arXiv subjects

S. R. White

Publications and source records attributed to S. R. White.

At least 19 recordsLinked to original sources

arXiv2017

Hybrid-space density matrix renormalization group study of the doped two-dimensional Hubbard model

The performance of the density matrix renormalization group (DMRG) is strongly influenced by the choice of the local basis of the underlying physical lattice. We demonstrate that, for the two-dimensional Hubbard model, the hybrid real-momentum space formulation of the DMRG is computationally more efficient than the standard real-space formulation. In particular, we show that the computational cost for fixed bond dimension of the hybrid-space DMRG is approximately independent of the width of the lattice, in contrast to the real-space DMRG, for which it is proportional to the width squared. We apply the hybrid-space algorithm to calculate the ground state of the doped two-dimensional Hubbard model on cylinders of width four and six sites; at $n=0.875$ filling, the ground state exhibits a striped charge-density distribution with a wavelength of eight sites for both $U/t=4.0$ and $U/t=8.0$. We find that the strength of the charge ordering depends on $U/t$ and on the boundary conditions.Furthermore, we investigate the magnetic ordering as well as the decay of the static spin, charge, and pair-field correlation functions.

cond-mat.str-el↗

arXiv2015

One hole in the two-leg t-J ladder and adiabatic continuity to the non-interacting limit

We have carried out density-matrix-renormalization group (DMRG) calculations for the problem of one doped hole in a two-leg $t-J$ ladder. Recent studies have concluded that exotic "Mott" physics --- arising from the projection onto the space of no double-occupied sites --- is manifest in this model system, leading to charge localization and a new mechanism for charge modulation. In contrast, we show that there is no localization and that the charge density modulation arises when the minimum in the quasiparticle dispersion moves away from $π$. Although singular changes in the quasiparticle dispersion do occur as a function of model parameters, all the DMRG results can be qualitatively understood from a non-interacting "band-structure" perspective.

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arXiv2012

Hubbard-model description of the high-energy spin-spectral-weight distribution in La(2)CuO(4)

The spectral-weight distribution in recent neutron scattering experiments on the parent compound La$_2$CuO$_4$ (LCO), which are limited in energy range to about 450\,meV, is studied in the framework of the Hubbard model on the square lattice with effective nearest-neighbor transfer integral $t$ and on-site repulsion $U$. Our study combines a number of numerical and theoretical approaches, including, in addition to standard treatments, density matrix renormalization group calculations for Hubbard cylinders and a suitable spinon approach for the spin excitations. Our results confirm that the $U/8t$ magnitude suitable to LCO corresponds to intermediate $U$ values smaller than the bandwidth $8t$, which we estimate to be $8t \approx 2.36$ eV for $U/8t\approx 0.76$. This confirms the unsuitability of the conventional linear spin-wave theory. Our theoretical studies provide evidence for the occurrence of ground-state d-wave spinon pairing in the half-filled Hubbard model on the square lattice. This pairing applies only to the rotated-electron spin degrees of freedom, but it could play a role in a possible electron d-wave pairing formation upon hole doping. We find that the higher-energy spin spectral weight extends to about 566 meV and is located at and near the momentum $[π,π]$. The continuum weight energy-integrated intensity vanishes or is extremely small at momentum $[π,0]$. This behavior of this intensity is consistent with that of the spin waves observed in recent high-energy neutron scattering experiments, which are damped at the momentum $[π,0]$. We suggest that future LCO neutron scattering experiments scan the energies between 450 meV and 566 meV and momenta around $[π,π]$.

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arXiv2012

Stripe Structures in the t-t'-J Model

Here, based upon density matrix renormalization group calculations, we discuss the structure of the stripes found in the doped $t$-$t^\prime$-$J$ model and the physics that underlies their formation.

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arXiv2012

Charge dynamics in half-filled Hubbard chains with finite on-site interaction

We study the charge dynamic structure factor of the one-dimensional Hubbard model with finite on-site repulsion U at half filling. Numerical results from the time-dependent density matrix renormalization group are analyzed by comparison with the exact spectrum of the model. The evolution of the line shape as a function of U is explained in terms of a relative transfer of spectral weight between the two-holon continuum that dominates in the limit U\to \infty and a subset of the two-holon-two-spinon continuum that reconstructs the electron-hole continuum in the limit U\to 0. Power-law singularities along boundary lines of the spectrum are described by effective impurity models that are explicitly invariant under spin and η-spin SU(2) rotations. The Mott-Hubbard metal-insulator transition is reflected in a discontinuous change of the exponents of edge singularities at U=0. The sharp feature observed in the spectrum for momenta near the zone boundary is attributed to a Van Hove singularity that persists as a consequence of integrability.

cond-mat.str-el↗

arXiv2012

Spin-spectral-weight distribution and energy range of the parent compound La$_2$CuO$_4$

The spectral-weight distribution in recent neutron scattering experiments on the parent compound La$_2$CuO$_4$ (LCO), which are limited in energy range to about 450\,meV, is studied in the framework of the Hubbard model on the square lattice. We find that the higher-energy weight extends to about 566\,meV and is located at and near the momentum $[π,π]$. Our results confirm that the $U/t$ value suitable to LCO is in the range $U/t\in (6,8)$. The continuum weight energy-integrated intensity vanishes or is extremely small at momentum $[π,0]$. This behavior of the intensity is consistent with that of spin waves, which are damped at $[π,0]$.

cond-mat.str-el↗

arXiv2011

Stripes in the two-dimensional t-J model with infinite projected entangled-pair states

We simulate the $t$ $J$ model in two dimensions by means of infinite projected entangled-pair states (iPEPS) generalized to arbitrary unit cells, finding results similar to those previously obtained by the density-matrix renormalization group (DMRG) for wide ladders. In particular, we show that states exhibiting stripes, {\it i.e}. a unidirectional modulation of hole-density and antiferromagnetic order with a $π$-phase shift between adjacent stripes, have a lower variational energy than uniform phases predicted by variational and fixed-node Monte Carlo simulations. For a fixed unit-cell size the energy per hole is minimized for a hole density $ρ_l\sim 0.5$ per unit length of a stripe. The superconducting order parameter is maximal around $ρ_l\sim 0.75-0.8$.

cond-mat.str-el↗

arXiv2008

Trionic and quartetting phases in one-dimensional multicomponent ultracold fermions

We investigate the possible formation of a molecular condensate, which might be, for instance, the analogue of the alpha condensate of nuclear physics, in the context of multicomponent cold atoms fermionic systems. A simple paradigmatic model of N-component fermions with contact interactions loaded into a one-dimensional optical lattice is studied by means of low-energy and numerical approaches. For attractive interaction, a quasi-long-range molecular superfluid phase, formed from bound-states made of N fermions, emerges at low density. We show that trionic and quartetting phases, respectively for N=3,4, extend in a large domain of the phase diagram and are robust against small symmetry-breaking perturbations.

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arXiv2008

Molecular superfluid phase in systems of one-dimensional multicomponent fermionic cold atoms

We study a simple model of N-component fermions with contact interactions which describes fermionic atoms with N=2F+1 hyperfine states loaded into a one-dimensional optical lattice. We show by means of analytical and numerical approaches that, for attractive interaction, a quasi-long-range molecular superfluid phase emerges at low density. In such a phase, the pairing instability is strongly suppressed and the leading instability is formed from bound-states made of N fermions. At small density, the molecular superfluid phase is generic and exists for a wide range of attractive contact interactions without an SU(N) symmetry between the hyperfine states.

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arXiv2007

Diamagnetism of doped two-leg ladders and probing the nature of their commensurate phases

We study the magnetic orbital effect of a doped two-leg ladder in the presence of a magnetic field component perpendicular to the ladder plane. Combining both low-energy approach (bosonization) and numerical simulations (density-matrix renormalization group) on the strong coupling limit (t-J model), a rich phase diagram is established as a function of hole doping and magnetic flux. Above a critical flux, the spin gap is destroyed and a Luttinger liquid phase is stabilized. Above a second critical flux, a reentrance of the spin gap at high magnetic flux is found. Interestingly, the phase transitions are associated with a change of sign of the orbital susceptibility. Focusing on the small magnetic field regime, the spin-gapped superconducting phase is robust but immediately acquires algebraic transverse (i.e. along rungs) current correlations which are commensurate with the 4k_F density correlations. In addition, we have computed the zero-field orbital susceptibility for a large range of doping and interactions ratio J/t : we found strong anomalies at low J/t only in the vicinity of the commensurate fillings corresponding to delta = 1/4 and 1/2. Furthermore, the behavior of the orbital susceptibility reveals that the nature of these insulating phases is different: while for delta = 1/4 a 4k_F charge density wave is confirmed, the delta = 1/2 phase is shown to be a bond order wave.

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arXiv2007

Dynamical structure factor at small q for the XXZ spin-1/2 chain

We combine Bethe Ansatz and field theory methods to study the longitudinal dynamical structure factor S^{zz}(q,omega) for the anisotropic spin-1/2 chain in the gapless regime. Using bosonization, we derive a low energy effective model, including the leading irrelevant operators (band curvature terms) which account for boson decay processes. The coupling constants of the effective model for finite anisotropy and finite magnetic field are determined exactly by comparison with corrections to thermodynamic quantities calculated by Bethe Ansatz. We show that a good approximation for the shape of the on-shell peak of S^{zz}(q,omega) in the interacting case is obtained by rescaling the result for free fermions by certain coefficients extracted from the effective Hamiltonian. In particular, the width of the on-shell peak is argued to scale like delta omega_{q} ~ q^2 and this prediction is shown to agree with the width of the two-particle continuum at finite fields calculated from the Bethe Ansatz equations. An exception to the q^2 scaling is found at finite field and large anisotropy parameter (near the isotropic point). We also present the calculation of the high-frequency tail of S^{zz}(q,ω) in the region delta omega_{q}<< omega-vq << J using finite-order perturbation theory in the band curvature terms. Both the width of the on-shell peak and the high-frequency tail are compared with S^{zz}(q,omega) calculated by Bethe Ansatz for finite chains using determinant expressions for the form factors and excellent agreement is obtained. Finally, the accuracy of the form factors is checked against the exact first moment sum rule and the static structure factor calculated by Density Matrix Renormalization Group (DMRG).

cond-mat.str-el↗

arXiv2007

Boson decay and the dynamical structure factor for the XXZ chain at finite magnetic field

We study the longitudinal dynamical structure factor $S^{zz}(q,ω)$ for the anisotropic spin-1/2 (XXZ) chain at finite magnetic field using bosonization. The leading irrelevant operators in the effective bosonic model stemming from band curvature describe boson decay processes and lead to a high-frequency tail and a finite width $γ_q$ of the on-shell peak for $S^{zz}(q,ω)$. We use the Bethe ansatz to show that $γ_q\sim q^2$ for $q\ll 1$ and to calculate the amplitudes of the leading irrelevant operators in the effective field theory.

cond-mat.str-el↗

arXiv2006

Probing the pairing symmetry and pair charge stiffness of doped $t-J$ ladders

We perform the numerical equivalent of a phase sensitive experiment on doped $t-J$ ladders. We apply proximity effect fields with different complex phases at both ends of an open system and we study the transport of Cooper pairs. Measuring the response of the system and the induced Josephson current, Density Matrix Renormalization Group calculations show how, depending on the doping fraction, the rung-leg parity of the pair field changes from minus to plus as the density of holes is increased. We also study the pair charge stiffness, and we observe a supression of the superconductivity in the region where static stripes appear. We compare our results with predictions from bosonization and renormalization group analysis.

cond-mat.str-el↗

arXiv2006

Cooper-pair transport through a Hubbard chain sandwiched between two superconductors: Density matrix renormalization group calculations

We present a numerical approach to study the coherent transport of Cooper pairs through a Hubbard chain, and study the role of the contacts in achieving perfect Andreev reflection. We calculate the pair transport using the Density Matrix Renormalization Group by measuring the response of the system to quantum pair fields with complex phases on the two ends of an open system. This approach gives an effective superfluid weight which is in close agreement with the Bethe Ansatz results for the superfluid weight for closed Hubbard rings.

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arXiv2006

Zeeman effect in superconducting two-leg ladders: irrational magnetization plateaus and exceeding the Pauli limit

The effect of a parallel magnetic field on superconducting two-leg ladders is investigated numerically. The magnetization curve displays an irrational plateau at a magnetization equal to the hole density. Remarkably, its stability is fundamentally connected to the existence of a well-known magnetic resonant mode. Once the zero-field spin gap is suppressed by the field, pairs acquire a finite momentum characteristic of a Fulde-Ferrell-Larkin-Ovchinnikov phase. In addition, S^z=0 triplet superconducting correlations coexist with singlet ones above the irrational plateau. This provides a simple mechanism in which the Pauli limit is exceeded as suggested by recent experiments.

cond-mat.str-el↗

arXiv2006

The dynamical spin structure factor for the anisotropic spin-1/2 Heisenberg chain

The longitudinal spin structure factor for the XXZ-chain at small wave-vector q is obtained using Bethe Ansatz, field theory methods and the Density Matrix Renormalization Group. It consists of a peak with peculiar, non-Lorentzian shape and a high-frequency tail. We show that the width of the peak is proportional to q^2 for finite magnetic field compared to q^3 for zero field. For the tail we derive an analytic formula without any adjustable parameters and demonstrate that the integrability of the model directly affects the lineshape.

cond-mat.str-el↗

arXiv2006

Methods for Time Dependence in DMRG

A major advance in density-matrix renormalization group (DMRG) calculations has been achieved by the invention of highly efficient DMRG techniques for the simulation of real-time dynamics of strongly correlated quantum systems in one dimension. Starting from established linear-response techniques in DMRG and early attempts at real-time dynamics, we go on to review two current methods which both implement the idea of adapting the effective Hilbert space of DMRG to the quantum state evolving in time. We also give an outlook on extensions to finite temperature calculations.

cond-mat.str-el↗

arXiv2005

Measuring orbital interaction using quantum information theory

Quantum information theory gives rise to a straightforward definition of the interaction of electrons $I_{p,q}$ in two orbitals $p$, $q$ for a given many-body wave function. A convenient way to calculate the von Neumann entropies needed is presented in this work, and the orbital interaction $I_{p,q}$ is successfully tested for different types of chemical bonds. As an example of an application of $I_{p,q}$ beyond the interpretation of wave functions, $I_{p,q}$ is then used to investigate the ordering problem in the density-matrix renormalization group.

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