arXiv subjects

E. M. Stoudenmire

Publications and source records attributed to E. M. Stoudenmire.

25 records · Page 2Linked to original sources

arXiv2011

Studying Two Dimensional Systems With the Density Matrix Renormalization Group

The Density Matrix Renormalization Group (DMRG) method scales exponentially in the system width for models in two dimensions, but remains one of the most powerful methods for studying 2D systems with a sign problem. Reviewing past applications of DMRG in 2D demonstrates its success in treating a wide variety of problems, although it remains underutilized in this setting. We present techniques for performing cutting edge 2D DMRG studies including methods for ensuring convergence, extrapolating finite-size data and extracting gaps and excited states. Finally, we compare the current performance of a recently developed tensor network method to 2D DMRG.

cond-mat.str-el↗

arXiv2011

Interaction Effects in Topological Superconducting Wires Supporting Majorana Fermions

Among the broad spectrum of systems predicted to exhibit topological superconductivity and Majorana fermions, one-dimensional wires with strong spin-orbit coupling provide one of the most promising experimental candidates. Here we investigate the fate of the topological superconducting phase in such wires when repulsive interactions are present. Using a combination of Density Matrix Renormalization Group, bosonization, and Hartree-Fock techniques, we demonstrate that while interactions degrade the bulk gap -consistent with recent results of Gangadharaiah et al.- they also greatly expand the parameter range over which the topological phase arises. In particular, we show that with interactions this phase can be accessed over a broader chemical potential window, thereby leading to greater immunity against disorder-induced chemical potential fluctuations in the wire. We also suggest that in certain wires strong interactions may allow Majorana fermions to be generated without requiring a magnetic field.

cond-mat.supr-con↗

arXiv2010

Minimally Entangled Typical Thermal State Algorithms

We discuss a method based on sampling minimally entangled typical thermal states (METTS) that can simulate finite temperature quantum systems with a computational cost comparable to ground state DMRG. Detailed implementations of each step of the method are presented, along with efficient algorithms for working with matrix product states and matrix product operators. We furthermore explore how properties of METTS can reveal characteristic order and excitations of systems and discuss why METTS form an efficient basis for sampling. Finally, we explore the extent to which the average entanglement of a METTS ensemble is minimal.

cond-mat.str-el↗

arXiv2009

Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAl$_{2-x}$Ga$_x$O$_4$: Lattice expansion and site disorder

The spinels CoB$_2$O$_4$ with magnetic Co$^{2+}$ ions on the diamond lattice A site can be frustrated because of competing near-neighbor ($J_1$) and next-near neighbor ($J_2$) interactions. Here we describe attempts to tune the relative strengths of these interactions by substitution on the non-magnetic B-site. The system we employ is CoAl$_{2-x}$Ga$_x$O$_4$, where Al is systematically replaced by the larger Ga, ostensibly on the B site. As expected, Ga substitution expands the lattice, resulting in Co atoms on the A-site being pushed further from one other and thereby weakening magnetic interactions. In addition, Ga distributes between the B and the A site in a concentration dependent manner displacing an increasing amount of Co from the A site with increasing $x$. This increased inversion, which is confirmed by neutron diffraction studies carried out at room temperature, affects magnetic ordering very significantly, and changes the nature of the ground state. Modeling of the magnetic coupling illustrates the complexity that arises from the cation site disorder.

cond-mat.mtrl-sci↗

arXiv2009

Quadrupolar correlations and spin freezing in S = 1 triangular lattice antiferromagnets

Motivated by experiments on NiGa2S4, we discuss characteristic (finite temperature) properties of spin S = 1 quantum antiferromagnets on the triangular lattice. Several recent theoretical studies have suggested the possibility of quadrupolar (spin-nematic) ground states in the presence of sufficient biquadratic exchange. We argue that quadrupolar correlations are substantially more robust than the spin-nematic ground state, and give rise to a two peak structure of the specific heat. We characterize this behavior by a novel T > 0 semiclassical approximation, which is amenable to efficient Monte Carlo simulations. Turning to low temperatures, we consider the effects of weak disorder on incommensurate magnetic order, which is present when interactions beyond nearest neighbor exchange are substantial. We show that non-magnetic impurities act as random fields on a component of the order parameter, leading to the disruption of long-range magnetic order even when the defects are arbitrarily weak. Instead, a gradual freezing phenomena is expected on lowering the temperature, with no sharp transition but a rapid slowing of dynamics and the development of substantial spin-glass-like correlations. We discuss these observations in relation to measurements of NiGa2S4.

cond-mat.str-el↗

arXiv2007

Ordered Phases of the Anisotropic Kagome Lattice Antiferromagnet in a Field

The antiferromagnetic Heisenberg model on an anisotropic kagome lattice may be a good minimal model for real magnetic systems as well as a limit from which the isotropic case can be better understood. We therefore study the nearest-neighbor Heisenberg antiferromagnet on an anisotropic kagome lattice in a magnetic field. Such a system should be well described by weakly interacting spin chains, and we motivate a general form for the interaction by symmetry considerations and by perturbatively projecting out the inter-chain spins. In the spin 1/2 case, we find that the system exhibits a quantum phase transition from a ferrimagnetic ordered state to an XY ordered state as the field is increased. Finally, we discuss the appearance of magnetization plateaux in the ferrimagnetic phase.

cond-mat.str-el↗

arXiv2004

Magnetoresistive Effects in Ferromagnet-Superconductor Multilayers

We consider a nanoscale system consisting of Manganite-ferromagnet and Cuprate-superconductor multilayers in a spin valve configuration. The magnetization of the bottom Manganite-ferromagnet is pinned by a Manganite-antiferromagnet. The magnetization of the top Manganite-ferromagnet is coupled to the bottom one via indirect exchange through the superconducting layers. We study the behavior of the critical temperature and the magnetoresistance as a function of an externally applied parallel magnetic field, when the number of Cuprate-superconductor layers are changed. There are two typical behaviors in the case of a few monolayers of the Cuprates: a) For small magnetic fields, the critical temperature and the magnetoresistance change abruptly when the flipping field of the top Manganite-ferromagnet is reached. b) For large magnetic fields, the multilayered system re-enters the zero-resistance (superconducting) state after having become resistive (normal).

cond-mat.supr-con↗