiv subjects

X. C. Xie

Publications and source records attributed to X. C. Xie.

At least 145 records · Page 8Linked to original sources

arXiv2020

The Discovery of Tunable Universality Class in Superconducting $β$-W Thin Films

The interplay between quenched disorder and critical behavior in quantum phase transitions is conceptually fascinating and of fundamental importance for understanding phase transitions. However, it is still unclear whether or not the quenched disorder influences the universality class of quantum phase transitions. More crucially, the absence of superconducting-metal transitions under in-plane magnetic fields in 2D superconductors imposes constraints on the universality of quantum criticality. Here, we discover the tunable universality class of superconductor-metal transition by changing the disorder strength in $β$-W films with varying thickness. The finite-size scaling uncovers the switch of universality class: quantum Griffiths singularity to multiple quantum criticality at a critical thickness of $t_{c \perp 1}\sim 8 nm$ and then from multiple quantum criticality to single criticality at $t_{c\perp 2}\sim 16 nm$. Moreover, the superconducting-metal transition is observed for the first time under in-plane magnetic fields and the universality class is changed at $t_{c \parallel }\sim 8 nm$. The discovery of tunable universality class under both out-of-plane and in-plane magnetic fields provides broad information for the disorder effect on superconducting-metal transitions and quantum criticality.

cond-mat.supr-con↗

arXiv2020

Moiré Flat Bands of Twisted Few-layer Graphite

We report that the twisted few layer graphite (tFL-graphite) is a new family of moiré heterostructures (MHSs), which has richer and highly tunable moiré flat band structures entirely distinct from all the known MHSs. A tFL-graphite is composed of two few-layer graphite (Bernal stacked multilayer graphene), which are stacked on each other with a small twisted angle. The moiré band structure of the tFL-graphite strongly depends on the layer number of its composed two van der Waals layers. Near the magic angle, a tFL-graphite always has two nearly flat bands coexisting with a few pairs of narrowed dispersive (parabolic or linear) bands at the Fermi level, thus, enhances the DOS at $E_F$. This coexistence property may also enhance the possible superconductivity as been demonstrated in other multiband superconductivity systems. Therefore, we expect strong multiband correlation effects in tFL-graphite. Meanwhile, a proper perpendicular electric field can induce several isolated nearly flat bands with nonzero valley Chern number in some simple tFL-graphites, indicating that tFL-graphite is also a novel topological flat band system.

cond-mat.mes-hall↗

arXiv2020

Scaling Behavior of the Quantum Phase Transition from a Quantum Anomalous Hall Insulator to an Axion Insulator

The phase transitions from one plateau to the next plateau or to an insulator in quantum Hall and quantum anomalous Hall (QAH) systems have revealed universal scaling behaviors. A magnetic-field-driven quantum phase transition from a QAH insulator to an axion insulator was recently demonstrated in magnetic topological insulator sandwich samples. Here, we show that the temperature dependence of the derivative of the longitudinal resistance on magnetic field at the transition point follows a characteristic power-law that indicates a universal scaling behavior for the QAH to axion insulator phase transition. Similar to the quantum Hall plateau to plateau transition, the QAH to axion insulator transition can also be understood by the Chalker-Coddington network model. We extract a critical exponent k~ 0.38 in agreement with recent high-precision numerical results on the correlation length exponent of the Chalker-Coddington model at v ~ 2.6, rather than the generally-accepted value of 2.33.

cond-mat.mes-hall↗

arXiv2020

Noise signatures for determining chiral Majorana fermion modes

The conductance measurement of a half quantized plateau in a quantum anomalous Hall insulator-superconductor structure is reported by a recent experiment [Q. L. He \textit{et al.}, Science 357, 294-299 (2017)], which suggests the existence of the chiral Majorana fermion modes. However, such half quantized conductance plateau may also originates from a disorder-induced metallic phase. To identify the exact mechanism, we study the transport properties of such a system in the presence of strong disorders. Our results show that the local current density distributions of these two mechanisms are different. In particular, the current noises measurement can be used to distinguish them without any further fabrication of current experimental setup.

cond-mat.mes-hall↗

arXiv2020

Quantum oscillation beyond the quantum limit in pseudospin Dirac materials

Recently, many unexpected fine structures in electric, magnetic, and thermoelectric responses at extremely magnetic fields in topological materials have attracted tremendous interest. We propose a new mechanism of quantum oscillation beyond the strong-field quantum limit for Dirac fermions. The amplitude of the oscillation is far larger than the usual Shubnikov--de Haas oscillation. The oscillation tends to be periodic in the magnetic field B, instead of 1/B. The period of the oscillation does not depend on the Fermi energy. These behaviors cannot be described by the famous Lifshitz-Kosevich formula. The oscillation arises from a mechanism that we refer to as the inversion of the lowest Landau level, resulted from the competition between the pseudospin Dirac-type Landau levels and real-spin Zeeman spitting beyond the quantum limit. This inversion gives rise to the oscillation of the Fermi energy and conductivity at extremely large magnetic fields. This mechanism will be useful for understanding the unexpected fine structures observed in the strong-field quantum limit in Dirac materials.

cond-mat.mes-hall↗

arXiv2020

Unconventional Hall Effect induced by Berry Curvature

Berry phase and Berry curvature play a key role in the development of topology in physics and do contribute to the transport properties in solid state systems. In this paper, we report the finding of novel nonzero Hall effect in topological material ZrTe5 flakes when in-plane magnetic field is parallel and perpendicular to the current. Surprisingly, both symmetric and antisymmetric components with respect to magnetic field are detected in the in-plane Hall resistivity. Further theoretical analysis suggests that the magnetotransport properties originate from the anomalous velocity induced by Berry curvature in a tilted Weyl semimetal. Our work not only enriches the Hall family but also provides new insights into the Berry phase effect in topological materials.

cond-mat.mes-hall↗

arXiv2020

Finite-thickness effect and spin polarization of the even-denominator fractional quantum Hall states

The spin-polarized even-denominator fractional quantum Hall (FQH) states in the second Landau level (LL), i.e. 5/2 and 7/2, may possess novel quasi-particle excitations obeying non-Abelian statistics. However, the spin polarization of the 7/2 FQH state has not been investigated experimentally and the spin polarization of the 5/2 FQH state from tilted field experiments remains controversial. Using a piezo-driven sample rotator with the lowest electron temperature down to 25 mK, we studied the energy gap of the even-denominator FQH states in the second LL by precise control of the tilted angles with a resolution less than 0.1°. We observed two different energy gap dependences on the in-plane magnetic field for 5/2, 7/2, other FQH states (7/3 and 8/3) in the second LL and reentrant integer quantum Hall (RIQH) states in the third LL. Though the transition fields vary from states, their corresponding in-plane magnetic lengths are comparable to the quantum well thickness of the sample, which may result from the influence of the finite-thickness effect. At low in-plane magnetic fields, before the conjectured finite-thickness effect starts to dominate, the energy gaps of both 5/2 and 7/2 states show a non-decreasing behavior, supporting spin-polarized ground states. Our results also suggest that the 7/3, 8/3 FQH states, and the RIQH states in the third LL are spin-polarized or partially spin-polarized.

cond-mat.mes-hall↗

arXiv2020

Transport study of the wormhole effect in three-dimensional topological insulators

Inside a three-dimensional strong topological insulator, a tube with $h/2e$ magnetic flux carries a pair of protected one-dimensional linear fermionic modes. This phenomenon is known as the "wormhole effect". In this work, we find that the "wormhole effect", as a unique degree of freedom, introduces exotic transport phenomena and thus manipulates the transport properties of topological insulators. Our numerical results demonstrate that the transport properties of a double-wormhole system can be manipulated by the wormhole interference. Specifically, the conductance and local density of states both oscillate with the Fermi energy due to the interference between the wormholes. Furthermore, by studying the multi-wormhole systems, we find that the number of wormholes can also modulate the differential conductance through a $\mathbb{Z}$$_{2}$ mechanism. Finally, we propose two types of topological devices in real applications, the "wormhole switch" device and the "traversable wormhole" device, which can be finely tuned by controlling the wormhole degree of freedom.

cond-mat.mes-hall↗

arXiv2020

Emergent $Z_2$ Topological Invariant and Robust Helical Edge States in Two-Dimensional Topological Metals

In this work, we study the disorder effect on topological metals that support a pair of helical edge modes deeply embedded inside the gapless bulk states. Strikingly, we predict that a quantum spin Hall (QSH) phase can be obtained from such topological metals without opening a global band gap. To be specific, disorder can lead to a pair of robust helical edge states which is protected by an emergent $Z_2$ topological invariant, giving rise to a quantized conductance plateau in transport measurements. These results are instructive for solving puzzles in {various transport experiments on QSH materials} that are intrinsically metallic. This work also will inspire experimental realization of the QSH effect in disordered topological metals.

cond-mat.mes-hall↗

arXiv2020

3D Quantum Hall Effect Manipulated by Chiral Landau Levels in Weyl Semimetals

We investigate the 3D quantum Hall effect in Weyl semimetals and elucidate a global picture of the edge states. The edge states hosting 3D quantum Hall effect are combinations of Fermi arcs and chiral bulk Landau levels parallel to the magnetic field. The Hall conductance, $σ_{xz}^H$, shows quantized plateaus at Weyl nodes while tuning the magnetic field. However, the chiral Landau levels manipulate the quantization of Weyl orbits, especially under a tilted magnetic field, and the resulting edge states lead to distinctive Hall transport phenomena. A tilted magnetic field contributes an intrinsic initial value to $σ_{xz}^H$ and such initial value is determined by the tilting angle $θ$. Particularly, even if the perpendicular magnetic field is fixed, $σ_{xz}^H$ will change its sign with an abrupt spatial shift of edge states when $θ$ exceeds a critical angle $θ_c$ in an experiment. Our work uncovers the unique edge-state nature of 3D quantum Hall effect in Weyl semimetals.

cond-mat.mes-hall↗

arXiv2020

Non-Abelian braiding of Dirac fermionic modes using topological corner states in higher-order topological insulator

We numerically demonstrate that the topological corner states residing in the corners of higher-order topological insulator possess non-Abelian braiding properties. Such topological corner states are Dirac fermionic modes other than Majorana zero-modes. We claim that Dirac fermionic modes protected by nontrivial topology also support non-Abelian braiding. An analytical description on such non-Abelian braiding is conducted based on the vortex-induced Dirac-type fermionic modes. The braiding operator for Dirac fermionic modes is also analytically derivated and compared with the Majorana zero-modes. Experimentally, such non-Abelian braiding operation on Dirac fermionic modes is proposed to be testified through topological electric circuit.

cond-mat.mes-hall↗

arXiv2019

Hinged Quantum Spin-Hall Effect in Antiferromagnetic Topological Insulators

In this work, we predict a hinged quantum spin-Hall (HQSH) effect featured by a pair of helical hinge modes in antiferromagnetic (AFM) topological insulator (TI) multilayers. This pair of helical hinge modes are localized on the hinges of the top and bottom surfaces of the AFM TI multilayers. Unlike the conventional QSH effect, the HQSH effect survives the breaking of time-reversal symmetry (TRS) and thus represents a different kind of topological phenomenon. The helical hinge modes are sustainable to inelastic scattering and TRS-breaking disorder, which can be observed in macroscopic samples. We show that this HQSH effect can be understood as a three-dimensional generalization of the Su-Schrieffer-Heeger model and its topology is characterized by the spin Chern number. At last, we propose that the HQSH effect can be realized in newly found intrinsic AFM TI materials (MnBi$_2$Te$_4$)$_m$(Bi$_2$Te$_3$)$_n$ or magnetic-doped TI multilayers by current experimental setups.

cond-mat.mes-hall↗

arXiv2019

Hydrogen-induced magnetism in graphene: a simple effective model description

Hydrogen adatoms induced magnetic moment on graphene has been observed in atomic scale in a recent experiment [Gonzalez-Herrero et al., Science 352, 437 (2016)]. Here, we demonstrate that all the experimental phenomena can be simply and well described by an equivalent Anderson impurity model, where the electronic correlations on both carbon and hydrogen atoms are represented by an effective on-site Coulomb interaction on hydrogen adatom $U_H$. This simple equivalent model works so well is because that the main effect of Coulomb interaction on both carbon and hydrogen atoms is just to split the impurity resonance states with different spin. This effective Anderson impurity model can also well reproduce all the behaviours of hydrogen dimer on graphene observed in experiment. We thus argue that it should be a general model to describe the hydrogen adatoms on graphene with various adsorption configurations, especially for their magnetic properties.

cond-mat.mes-hall↗

arXiv2019

Numerical study of the negative nonlocal resistance and the backflow current in a ballistic graphene system

Besides the giant peak of the nonlocal resistance $R_{NL}$, an anomalous negative value of $R_{NL}$ has been observed in graphene systems, while its formation mechanism is not quite understood yet. In this work, utilizing the non-equilibrium Green's function method, we calculate the local-current flow in an H-shaped non-interacting graphene system locating in the ballistic regime. Similar to the previous conclusions made from the viscous regime, the numerical results show that a local-current vortex appears between the nonlocal measuring terminals, which induces a backflow current and a remarkable negative voltage drop at the probe. Specifically, the stronger the vortex exhibits, the more negative $R_{NL}$ manifests. Besides, a spin-orbital coupling is added as an additional tool to study this exotic vortex, which is not a driving force for the arising vortex at all. Moreover, a breakdown of the nonlocal Wiedemann-Franz law is obtained in this ballistic system, and two experimental criteria are further provided to confirm the existence of this exotic vortex. Notably, a discussion is made that the vortex actually originates from the collision between the flowing current and the boundaries, due to the long electron mean free path and the consequent ballistic transport caused by the specific linear spectrum of graphene.

cond-mat.mes-hall↗

arXiv2019

Quantized thermal Hall conductance from edge current calculations in lattice models

The quantized thermal Hall effect is an important probe for detecting chiral topological order and revealing the nature of chiral gapless edge states. The standard Kubo formula approach for the thermal Hall conductance $κ_{xy}$ based on the linear-response theory faces difficulties in practical application due to the lack of a reliable numerical method for calculating dynamical quantities in microscopic models at finite temperature. In this work, we propose an approach for calculating $κ_{xy}$ in two-dimensional lattice models displaying chiral topological order. Our approach targets at the edge current localized at the boundary which involves only thermal averages of local operators in equilibrium, thus drastically lowering the barrier for the calculation of $κ_{xy}$. We use the chiral $p$-wave superconductor (with and without disorder) and the Hofstadter model as benchmark examples to illustrate several sources of finite-size effects, and we suggest the infinite (or sufficiently long) strip as the best geometry for carrying out numerical simulations.

cond-mat.str-el↗

arXiv2019

3/2 Fractional Quantum Hall Plateau in Confined Two-Dimensional Electron Gas

Even-denominator fractional quantum Hall (FQH) states, such as 5/2 and 7/2, have been well known in a two-dimensional electron gas (2DEG) for decades and are still investigated as candidates of non-Abelian statistics. In this paper, we present the observation of a 3/2 FQH plateau in a single-layer 2DEG with lateral confinement at a bulk filling factor of 5/3. The 3/2 FQH plateau is quantized at (h/e^2 )/(3/2) within 0.02%, and can survive up to 300 mK. This even-denominator FQH plateau may imply intriguing edge structure and excitation in FQH system with lateral confinement. The observations in this work demonstrate that understanding the effect of the lateral confinement on the many-body system is critical in the pursuit of important theoretical proposals involving edge physics, such as the demonstration of non-Abelian statistics and the realization of braiding for fault-tolerant quantum computation.

cond-mat.mes-hall↗

arXiv2019

Microscopic theory of in-plane critical field in two-dimensional Ising superconducting systems

We study the in-plane critical magnetic field of two-dimensional Ising superconducting systems, and propose the microscopic theory for these systems with or without inversion symmetry. Protected by certain specific spin-orbit interaction which polarizes the electron spin to the out-of-plane direction, the in-plane critical fields largely surpass the Pauli limit and show remarkable upturn in the zero temperature limit. The impurity scattering and Rashba spin-orbit coupling, treated on equal-footing in the microscopic framework, both weaken the critical field but in qualitatively different manners. The microscopic theory is consistent with recent experimental results in stanene and Pb superconducting ultra-thin films.

cond-mat.supr-con↗

arXiv2019

Anomalous Hall effect mechanisms in quasi-2D van der Waals ferromagnet Fe0.29TaS2

The recent emergence of two-dimensional (2D) van der Waals ferromagnets has provided a new platform for exploring magnetism in the flatland and for designing 2D ferromagnet-based spintronics devices. Despite intensive studies, the anomalous Hall effect (AHE) mechanisms in 2D van der Waals ferromagnets have not been investigated yet. In this paper, we report the AHE mechanisms in quasi-2D van der Waals ferromagnet Fe0.29TaS2 via systematically measuring Fe0.29TaS2 devices with thickness from 14 nm to bulk single crystal. The AHE mechanisms are investigated via the scaling relationship between the anomalous Hall and channel conductivities. As the Fe0.29TaS2 thickness decreases, the major AHE mechanism changes from extrinsic scattering to intrinsic contribution. The crossover of the AHE mechanisms is found to be highly associated with the channel conductivities as the Fe0.29TaS2 thickness varies.

cond-mat.mtrl-sci↗