arXiv subjects

B. S. Shastry

Publications and source records attributed to B. S. Shastry.

16 recordsLinked to original sources

arXiv2018

Band-edge quasiparticles from electron phonon coupling and resistivity saturation

We address the problem of resistivity saturation observed in materials such as the A-15 compounds. To do so, we calculate the resistivity for the Hubbard-Holstein model in infinite spatial dimensions to second order in on-site repulsion $U\leq D$ and to first order in (dimensionless) electron-phonon coupling strength $λ\leq0.5$, where $D$ is the half-bandwidth. We identify a unique mechanism to obtain two parallel quantum conducting channels: low-energy and band-edge high-energy quasi-particles. We identify the source of the hitherto unremarked high-energy quasi-particles as a positive slope in the frequency-dependence of the real part of the electron self-energy. In the presence of phonons, the self-energy grows linearly with the temperature at high-$T$, causing the resistivity to saturate. As $U$ is increased, the saturation temperature is pushed to higher values, offering a mechanism by which electron-correlations destroy saturation.

cond-mat.str-el↗

arXiv2013

Extremely correlated Fermi liquid theory meets Dynamical mean-field theory: Analytical insights into the doping-driven Mott transition

We consider a doped Mott insulator in the large dimensionality limit within both the recently developed Extremely Correlated Fermi Liquid (ECFL) theory and the Dynamical Mean-Field Theory (DMFT). We show that the general structure of the ECFL sheds light on the rich frequency-dependence of the DMFT self-energy. Using the leading Fermi-liquid form of the two key auxiliary functions introduced in the ECFL theory, we obtain an analytical ansatz which provides a good quantitative description of the DMFT self-energy down to hole doping level 0.2. In particular, the deviation from Fermi-liquid behavior and the corresponding particle-hole asymmetry developing at a low energy scale are well reproduced by this ansatz. The DMFT being exact at large dimensionality, our study also provides a benchmark of the ECFL in this limit. We find that the main features of the self-energy and spectral line-shape are well reproduced by the ECFL calculations in the O(λ^2) `minimal scheme', for not too low doping level >0.3. The DMFT calculations reported here are performed using a state-of-the-art numerical renormalization-group impurity solver, which yields accurate results down to an unprecedentedly small doping level 0.001.

cond-mat.str-el↗

arXiv2011

Extremely Correlated Fermi Liquid Description of Normal State ARPES in Cuprates

The normal state single particle spectral function of the high temperature superconducting cuprates, measured by the angle resolved photoelectron spectroscopy (ARPES), has been considered both anomalous and crucial to understand. Here we show that an unprecedentedly detailed description of the data is provided by a spectral function arising from the Extremely Correlated Fermi Liquid state of the t-J model proposed recently by Shastry. The description encompasses both laser and conventional synchrotron ARPES data on optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, and also conventional synchrotron ARPES data on the La$_{1.85}$Sr$_{0.15}$CuO$_4$ materials. {\em It fits all data sets with the same physical parameter values}, satisfies the particle sum rule and successfully addresses two widely discussed "kink" anomalies in the dispersion.

cond-mat.str-el↗

arXiv2005

Degeneracy and Strong Fluctuation-Induced First-Order Phase Transition in the Dipolar Pyrochlore Antiferromagnet

We show that a continuous set of degenerate critical soft modes strongly enhances the first-order character of a fluctuation-induced first-order transition in the pyrochlore dipolar Heisenberg antiferromagnet. Such a degeneracy seems essential to explain the strong first-order transition recently observed in Gd(2)Sn(2)O(7). We present some evidence from Monte-Carlo simulations and a perturbative renormalization group expansion.

cond-mat.mtrl-sci↗

arXiv2003

Hubbard Model on Decorated Lattices

We introduce a family of lattices for which the Hubbard model and its natural extensions can be quasi-exactly solved, i.e. solved for the ground and low energy states. In particular, we show rigorously that the ground state of the Hubbard model with off-site Coulomb repulsions on a decorated Kagomè lattice is an ordered array of local currents. The low energy theory describing this chiral state is an $S=1/2$ XY model, where each spin degree of freedom represents the two possible chiralities of each local current.

cond-mat.str-el↗

arXiv2002

The Origin of Degeneracies and Crossings in the 1d Hubbard Model

The paper is devoted to the connection between integrability of a finite quantum system and degeneracies of its energy levels. In particular, we analyze in detail the energy spectra of finite Hubbard chains. Heilmann and Lieb demonstrated that in these systems there are crossings of levels of the same parameter independent symmetry. We show that this apparent violation of the Wigner-von Neumann noncrossing rule follows directly from the existence of nontrivial conservation laws and is a characteristic signature of quantum integrability. The energy spectra of Hubbard chains display many instances of permanent (at all values of the coupling) twofold degeneracies that cannot be explained by parameter independent symmetries. We relate these degeneracies to the different transformation properties of the conserved currents under spatial reflections and the particle-hole transformation and estimate the fraction of doubly degenerate states. We also discuss multiply degenerate eigenstates of the Hubbard Hamiltonian. The wave functions of many of these states do not depend on the coupling, which suggests the existence of an additional parameter independent symmetry.

cond-mat.str-el↗

arXiv2001

Multiple Field-Induced Phase Transitions in a Geometrically-Frustrated Dipolar Magnet - Gd2Ti2O7

Field-driven phase transitions generally arise from competition between Zeeman energy and exchange or crystal-field anisotropy. Here we present the phase diagram of a frustrated pyrochlore magnet Gd2Ti2O7, where crystal field splitting is small compared to the dipolar energy. We find good agreement between zero-temperature critical fields and those obtained from a mean-field model. Here, dipolar interactions couple real-space and spin-space, so the transitions in Gd2Ti2O7 arise from field-induced "cooperative anisotropy" reflecting the broken spatial symmetries of the pyrochlore lattice.

cond-mat.stat-mech↗

arXiv2001

Spin dynamics of Sr_14Cu_24O_41 two-leg ladder studied by Raman spectroscopy

The two-magnon (2M) excitation at 3000 cm-1 in Sr_14Cu_24O_41 two-leg ladder is studied by Raman scattering. A slight anisotropy of the superexchange coupling J_{\perp}/J_{||} \approx 0.8 with J_{||} = 110 \pm 20 meV is proposed from the analysis of the magnetic scattering. The resonant coupling across the charge transfer gap increases the 2M intensity by orders of magnitude. The anisotropy of Raman scattering is dependent upon the excitation energy. The 2M relaxation is found to be correlated with the temperature dependent electronic Raman continuum at low frequencies.

cond-mat.str-el↗

arXiv2000

Ordering and partial ordering in holmium titanate and related systems

We take another look at two compounds which have been discussed as possible realizations of ``spin ice'', namely holmium titanate and dysprosium titanate. As we have earlier observed, holmium titanate does not display ice-like behaviour at low temperatures because the long ranged dipolar interactions between spins are strong compared to the nearest neighbour interactions. We show, analytically, that the true ground state of this system must be fully ordered, but simulations only reach partially ordered states because there are infinite energy barriers separating these from the true ground state. We also show that the true ground state of our model of dysprosium titanate is also fully ordered, and offer some explanations as to why simulations and experiments show ice-like behaviour. We discuss the effect on these systems of an applied magnetic field. Finally, we discuss several other models which show similar partial or full ordering in their ground states, including the well known Ising model on the fcc lattice.

cond-mat.stat-mech↗

arXiv1999

Ising pyrochlore magnets: Low temperature properties, ice rules and beyond

Pyrochlore magnets are candidates for spin-ice behavior. We present theoretical simulations of relevance for the pyrochlore family R2Ti2O7 (R= rare earth) supported by magnetothermal measurements on selected systems. By considering long ranged dipole-dipole as well as short-ranged superexchange interactions we get three distinct behaviours: (i) an ordered doubly degenerate state, (ii) a highly disordered state with a broad transition to paramagnetism, (iii) a partially ordered state with a sharp transition to paramagnetism. Thus these competing interactions can induce behaviour very different from conventional ``spin ice''. Closely corresponding behaviour is seen in the real compounds---in particular Ho2Ti2O7 corresponds to case (iii) which has not been discussed before, rather than (ii) as suggested earlier.

cond-mat.stat-mech↗

arXiv1999

Many-chain Generalization of Gauge-Coupled Fermionic Systems

We present a many chain generalization of a recent work of ours, wherein an arbitrary number of fermionic chains are coupled via a Gauge interaction. Central to this construction is the role of an antisymmetric tensor which enters the couplings and helps define a pseudo-unitary transformation, that maps the models on to ``free fermi'' models with twisted boundary conditions.

cond-mat.str-el↗

arXiv1998

The 2-d Coulomb Gas on a 1-d lattice

The statistical mechanics of a two dimensional Coulomb gas confined to one dimension is studied, wherein hard core particles move on a ring. Exact self-duality is shown for a version of the sine-Gordon model arising in this context, thereby locating the transition temperature exactly. We present asymptotically exact results for the correlations in the model and characterize the low and high temperature phases. Numerical simulations provide support to these renormalization group calculations. Connections with other interesting problems, the quantum Brownian motion of a particle in a periodic potential and impurity problems, are pointed out.

cond-mat↗

arXiv1997

A new class of exactly solvable interacting fermion models in one dimension

We investigate a model containing two species of one-dimensional fermions interacting via a gauge field determined by the positions of all particles of the opposite species. The model can be solved exactly via a simple unitary transformation. Nevertheless, correlation functions exhibit nontrivial interaction-dependent exponents. A similar model defined on a lattice is introduced and solved. Various generalizations, e.g. to the case of internal symmetries of the fermions, are discussed. The present treatment also clarifies certain aspects of Luttinger's original solution of the ``Luttinger model''.

cond-mat↗

arXiv1995

Random Matrix Model and the Calogero-Sutherland Model: A Novel Current-Density Mapping

We investigate the relation between the invariant correlators of random matrix theory and correlators of the integrable one-dimensional systems. Starting from the relation between correlators for the coupling strengths $λ=1/ 2$, $1$, and $2$, we explore the local current-density mapping applicable to arbitrary $λ$ including {\em irrational\/} values, which results from the novel structure of the Calogero-Sutherland model. We find an interesting and novel relationship between equal time current and density correlations for any coupling, which exist {\em in addition} to the usual Ward Identities for this class of systems.

cond-mat↗

arXiv1994

Does the attractive Hubbard model support larger persistent currents than the repulsive one ?

We consider a one-dimensional Hubbard model in the presence of disorder. We compute the charge stiffness for a mesoscopic ring, as a function of the size $L$, which is a measure of the permanent currents. We find that for finite disorder the permanent currents of the system with repulsive interactions are larger than those of the system with attractive interactions. This counter intuitive result is due to the fact that local density fluctuations are reduced in the presence of repulsive interactions.

cond-mat↗

arXiv1993

Exact Dynamical Correlations of the 1/r^2 Model

We present exact results for the dynamical structure function, i.e.~the density-density correlations for the 1/r^2 system of interacting particles at three special values of the coupling constant. The results are interpreted in terms of exact excitations of the model which are available from Bethe's Ansatz, thereby throwing light on the quasi-particle content of the elementary excitations. We also obtain the first moment of the discrete version of the model, which provides a non-trivial check on its structure function. We show that the property of spectrum saturation is a common feature of both versions.

cond-mat↗