arXiv subjects

Qifeng Liang

Publications and source records attributed to Qifeng Liang.

11 recordsLinked to original sources

arXiv2026

The Locality Cost of Fully Flat Hopf Insulators

Hopf topology permits a strictly finite-range Hamiltonian with one exactly flat topological band. We prove, however, that extending flatness to the complete two-band spectrum necessarily sacrifices strict locality or the gap: any gapped, Hermitian, translationally invariant two-band Hamiltonian with strictly finite-range hopping and two exactly flat bands has vanishing Hopf invariant. Equivalently, within this two-band setting, a Hopf band admits no compactly supported, translation-covariant, orthonormal Wannier generator. For factorized one-flat-band Hopf parents, the unavoidable partner dispersion equals the Gram symbol of translated compact localized states and encodes their nonorthogonality. Full flattening converts this dispersion into exponentially decaying but infinitely supported hopping. Model-independent bounds provide a sufficient criterion for finite-range approximants to retain the Hopf phase. An explicit model yields the axial decay length $\xi_z/a=1/\ln 2$, parameter-free hopping tails, and residual bandwidths testable in circuit and photonic lattices. Hopf topology therefore does not prohibit a flat band but forces the locality--flatness cost to appear as either partner-band dispersion or nonlocal hopping.

cond-mat.mtrl-sci↗

arXiv2024

Band-Structure-Independent Topology from Nonsymmorphic Wannier Complexes

Nonsymmorphic symmetries can enforce band connectivity that obstructs a single-band Wannier description. We show that a fractional translation $\mathcal{L}$ connecting distinct high-symmetry Wyckoff positions generically renders the Wannier center of an individual band gauge ill-defined, requiring a symmetry-enforced multiband object -- a Wannier complex. We formulate a real-space topological classification of Wannier complexes and show that, when $\mathcal{L}$ is combined with certain point-group symmetries (notably $C_4$ and $C_3$), all symmetry-allowed Wannier-complex configurations carry a nontrivial quantized total electric polarization. This yields boundary phenomena that persist across symmetry-preserving deformations of the Hamiltonian, including parameter regimes with and without bulk gaps. We demonstrate the mechanism in minimal tight-binding models exhibiting M{\"o}bius-twisted Wilson-loop structures and higher-order corner modes, and propose experimental signatures in a dielectric photonic crystal and a first-principles electronic platform octa-graphene, accompanied by a three-dimensional extension.

cond-mat.supr-con↗

arXiv2020

High-Pressure Crystal Growth, Superconducting Properties, and Electronic Band Structure of Nb2P5

Orthorhombic (space group: Pnma) Nb2P5 is a high-pressure phase that is quenchable to ambient pressure, which could viewed as the zigzag infinite P chain-inserted NbP2. We report herein the high-pressure crystal growth of Nb2P5 and the discovery of its superconducting transition at Tc ~ 2.6 K. The electrical resistivity, magnetization, and specific heat capacity measurements on the high-quality crystal unveiled a conventional type-II weakly coupled s-wave nature of the superconductivity, with the upper critical field Hc2(0) ~ 0.5 T, the electron-phonon coupling strength λep ~ 0.5 - 0.8, and the Ginzburg-Landau parameter \k{appa} ~ 100. The ab initio calculations on the electronic band structure unveiled nodal-line structures protected by different symmetries. The one caused by band inversion, for example, on the Γ-X and U-R paths of the Brillouin zone, likely could bring nontrivial topology and hence possible nontrivial surface state on the surface. The surface states on the (100), (010) and (110) surfaces were also calculated and discussed. The discovery of the phosphorus-rich Nb2P5 superconductor would be instructive for the design of more metal phosphides superconductors which might host unconventional superconductivity or potential technical applications.

cond-mat.supr-con↗

arXiv2019

Robust magnetoresistance in TaAs2 under pressure up to about 37 GPa

The extremely large magnetoresistance (XMR) in nonmagnetic semimetals has inspired growing interest owing to both intriguing physics and potential applications. We report results of synchrotron X-ray diffraction (SXRD) and electrical transport measurements on TaAs2 under pressure up to ~ 37 GPa, which revealed an anisotropic compression of the unit cell, formation of unusual As-As bonds above 9.5 GPa, and enhancement of metallicity. Interestingly, the MR of TaAs2 under pressure changed gently, which at 1.7 GPa is 96.6% and at 36.6 GPa is still 36.7%. The almost robust MR under pressure could be related to the nearly stable electronic structure unveiled by the ab initio calculations. The discovery would expand the potential use of XMR even under high pressure.

cond-mat.mtrl-sci↗

arXiv2019

Quantum oscillations and nontrivial topological state in a compensated semimetal TaP2

We report systematic magneto-transport measurements and ab initio calculations on single-crystalline TaP2, a new member of the transition-metal dipnictides. We observed unsaturated magnetoresistance (MR) reaching ~ 700% at a magnetic field (B) of 9 T at 2 K along with striking Shubnikov-de Hass (SdH) oscillations. Our analysis on the SdH oscillations reveals nonzero Berry phase, indicating nontrivial band topology. The analysis also uncovers three fundamental magnetic oscillation frequencies of 72 T, 237 T, and 356 T, consistent with the theoretical calculations which reveal one hole pocket and two electron pockets at the L point and one electron pocket at the Z point of the Brillouin zone. We also found negative longitudinal MR (n-MR) within a narrow window of the angles between B and the electric current (I). The n-MR could be fitted with the Adler-Bell-Jackiw chiral anomaly equation but the origin remains yet ambiguous. The ab inito calculations suggest TaP2 as a weak topological insulator with the Z2 indices of (0; 111), which exhibits topological surface states on the (001) surface.

cond-mat.str-el↗

arXiv2017

Majorana phi0 Junction in a Disordered Spin-orbit Coupling Nanowire with Tilted Magnetic Field

Majorana Josephson junctions in nanowire systems exhibit a pseudo-$4π$ period current-phase relation in the clean limit. In this work, we study how this current-phase relation responds to a tilted magnetic field in a disordered Majorana Josephson junction within the Bogoliubov-de Gennes approach. We show that the tilted magnetic field induces a $ϕ_0$ phase shift to the current-phase relation. Most importantly, we find that this $ϕ_0$-junction behavior is robust even in the presence of disorders.

cond-mat.mes-hall↗

arXiv2014

Tunable Semimetallic State in Compressive-strained SrIrO3 Films Revealed by Transport Behaviors

Orthorhombic SrIrO3 is a typical spin-orbit-coupling correlated metal that shows diversified physical properties under the external stimuli. Here nonlinear Hall effect and weakly temperature-dependent resistance are observed in a SrIrO3 film epitaxially grown on SrTiO3 substrate. It infers that orthorhombic SrIrO3 is a semimetal oxide. However, linear Hall effect and insensitive-temperature-dependent resistance are observed in SrIrO3 films grown on (La,Sr)(Al,Ta)O3 (LSAT) substrates, suggesting a tunable semimetallic state due to band structure change in SrIrO3 films under different compressive strain. The mechanism of this evolution is explored in detail through strain-state analysis by reciprocal space mapping and electron diffraction, carrier density and mobility calculations, as well as electronic band structure evolution under compressive strain (predicted by tight-binding approximation). It might suggest that the strain-induced band shift leads to the semimetallic tuning in the SrIrO3 film grown on from SrTiO3 to LSAT substrates. Our findings illustrate the tunability of SrIrO3 properties and pave the way to induce novel physical states in SrIrO3 such as the proposed topological insulator state in heterostructures.

cond-mat.mtrl-sci↗

arXiv2012

Analytical solutions for the Rabi model

The Rabi model that describes the fundamental interaction between a two-level system with a quantized harmonic oscillator is one of the simplest and most ubiquitous models in modern physics. However, this model has not been solved exactly because it is hard to find a second conserved quantity besides the energy. Here we present a unitary transformation to map this unsolvable Rabi model into a solvable Jaynes-Cummings-like model by choosing a proper variation parameter. As a result, the analytical energy spectrums and wavefunctions including both the ground and the excited states can be obtained easily. Moreover, these explicit results agree well with the direct numerical simulations in a wide range of the experimental parameters. In addition, based on our obtained energy spectrums, the recent experimental observation of Bloch-Siegert in the circuit quantum electrodynamics with the ultrastrong coupling can be explained perfectly. Our results have the potential application in the solid-state quantum information processing.

quant-ph↗

arXiv2011

Quantum resonant effect of the strongly-driven spin-boson model

In this paper we discuss both analytically and numerically the rich quantum dynamics of the spin-boson model driven by a time-independent field of photon. Interestingly, we predict a new Rabi oscillation, whose period is inversely proportional to the driving amplitude. More surprisingly, some nonzero resonant peaks are found for some special values of the \emph{strong} driving regime. Moreover, for the different resonant positions, the peaks have different values. Thus, an important application of this resonance effect is to realize the precision measurement of the relative parameters in experiment. We also illustrate that this resonant effect arises from the interference of the nontrivial periodic phase factors induced by the evolution of the coherent states in two different subspaces. Our predictions may be, in principle, observed in the solid-state cavity quantum electrodynamics with the ultrastrong coupling if the driving magnitude of the photon field is sufficiently large.

quant-ph↗

arXiv2011

Analytical ground state for the Jaynes-Cummings model with the ultrastrong coupling

We present a generalized variational method to analytically obtain the ground-state properties of the unsolvable Jaynes-Cummings model with the ultrastrong coupling. An explicit expression for the ground-state energy, which agrees well with the numerical simulation in a wide range of the experimental parameters, is given. In particular, the introduced method can successfully solve this Jaynes-Cummings model with the positive detuning (the atomic resonant level is larger than the photon frequency), which can not be treated in the adiabatical approximation and the generalized rotating-wave approximation. Finally, we also demonstrate analytically how to control the mean photon number by means of the current experimental parameters including the photon frequency, the coupling strength, and especially the atomic resonant level.

quant-ph↗

arXiv2009

Enhanced spin-orbit coupling in hydrogenated and fluorinated graphenes studied from first principles

The spin-orbit couplings (SOCs) of hydrogenated and fluorinated graphenes are calculated from the first principles method. It is found that the SOC-induced band splittings near their Fermi energies can be significantly enhanced to the order of 10$^{-2}$ eV from the original about 10$^{-6}$ eV of the pure raphene, which is comparable to those found in the diamond and even the archetypal semiconductors. And two different mechanisms are proposed to explain the SOC enhancements in these two systems. The huge SOC enhancements are found to come not only from the sp$^3$ hybridization of carbon atoms, but also from the larger intrinsic SOC of the fluorine atom than the carbon one. We hope many interesting phenomena caused by the SOCs (e.g. the spin Hall effect) can be observed experimentally in these systems.

cond-mat.mes-hall↗