arXiv subjects

Huan-Qiang Zhou

Publications and source records attributed to Huan-Qiang Zhou.

At least 73 records · Page 4Linked to original sources

arXiv2009

Kosterlitz-Thouless Phase Transition and Ground State Fidelity: a Novel Perspective from Matrix Product States

The Kosterlitz-Thouless transition is studied from the representation of the systems's ground state wave functions in terms of Matrix Product States for a quantum system on an infinite-size lattice in one spatial dimension. It is found that, in the critical regime for a one-dimensional quantum lattice system with continuous symmetry, the newly-developed infinite Matrix Product State algorithm automatically leads to infinite degenerate ground states, due to the finiteness of the truncation dimension. This results in \textit{pseudo} continuous symmetry spontaneous breakdown, which allows to introduce a pseudo-order parameter that must be scaled down to zero, in order to be consistent with the Mermin-Wegner theorem. We also show that the ground state fidelity per lattice site exhibits a \textit{catastrophe point}, thus resolving a controversy regarding whether or not the ground state fidelity is able to detect the Kosterlitz-Thouless transition.

cond-mat.stat-mech↗

arXiv2009

Spontaneous symmetry breaking and bifurcations in ground state fidelity for quantum lattice systems

Spontaneous symmetry breaking occurs in a system when its Hamiltonian possesses a certain symmetry, whereas the ground state wave functions do not preserve it. This provides such a scenario that a bifurcation, which breaks the symmetry, occurs when some control parameter crosses its critical value. It is unveiled that the ground state fidelity per lattice site exhibits such a bifurcation for quantum lattice systems undergoing quantum phase transitions. The significance of this result lies in the fact that the ground state fidelity per lattice site is \textit{universal}, in the sense that it is model-independent, in contrast to (model-dependent) order parameters. This fundamental quantity may be computed by exploiting the newly-developed tensor network algorithms on infinite-size lattices. We illustrate the scheme in terms of the quantum Ising model in a transverse magnetic field and the spin 1/2 XYX model in an external magnetic field on an infinite-size lattice in one spatial dimension.

cond-mat.stat-mech↗

arXiv2008

Singularities in ground state fidelity and quantum phase transitions for the Kitaev model

The ground state fidelity per lattice site is shown to be able to detect quantum phase transitions for the Kitaev model on the honeycomb lattice, a prototypical example of quantum lattice systems with topological order. It is found that, in the thermodynamic limit, the ground state fidelity per lattice site is non-analytic at the phase boundaries: the second-order derivative of its logarithmic function with respect to a control parameter describing the interaction between neighboring spins is logarithmically divergent. A finite size scaling analysis is performed, which allows us to extract the correlation length critical exponent from the scaling behaviors of the ground state fidelity per lattice site.

cond-mat.str-el↗

arXiv2008

Deriving local order parameters from tensor network representations

A large class of quantum phase transitions for quantum lattice systems are characterized by local order parameters. It is shown that local order parameters may be systematically constructed from tensor network representations of quantum many-body ground state wave functions by investigating the reduced density matrices for local areas on an infinite-size lattice. Depending on whether or not the system symmetries are spontaneously broken, and whether or not the ground state fidelity per lattice site is continuous, there are four categories of quantum phase transitions for systems with local order parameters. Quantum phase transitions characterized by nonlocal order parameters are discussed, aiming at better understanding quantum systems with topological order.

cond-mat.stat-mech↗

arXiv2008

Ground state fidelity from tensor network representations

For any D-dimensional quantum lattice system, the fidelity between two ground state many-body wave functions is mapped onto the partition function of a D-dimensional classical statistical vertex lattice model with the same lattice geometry. The fidelity per lattice site, analogous to the free energy per site, is well-defined in the thermodynamic limit and can be used to characterize the phase diagram of the model. We explain how to compute the fidelity per site in the context of tensor network algorithms, and demonstrate the approach by analyzing the two-dimensional quantum Ising model with transverse and parallel magnetic fields.

cond-mat.stat-mech↗

arXiv2007

Singularities in fidelity surfaces for quantum phase transitions: a geometric perspective

The fidelity per site between two ground states of a quantum lattice system corresponding to different values of the control parameter defines a surface embedded in a Euclidean space. The Gaussian curvature naturally quantifies quantum fluctuations that destroy orders at transition points. It turns out that quantum fluctuations wildly distort the fidelity surface near the transition points, at which the Gaussian curvature is singular in the thermodynamic limit. As a concrete example, the one-dimensional quantum Ising model in a transverse field is analyzed. We also perform a finite size scaling analysis for the transverse Ising model of finite sizes. The scaling behavior for the Gaussian curvature is numerically checked and the correlation length critical exponent is extracted, which is consistent with the conformal invariance at the critical point.

cond-mat.stat-mech↗

arXiv2007

Renormalization group flows and quantum phase transitions: fidelity versus entanglement

We compare the roles of fidelity and entanglement in characterizing renormalization group flows and quantum phase transitions. It turns out that the scaling parameter extracted from fidelity for different ground states succeeds to capture nontrivial information including stable and unstable fixed points, whereas the von Neumann entropy as a bipartite entanglement measure (or equivalently, majorization relations satisfied by the spectra of the reduced density matrix along renormalization group flows) often fails, as far as the intrinsic irreversibility-information loss along renormalization group flows-is concerned. We also clarify an intimate connection between the von Neumman entropy, majorization relations, and fidelity. The relevance to Zamolodchikov's c theorem is indicated.

cond-mat.stat-mech↗

arXiv2007

Fidelity approach to quantum phase transitions: finite size scaling for quantum Ising model in a transverse field

We analyze the scaling parameter, extracted from the fidelity for two different ground states, for the one-dimensional quantum Ising model in a transverse field near the critical point. It is found that, in the thermodynamic limit, the scaling parameter is singular, and the derivative of its logarithmic function with respect to the transverse field strength is logarithmically divergent at the critical point. The scaling behavior is confirmed numerically by performing a finite size scaling analysis for systems of different sizes, consistent with the conformal invariance at the critical point. This allows us to extract the correlation length critical exponent, which turns out to be universal in the sense that the correlation length critical exponent does not depend on either the anisotropic parameter or the transverse field strength.

cond-mat.stat-mech↗

arXiv2007

Fidelity and quantum phase transitions

It is shown that the fidelity, a basic notion of quantum information science, may be used to characterize quantum phase transitions, regardless of what type of internal order is present in quantum many-body states. If the fidelity of two given states vanishes, then there are two cases: (1) they are in the same phase if the distinguishability results from irrelevant local information; or (2) they are in different phases if the distinguishability results from relevant long-distance information. The different effects of irrelevant and relevant information are quantified, which allows us to identify unstable and stable fixed points (in the sense of renormalization group theory). A physical implication of our results is the occurrence of the orthogonality catastrophe near the transition points.

cond-mat.stat-mech↗

arXiv2005

Entanglement and boundary critical phenomena

We investigate boundary critical phenomena from a quantum information perspective. Bipartite entanglement in the ground state of one-dimensional quantum systems is quantified using the Renyi entropy S_alpha, which includes the von Neumann entropy (alpha=1) and the single-copy entanglement (alpha=infinity) as special cases. We identify the contribution from the boundary entropy to the Renyi entropy, and show that there is an entanglement loss along boundary renormalization group (RG) flows. This property, which is intimately related to the Affleck-Ludwig g-theorem, can be regarded as a consequence of majorization relations between the spectra of the reduced density matrix along the boundary RG flows. We also point out that the bulk contribution to the single-copy entanglement is half of that to the von Neumann entropy, whereas the boundary contribution is the same.

cond-mat.str-el↗

arXiv2005

Quantifying ground state entanglement of the BCS model

The concept of local concurrence is used to quantify the entanglement between a single qubit and the remainder of a multi-qubit system. For the ground state of the BCS model in the thermodynamic limit the set of local concurrences completely describe the entanglement. As a measure for the entanglement of the full system we investigate the Average Local Concurrence (ALC). We find that the ALC satisfies a simple relation with the order parameter. We then show that for finite systems with fixed particle number, a relation between the ALC and the condensation energy exposes a threshold coupling. Below the threshold, entanglement measures besides the ALC are significant.

quant-ph↗

arXiv2005

Geometric phases of scattering states in a ring geometry: adiabatic pumping in mesoscopic devices

Geometric phases of scattering states in a ring geometry are studied based on a variant of the adiabatic theorem. Three time scales, i.e., the adiabatic period, the system time and the dwell time, associated with adiabatic scattering in a ring geometry plays a crucial role in determining geometric phases, in contrast to only two time scales, i.e., the adiabatic period and the dwell time, in an open system. We derive a formula connecting the gauge invariant geometric phases acquired by time-reversed scattering states and the circulating (pumping) current. A numerical calculation shows that the effect of the geometric phases is observable in a nanoscale electronic device.

cond-mat.mes-hall↗

arXiv2004

Energy level statistics for models of coupled single-mode Bose--Einstein condensates

We study the distribution of energy level spacings in two models describing coupled single-mode Bose-Einstein condensates. Both models have a fixed number of degrees of freedom, which is small compared to the number of interaction parameters, and is independent of the dimensionality of the Hilbert space. We find that the distribution follows a universal Poisson form independent of the choice of coupling parameters, which is indicative of the integrability of both models. These results complement those for integrable lattice models where the number of degrees of freedom increases with increasing dimensionality of the Hilbert space. Finally, we also show that for one model the inclusion of an additional interaction which breaks the integrability leads to a non-Poisson distribution.

quant-ph↗

arXiv2004

Quantum phase transitions and magnetization of an integrable spin ladder with new parameters in bridging to real compounds

We study the field-induced quantum phase transitions (QPT) and the relevant magnetic properties of a spin-1/2 two-leg integrable spin ladder (ISL), of which the system parameters in bridging to the real compounds are determined by setting the extra interactions in the Hamiltonian of the ISL relative to the Heisenberg spin ladder to vanish in the expectation in the ground state (GS). Such an ISL analytically has the correct leading terms of both the critical fields of the two QPT's as in the real strongly-coupled compounds: gμ_BH_{c1}=J_{\perp}-J_{\parallel} and gμ_BH_{c2}=J_{\perp}+2J_{\parallel} in terms of the experimental leg (J_{\parallel}) and rung (J_{\perp}) interactions. The symmetric magnetization inflection point is located at gμ_BH_{IP}=J_{\perp}+J_{\parallel}/2. The magnetizations for the GS and at finite temperatures, as well as the susceptibility, show good agreements in various comparisons with the finite-site exact diagonalization, the transfer-matrix renormalization group numerical result, the perturbation theory, and the compounds (5IAP)_2CuBr_4\cdot 2H_2O, Cu_2(C_5H_{12}N_2)_2Cl_4 and (C_5H_{12}N)_2CuBr_4.

cond-mat.str-el↗

arXiv2003

Measuring geometric phases of scattering states in nanoscale electronic devices

We show how a new quantum property, a geometric phase, associated with scattering states can be exhibited in nanoscale electronic devices. We propose an experiment to use interference to directly measure the effect of the new geometric phase. The setup involves a double path interferometer, adapted from that used to measure the phase evolution of electrons as they traverse a quantum dot (QD). Gate voltages on the QD could be varied cyclically and adiabatically, in a manner similar to that used to observe quantum adiabatic charge pumping. The interference due to the geometric phase results in oscillations in the current collected in the drain when a small bias across the device is applied. We illustrate the effect with examples of geometric phases resulting from both Abelian and non-Abelian gauge potentials.

cond-mat.mes-hall↗

arXiv2003

Phase diagram and symmetry breaking of SU(4) spin-orbital chain in a generalized external field

The ground state phases of a one-dimensional SU(4) spin-orbital Hamiltonian in a generalized external field are studied on the basis of Bethe-ansatz solution. Introducing three Landé $g$ factors for spin, orbital and their products in the SU(4) Zeeman term, we discuss systematically the various symmetry breaking. The magnetization versus external field are obtained by solving Bethe-ansatz equations numerically. The phase diagrams corresponding to distinct residual symmetries are given by means of both numerical and analytical methods.

cond-mat.str-el↗

arXiv2003

Gauge Fields, Geometric Phases, and Quantum Adiabatic Pumps

Quantum adiabatic pumping of charge and spin between two reservoirs (leads) has recently been demonstrated in nanoscale electronic devices. Pumping occurs when two or more system parameters are varied in a cyclic manner and sufficiently slowly that the quantum system always remains in its ground state. We show that quantum pumping has a natural geometric representation in terms of gauge fields (both Abelian and non-Abelian) defined on the space of system parameters. We make explicit the similarities and differences with Berry's geometric phase. Tunneling from a scanning tunneling microscope tip through a magnetic atom could be used to demonstrate the non-Abelian character of the gauge field.

cond-mat.mes-hall↗

arXiv2003

Quantum Transport and Integrability of the Anderson Model for a Quantum Dot with Multiple Leads

We show that an Anderson Hamiltonian describing a quantum dot connected to multiple leads is integrable. A general expression for the non-linear conductance is obtained by combining the Bethe ansatz exact solution with Landauer-Büttiker theory. In the Kondo regime, a closed form expression is given for the matrix conductance at zero temperature and when all the leads are close to the symmetric point. A bias-induced splitting of the Kondo resonance is possible for three or more leads. Specifically, for $N$ leads, with each at a different chemical potential, there can be $N-1$ Kondo peaks in the conductance.

cond-mat.mes-hall↗