arXiv subjects

Qiang Han

Publications and source records attributed to Qiang Han.

33 records · Page 2Linked to original sources

arXiv2018

Lattice Energetics and Correlation-Driven Metal-Insulator Transitions: The Case of Ca$_2$RuO$_4$

This Letter uses density functional, dynamical mean field, and Landau-theory methods to elucidate the interplay of electronic and structural energetics in the Mott metal-insulator transition. A Landau-theory free energy is presented that incorporates the electronic energetics, the coupling of the electronic state to local distortions and the coupling of local distortions to long-wavelength strains. The theory is applied to Ca$_2$RuO$_4$. The change in lattice energy across the metal-insulator transition is comparable to the change in electronic energy. Important consequences are a strongly first order transition, a sensitive dependence of the phase boundary on pressure and that the geometrical constraints on in-plane lattice parameter associated with epitaxial growth on a substrate typically change the lattice energetics enough to eliminate the metal-insulator transition entirely.

cond-mat.str-el↗

arXiv2018

Topological invariants in terms of Green's function for the interacting Kitaev chain

The one dimensional closed interacting Kitaev chain and the dimerized version are studied. The topological invariants in terms of Green's function are calculated by the density matrix renormalization group method and the exact diagonalization method. For the interacting Kitaev chain, we point out that the calculation of topological invariant in the charge density wave phase must consider the dimerized configuration of the ground states. The variation of topological invariant are attributed to the poles of eigenvalues of the zero-frequency Green's functions. For the interacting dimerized Kitaev chain, we show that the topological invariant defined by the Green's functions can distinguish more topological nonequivalent phases than the fermion parity.

cond-mat.str-el↗

arXiv2018

Effect of interaction on the Majorana zero modes in the Kitaev chain at half filling

The one dimension interacting Kitaev chain at half filling is studied. The symmetry of the Hamiltonian is examined by dual transformations and various physical quantities as functions of the fermion-fermion interaction $U$ are calculated systematically using the density matrix renormalization group method. A special value of interaction $U_p$ is revealed in the topological region of the phase diagram. We show that at $U_p$ the ground states are strictly two-fold degenerate even though the chain length is finite and the zero-energy peak due to the Majorana zero modes is maximally enhanced and exactly localized at the end sites. $U_p$ may be attractive or repulsive depending on other system parameters. We also give a qualitative understanding of the effect of interaction under the self-consistent mean field framework.

cond-mat.supr-con↗

arXiv2015

Ferromagnetism and correlation strength in cubic barium ruthenate in comparison to strontium and calcium ruthenate: a dynamical mean field study

We present density functional plus dynamical mean field studies of cubic BaRuO$_3$ using interaction parameters previously found to be appropriate for the related materials CaRuO$_3$ and SrRuO$_3$. The calculated variation in transition temperature between the Ba and Sr compounds is consistent with experiment, confirming the assignment of the compounds to the Hund's metal family of materials, and also confirming the appropriateness of the values for the interaction parameters previously estimated and the appropriateness of the single-site dynamical mean field approximation for these materials. The results provide insights into the origin of magnetism and the role of the van Hove singularity in the physics of Hund's metals.

cond-mat.str-el↗

arXiv2012

An efficient Monte Carlo algorithm for the evaluation of Renyi entanglement entropy of a general quantum dimer model at the R-K point

A highly efficient and simple to implement Monte Carlo algorithm is proposed for the evaluation of the Renyi entanglement entropy(REE) of quantum dimer model(QDM) at the Rokhsar-Kivelson(R-K) point. It makes possible the evaluation of REE at the R-K point to the thermodynamic limit for a general QDM. We apply the algorithm to QDM on both triangular and square lattice as demonstrations and find the REE on both lattices follow perfect linear scaling in the thermodynamic limit, apart from an even-odd oscillation in the latter case. We also evaluate the topological entanglement entropy(TEE) on both lattices with a subtraction procedure. While the expected TEE of $\ln2$ is clearly demonstrated for QDM on triangular lattice, a strong oscillation of the result is found for QDM on square lattice, which implies the relevance of boundary perturbation in such a critical system.

cond-mat.str-el↗

arXiv2012

Exact analysis on the singularity of the joint density of states and its relationship to the quasiparticle interference

Singularities of the joint density of states (JDOS) and Fourier-transformed local density of states (FT-LDOS) correspond to the hot spots in quasiparticle interference patterns. In this paper the singularity of JDOS is analyzed exactly, with three types of singularities being classified. In particular, the third type of singularities are found exactly to be envelopes of the contours of constant energy. Remarkably, we show that JDOS and FT-LDOS have the same singular points. Approaching to the singular points, both quantities diverge complementarily in an inverse-square-root manner if the joint curvature is nonzero. The relative magnitude of divergence is governed by the joint curvature as well as the product of the quasiparticle velocities. If the joint curvature of certain singularity is zero, the divergence has a higher order than -1/2.

cond-mat.supr-con↗

arXiv2012

Generalized joint density of states and its application to exploring the pairing symmetry of superconductors

We introduce a generalized joint density of states (GJDOS), which incorporates the coherent factor into the JDOS, to study quasiparticle interference (QPI) in superconductors. The intimate relation between the Fourier-transformed local density of states and GJDOS is revealed: they corre- spond respectively to the real and imaginary parts of a generalized impurity-response function, and particularly share the same angular factors and singular boundaries, as seen from our approximate analytic results for d-wave superconductors. Remarkably, our numerical GJDOS analysis agrees well with the QPI patten of d-wave cuprates and s\pm-wave iron-based superconductors. Moreover, we illustrate that the present GJDOS scenario can uncover the sign features of the superconducting gap and thus can be used to explore the possible pairing symmetry of the KxFe2-ySe2 superconductors.

cond-mat.supr-con↗

arXiv2011

Unconventional Fulde-Ferrel-Larkin-Ovchinnikov states in spin-orbit coupled condensates: exact results

We find that a model Hamiltonian of s-wave superconductors in the presence of spin-orbit interactions and a Zeeman field is exactly solvable. Most intriguingly, based on the exact solutions, an unconventional type of Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) ground state is rigorously revealed, in which the center-of-mass momentum of the fermion pair is proportional to the Zeeman field. We also elaborate on the drifting effect of the Zeeman field on the spin-orbit-coupled Bose-Einstein condensate.

cond-mat.supr-con↗

arXiv2011

Monte Carlo study of thermal fluctuations and Fermi-arc formation in d-wave superconductors

From the perspective of thermal fluctuations, we investigate the pseudogap phenomena in underdoped high-temperature curpate superconductors. We present a local update Monte Carlo procedure based on the Green's function method to sample the fluctuating pairing field. The Chebyshev polynomial method is applied to calculate the single-particle spectral function directly and efficiently. The evolution of Fermi arcs as a function of temperature is studied by examining the spectral function at Fermi energy as well as the loss of spectral weight. Our results signify the importance of the vortex-like phase fluctuation on the formation of Fermi arcs.

cond-mat.supr-con↗

arXiv2011

Joint density of states and its relationship with quasiparticle interference pattern of d-wave superconductors

We present analytic analyses for the joint density of states (JDOS) and the elastic scattering susceptibility (ESS) for d-wave superconductors under the linear dispersion approximation. The profile of the JDOS, determined by the curves on which the JDOS diverges, is found to be the envelope function related to the contour of constant energy of the Bogoliubov quasiparticles. We derive the explicit form of the envelope functions. The intrinsic relationship of the JDOS patten and the octet model of ESS is clearly revealed. It is also found that apexes of the envelope function coincide with the octet vectors when the Dirac-cone anisotropy is strong, which is relevant to the d-wave cuprates.

cond-mat.supr-con↗

arXiv2011

Exact solutions for a type of electron pairing model with spin-orbit interactions and Zeeman coupling

A type of electron pairing model with spin-orbit interactions or Zeeman coupling is solved exactly in the framework of Richardson ansatz. Based on the exact solutions for the case with spin-orbit interactions, it is shown rigorously that the pairing order parameter has always the $p+ip$-wave symmetry regardless of the strength of pairing interaction, justifying an important conclusion obtained by the mean field theory. Analytical results are derived for two simple systems with respectively spin-orbit interactions and Zeeman coupling.

cond-mat.supr-con↗

arXiv2010

Pseudogap and Fermi-arc Evolution in the Phase-fluctuation Scenario

Pseudogap phenomena and the formation of Fermi arcs in underdoped cuprates are numerically studied in the presence of phase fluctuations that are simulated by an XY model. Most importantly the spectral function for each Monte Carlo sample is calculated directly and efficiently by the Chebyshev polynomials without having to diagonalize the fermion Hamiltonian, which enables us to handle a system large enough to achieve sufficient momentum/energy resolution. We find that the momentum dependence of the energy gap is identical to that of a pure d-wave superconductor well below the KT-transition temperature ($T_{KT}$), while displays an upturn deviation from $\cos k_x - \cos k_y$ with increasing temperature. An abrupt onset of the Fermi arcs is observed above $T_{KT}$ and the arc length exhibits a similar temperature dependence to the thermally activated vortex excitations.

cond-mat.supr-con↗

arXiv2010

On the origin of the Fermi arc phenomena in the underdoped cuprates: signature of KT-type superconducting transition

We study the effect of thermal phase fluctuation on the electron spectral function $A(k,ω)$ in a d-wave superconductor with Monte Carlo simulation. The phase degree of freedom is modeled by a XY-type model with build-in d-wave character. We find a ridge-like structure emerges abruptly on the underlying Fermi surface in $A(k,ω=0)$ above the KT-transition temperature of the XY model. Such a ridge-like structure, which shares the same characters with the Fermi arc observed in the pseudogap phase of the underdoped cuprates, is found to be caused by the vortex-like phase fluctuation of the XY model.

cond-mat.supr-con↗

arXiv2009

Method of studying the Bogoliubov-de Gennes equations for the superconducting vortex lattice state

In this paper, we present a method to construct the eigenspace of the normal-state electrons moving in a 2D square lattice in presence of a perpendicular uniform magnetic field which imposes (quasi)-periodic boundary conditions for the wave functions in the magnetic unit cell. An exact unitary transformations are put forward to correlate the discrete eigenvectors of the 2D electrons with those of the Harper's equation. The cyclic-tridiagonal matrix associated with the Harper's equation is then tridiagonalized by another unitary transformation. The obtained eigenbasis is utilized to expand the Bogoliubov-de Gennes equations for the superconducting vortex lattice state, which showing the merit of our method in studying the large-sized system. To test our method, we have applied our results to study the vortex lattice state of an s-wave superconductor.

cond-mat.supr-con↗

arXiv2003

Vortex State in Na_xCoO_2.yH_2O: p_x\pm ip_y-wave versus d_{x^2-y^2}\pm id_{xy}-wave Pairing

Based on an effective Hamiltonian specified in the triangular lattice with possible $p_x\pm ip_y$- or $d_{x^2-y^2}\pm id_{xy}$-wave pairing, which has close relevance to the newly discovered Na$_{0.35}$CoO$_2$$\cdot y$H$_2$O, the electronic structure of the vortex state is studied by solving the Bogoliubov-de Gennes equations. It is found that $p_x\pm ip_y$-wave is favored for the electron doping as the hopping integral $t<0$. The lowest-lying vortex bound states are found to have respectively zero and positive energies for $p_x\pm ip_y$- and $d_{x^2-y^2}\pm id_{xy}$-wave superconductors, whose vortex structures exhibit the intriguing six-fold symmetry. In the presence of strong on-site repulsion, the antiferromagnetic and ferromagnetic orders are induced around the vortex cores for the former and the latter, respectively, both of which cause the splitting of the LDOS peaks due to the lifting of spin degeneracy. STM and NMR measurements are able to probe the new features of vortex states uncovered in this work.

cond-mat.supr-con↗