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Ronny Thomale

Publications and source records attributed to Ronny Thomale.

265 records · Page 15Linked to original sources
arXiv2010

Stochastic Mean-Field Theory: Method and Application to the Disordered Bose-Hubbard Model at Finite Temperature and Speckle Disorder

We discuss the stochastic mean-field theory (SMFT) method which is a new approach for describing disordered Bose systems in the thermodynamic limit including localization and dimensional effects. We explicate the method in detail and apply it to the disordered Bose-Hubbard model at finite temperature, with on-site box disorder, as well as experimentally relevant unbounded speckle disorder. We find that disorder-induced condensation and reentrant behavior at constant filling are only possible at low temperatures, beyond the reach of current experiments [Pasienski et al., arXiv:0908.1182]. Including off-diagonal hopping disorder as well, we investigate its effect on the phase diagram in addition to pure on-site disorder. To make contact to present experiments on a quantitative level, we also combine SMFT with an LDA approach and obtain the condensate fraction in the presence of an external trapping potential.

cond-mat.dis-nn↗
arXiv2010

Non-local order in gapless systems: Entanglement Spectrum in Spin Chains

We show that the entanglement spectrum can be used to define non-local order in gapless spin systems. We find a gap that fully separates a series of generic, high `entanglement energy' levels, from a flat band of levels with specific multiplicities that uniquely define the ground-state, and remains finite in the thermodynamic limit. We pick the appropriate set of quantum numbers, and then partition the system in this space. This partition corresponds to a very non-local real-space cut. Despite the fact that the Laughlin state is bulk gapped while the antiferromagnetic spin chain state is bulk gapless, we show that the S=1/2 Heisenberg antiferromagnet in one dimension has an entanglement spectrum almost identical to that of the Laughlin Fractional Quantum Hall state in two dimensions, revealing the similar field theory of their low-energy edge and bulk excitations respectively.

cond-mat.str-el↗
arXiv2010

Color-charge separation in trapped SU(3) fermionic atoms

Cold fermionic atoms with three different hyperfine states with SU(3) symmetry confined in one-dimensional optical lattices show color-charge separation, generalizing the conventional spin charge separation for interacting SU(2) fermions in one dimension. Through time-dependent DMRG simulations, we explore the features of this phenomenon for a generalized SU(3) Hubbard Hamiltonian. In our numerical simulations of finite size systems, we observe different velocities of the charge and color degrees of freedom when a Gaussian wave packet or a charge (color) density response to a local perturbation is evolved. The differences between attractive and repulsive interactions are explored and we note that neither a small anisotropy of the interaction, breaking the SU(3) symmetry, nor the filling impedes the basic observation of these effects.

cond-mat.quant-gas↗
arXiv2010

Why some Iron-based superconductors are nodal while others are nodeless

The symmetry of the order parameter in iron-based superconductors, especially the presence or absence of nodes, is still a question of debate. While contradictory experiments can be explained by appropriately tuned theories of nodeless superconductivity in the iron-arsenide compounds, for LaOFeP all experiments clearly point to a nodal order parameter. We put forward a scenario that naturally explains the difference between the order-parameter character in these two sets of compounds, and use functional renormalization group (fRG) techniques to analyze it in detail. Our results show that, due to the orbital content of the electron and hole bands, nodal superconductivity on the electron pockets (hole pocket gaps are always nodeless) can naturally appear when the third hole pocket which lies at wavevector (pi,pi) in the unfolded Brillouin zone is absent, as is the case in LaOFeP. When present, the third hole pocket has overwhelming d_{xy} orbital character, and the intra-orbital interaction with the d_{xy} dominated part of the electron Fermi surface is enough to drive the superconductivity nodeless (of s^+- form). However, in its absence, pair hopping, inter-orbital, and electron-electron intra-orbital interactions render the gap on the electron pockets softly nodal.

cond-mat.supr-con↗
arXiv2009

Doping-dependent pairing symmetry in the Iron-Pnictides

We use the functional renormalization group method to analyze the phase diagram of a 4-band model for the iron-pnictides subject to band interactions with certain A_{1g} momentum dependence. We determine the parameter regimes where an extended s-wave pairing instability with and without nodes emerges. On the electron-doped side, the parameter regime in which a nodal gap appears is found to be much narrower than recently predicted in arXiv:0903.5547. On the hole-doped side, the extended s-wave pairing never becomes nodal: above a critical strength of the intra-band repulsion, the system favors an exotic extended d-wave instability on the enlarged hole pockets at much lower T_c. At half filling, we find that a strong momentum dependence of inter-band pair hopping yields an extended s-wave instability instead of spin-density wave (SDW) ordering. These results demonstrate that an interaction anisotropy around the Fermi surfaces generally leads to a pronounced sensitivity of the pairing state on the system parameters.

cond-mat.supr-con↗
arXiv2009

Parent Hamiltonian for the Chiral Spin Liquid

We present a method for constructing parent Hamiltonians for the chiral spin liquid. We find two distinct Hamiltonians for which the chiral spin liquid on a square lattice is an exact zero-energy ground state. We diagonalize both Hamiltonians numerically for 16-site lattices, and find that the chiral spin liquid, modulo its two-fold topological degeneracy, is indeed the unique ground state for one Hamiltonian, while it is not unique for the other.

cond-mat.str-el↗
arXiv2009

Spontaneous Parity Violation in a Quantum Spin Chain

We report on a spontaneous breakdown of parity in the ground state of a spin Hamiltonian involving nearest-neighbor interactions. This occurs for a one-dimensional model where spins transform under the gauge field representation of QCD, the eight-dimensional adjoint representation of SU(3). The ground state spontaneously violates parity and is two-fold degenerate. In addition, the model possesses a non-vanishing topological string order parameter which we explicate analytically.

cond-mat.str-el↗
arXiv2009

Non-Abelian Statistics in a Quantum Antiferromagnet

We propose a novel spin liquid state for a spin S=1 antiferromagnet in two dimensions. The ground state violates P and T, is a spin-singlet, and is fully invariant under the lattice symmetries. The spinon and holon excitations are deconfined and obey non-abelian statistics. We present preliminary numerical evidence that the universality class of this topological liquid can be stabilized by a local Hamiltonian involving three-spin interactions. We conjecture that spinons in spin liquids with spin larger than 1/2 obey non-abelian statistics in general.

cond-mat.str-el↗
arXiv2007

Numerical analysis of three-band models for CuO planes as candidates for a spontaneous T violating orbital current phase

Recently, we have numerically evaluated the current-current correlation function for the ground states of three-band models for the CuO planes of high-Tc superconductors at hole doping x=1/8 using systems with 24 sites and periodic boundary conditions. In this article, the numerical analysis is explicated in detail and extended to a wider range of parameters. Our results show no evidence for the time-reversal symmetry violating current patterns recently proposed by Varma. If such current patterns exist, our results indicate that the energy associated with the loop currents must be smaller than 5 meV per link even if the on-site chemical potential on the oxygen sites, which is generally assumed to be of the order of 3.6 eV, is taken to zero, as advocated by Varma. We also vary the inter-atomic Coulomb repulsion scale and find only a weak dependence on this parameter. So while our studies do not rule out the existence of such current patterns, they do rule out that quantum critical fluctuations of these patterns are responsible for phenomena occurring at significantly higher energies such as the superconductivity or the anomalous properties observed in the strange metal phase provided the CuO superconductors are adequately described by any of the three-band models discussed.

cond-mat.supr-con↗
arXiv2007

No evidence for spontaneous orbital currents in finite size studies of three-band models for CuO planes

We have numerically evaluated the current-current correlations for three-band models of the CuO planes in high-T_c superconductors at hole doping x=1/8. The results show no evidence for the orbital current patterns proposed by Varma. If such patterns exist, the associated energy is estimated to be smaller than 5 meV per link even if epsilon_p-epsilon_d=0. Assuming that the three-band models are adequate, quantum critical fluctuations of such patterns hence cannot be responsible for phenomena occurring at significantly higher energies, such as superconductivity or the anomalous properties of the pseudogap phase.

cond-mat.supr-con↗
arXiv2006

Charge excitations in SU(n) spin chains: exact results for the 1/r^2 model

We study the one- and two-holon excitations of the SU(3) Kuramoto-Yokoyama model on the level of explicit wave functions, and generalize the calculations to the case of SU(n). We obtain the exact energies and the single holon momenta, which we find fractionally spaced according to fractional statistics with statistical parameter g=1/n.

cond-mat.str-el↗
arXiv2006

Exact two-holon wave functions in the Kuramoto-Yokoyama model

We construct the explicit two-holon eigenstates of the SU(2) Kuramoto-Yokoyama model at the level of explicit wave functions. We derive the exact energies and obtain the individual holon momenta, which are quantized according to the half-Fermi statistics of the holons.

cond-mat.str-el↗