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At least 19 recordsLinked to original sources
arXiv2007

Magnetic structure of Sm2IrIn8

The magnetic structure of the intermetallic antiferromagnet Sm2IrIn8 was determined using x-ray resonant magnetic scattering (XRMS). Below TN = 14.2, Sm2IrIn8 has a commensurate antiferromagnetic structure with a propagation vector (1/2,0,0). The Sm magnetic moments lie in the ab plane and are rotated roughly 18 degrees away from the a axis. The magnetic structure of this compound was obtained by measuring the strong dipolar resonant peak whose enhancement was of over two orders of magnitude at the L2 edge. At the L3 edge both quadrupolar and dipolar features were observed in the energy line shape. The magnetic structure and properties of Sm2IrIn8 are found to be consistent with the general trend already seen for the Nd-, Tb- and the Ce-based compounds from the RmMnIn3m+2n family (R = rare earth; M=Rh or Ir, m = 1, 2; n = 0, 1), where the crystalline electrical field (CEF) effects determine the direction of magnetic moments and the TN evolution in the series. The measured Neel temperature for Sm2IrIn8 is slightly suppressed when compared to the TN of the parent cubic compound SmIn3.

cond-mat.str-el↗
arXiv2007

Collective excitations of hard-core Bosons at half filling on square and triangular lattices: Development of roton minima and collapse of roton gap

We study ground state properties and excitation spectra for hard-core Bosons on square and triangular lattices, at half filling, using series expansion methods. Nearest-neighbor repulsion between the Bosons leads to the development of short-range density order at the antiferromagnetic wavevector, and simultaneously a roton minima in the density excitation spectra. On the square-lattice, the model maps on to the well studied XXZ model, and the roton gap collapses to zero precisely at the Heisenberg symmetry point, leading to the well known spectra for the Heisenberg antiferromagnet. On the triangular-lattice, the collapse of the roton gap signals the onset of the supersolid phase. Our results suggest that the transition from the superfluid to the supersolid phase maybe weakly first order. We also find several features in the density of states, including two-peaks and a sharp discontinuity, which maybe observable in experimental realization of such systems.

cond-mat.str-el↗
arXiv2007

Electron-electron correlation in graphite

The full three dimensional dispersion of the pi-bands, Fermi velocities and effective masses are measured with angle resolved photoemission spectroscopy and compared to first-principles calculations. The band structure by density-functional theory strongly underestimates the slope of the bands and the trigonal warping effect. Including electron-electron calculation on the level of the GW approximation, however, yields remarkable agreement in the vicinity of the Fermi level. This demonstrates the breakdown of the independent electron picture in semi-metallic graphite and points towards a pronounced role of electron correlation for the interpretation of transport experiments and double-resonant Raman scattering for a wide range of carbon based materials.

cond-mat.mtrl-sci↗
arXiv2007

Pressure induced magnetic ordering in Yb2Pd2Sn with two quantum critical points

Pressure induced long range antiferromagnetic order is discovered in Yb2Pd2Sn by measuring the electrical resistivity under pressure up to 5.0 GPa. Magnetic ordering is observed above about 1.0 GPa, being the lowest pressure in Yb-intermetallics showing pressure induced magnetic ordering, Unexpectedly, ordering disappears above about 4.0 GPa, giving rise to the first observation of the appearance of two quantum critical points persisting in a broad range of pressure within a single material.

cond-mat.str-el↗
arXiv2007

The origin of the work function

In this paper we analyse the mechanisms responsible for the bonding of electrons to metal surfaces. We present and validate a method to measure the energy distribution of dense electron ensembles at ambient conditions. We have found sharp structures in the energy distribution of charge confined at metal surfaces. These structures correspond to the onset of space charge i.e. electron emission. The threshold energy for emission increases linear with the free electron density. We conclude that the work function is a direct measure of the Hartree-Fock exchange energy. Remarkably, the emissions that have been observed do not obey the Einstein-Millikan threshold relation for the photo electric effect. By discrimination between requirements for energy and momentum we deduce that electron emissions are trigged by photon momentum. We claim that the photo electric effect is the result of a phase transition in an electron gas that connects a surface bound, condensed state to a free continuum state. The assignment of a phase transition to the effect under consideration is justified by analyzing the behaviour of the Fermi-Dirac energy distribution upon raising the Fermi level. We predict an abrupt increment in the number of electrons in the continuum that exceeds 70 orders of magnitude. Both the observed threshold energy and the line shape of the transition are in good agreement with theoretical predictions.

cond-mat.str-el↗
arXiv2007

The Origin of Fermi Arcs in Cuprate Pseudogap States and Strong Constraints on Viable Theories of High-Temperature Superconductivity

A full Fermi surface exists in underdoped high-temperature superconductors if the temperature T lies above the pseudogap temperature T*. Below T* only arcs of Fermi surface survive, scaling with T/T* as T -> 0, with T* displaying strong doping dependence. There is no accepted explanation for this behavior. We show that generalizing the BCS theory of normal superconductivity to include d-wave pairs and antiferromagnetism leads to the origin and doping dependence of the T* scale, and a quantitative description of Fermi arcs. These results place strong constraints on viable theories of high-temperature superconductivity.

cond-mat.supr-con↗
arXiv2007

From semiconductors to quantum gravity: to centenary of Matvei Bronstein

Investigation of the many-body condensed-matter systems allows us to connect the microscopic physics at the atomic energy scale and the macroscopic physics emerging in the low-energy corner. It gives some hints on the mechanisms of the formation of the physical laws which our Universe obeys. The paper is devoted to the centenary of Matvei Petrovich Bronstein.

gr-qc↗
arXiv2007

Mott Fermionic "Quantum" Criticality Beyond Ginzburg-Landau-Wilson

The Mott critical point between a metal and a correlated insulator has usually been studied via density or spin density bosonic mode fluctuations according to the standard Ginzburg-Landau-Wilson phase transition paradigm. A moment's reflection leads to increasing doubts that such an approach should work as the transition is nonmagnetic, voiding the relevance of spin density modes. Charge density modes are irrelevelant since the long range Coulomb interaction leads to a large plasmon gap and their incompressibility. In solidarity with these doubts, recent measurements of the Mott critical point in low dimensional organic materials yield critical exponents in violent diasagreement with the bosonic mode criticality lore. We propose that fermionic fluctuations control the behavior of the Mott transition. The transition thus has an intrinsic quantum aspect despite being a finite temperature phase transition. We develop this hitherto unexplored physics, obtain experimental predictions and find agreement with one of the novel unexplained experimental exponents. We conclude that this Mott transition corresponds to a new universality class of finite temperature critical points that contains quantum effects and cannot be accounted for by conventional Ginzburg-Landau-Wilson wisdom.

cond-mat.str-el↗
arXiv2008

Analysis of long range order

A first principles analysis of order-disorder transition in alloys shows that ordering energy is a function of temperature due to thermal vibrations. The inter-nuclear potential energy term converges if zero point vibrations are incorporated and this method can replace the Ewald sum method. Core energy contributions to the ordering energy are stored exclusively in superlattice lines. The effect of electron-phonon interactions on ordering energy is of the same order of magnitude as ordering energy near transition temperatures and cannot be ignored. Ising model and variants are incorrect in explaining alloy phase transitions as they ignore the role of electron-phonon interactions without justification. A theoretical formalism that incorporates the Debye-Waller Factor component of electron-phonon interactions in electronic structure calculations already exists and must be adopted when modeling temperature dependent phenomena. It is suggested that DWF correction will account substantially for the discrepancy between experimental and theoretical ordering energy in Ni3V. Thermal vibrations alter magnetic ordering energy at finite temperatures. The role of electron-phonon interactions in alloy and magnetic phase transitions cannot be ignored and must be incorporated in all models. This will also ensure consistency with x-ray and electron diffraction (alloy transitions) and neutron diffraction (magnetic transitions) results. An isotope effect is predicted for (magnetic) phase transitions if the transition temperature is below Debye temperature. Recent observations of an isotope effect in magnetic phase transitions confirm our above conclusions and imply that the role of electron-phonon interactions must be incorporated in all theories and models of magnetism to avoid contradictions.

cond-mat.mtrl-sci↗
arXiv2007

Pressure induced Insulator-Metal transition in LaMnO$_3$

The recent observation of a insulator to metal transition (IMT) [I. Loa et al, PRL 87, 125501 (2001)] in pure LaMnO$_3$ at 32 GPa and room temperature, well above the Neel temperature (145 K) and below the Jahn-Teller transition temperature (780 K), opens the way to a study of the role of the orbital degrees of freedom on the electronic structure in a stoichiometric material. In this paper we focus our attention in the orbital aspects of the insulator to metal transition. We use a model Hamiltonian for the $e_g$ orbitals of Mn that includes the on site Coulomb repulsion $U$, the hopping $t$, and its dependence with pressure. In order to include in an appropriate way the strong correlations induced by the dominant electron-electron interactions we introduce auxiliary fields (Slave Bosons,SB) to the description of the low energy states. We use a O-Mn distance ($d$) dependence of $t$ and the pressure-$d$ relation from the experimental data to describe the evolution of the electronic structure with pressure. Our results confirm and make transparent the conclusion reached in previous ab-initio calculations: the inclusion of the Coulomb energy is necessary and constitutes an important factor enhancing the orbital polarization in these compounds.

cond-mat.str-el↗
arXiv2007

Bound States and Many-Body Effects in H-Shaped Quantum Wires

In this paper, bound states energies and corresponding wave functions of H-shaped quantum wires are calculated numerically in the presence of external magnetic and electric fields and within the Landau gauge. With a suitable definition of external confinement potential, we present a numerical algorithm to calculate the profile of probability distribution of charge carriers. Our analysis shows that in the presence of external electric and magnetic fields, bound state properties of carriers are sensitive functions of an asymmetric parameter $a=\frac{W_{x}}{W_{y}}$ which measures the relative width of the well in two directions. We also study many body effect of bandgap renormalization in this quasi one dimensional system within dynamical random phase approximation.

cond-mat.str-el↗
arXiv2007

Theory of Neutron Scattering in High-T$_c$ Cuprates: Two Component Spin-Fermion Model

Recent neutron scattering experiments have revealed that the generic form of the magnetic excitations in the high-Tc cuprates of wide range of doping has the so-called "hourglass" shape; it features both upward and downward excitations at the incommensurate (IC) momenta spanning from the resonance peak at the commensurate momentum $(π,π)$. We propose the two-component spin-fermion model as a minimal phenomenological model which has both local spins and itinerant fermions as independent degrees of freedom. Our calculations of the dynamic spin correlation function provide good agreement with experiments and show: (1) the upward dispersion branch of magnetic excitations is mostly due to the local spin excitations; (2) the downward dispersion branch is from collective spin excitations of fermions; (3) the resonance mode is a mixture of both degrees of freedom.

cond-mat.str-el↗
arXiv2007

Phase transitions in coupled two dimensional XY systems with spatial anisotropy

We study phase transitions of coupled two dimensional XY systems with spatial anisotropy and $U(1) \times \mathbb{Z}_2$ symmetry, motivated by spinless bosonic atoms trapped in square optical lattice on the metastable first excited $p-$level orbitals with anisotropic hopping amplitudes. The phase transitions of the system are generally split into an Ising transition and an XY transition, but the sequence and the nature of the transitions depend on the ratio between the anisotropic couplings. In the isotropic limit the XY variables are expected to be disordered before the Ising variables when thermal or quantum fluctuations are turned on gradually. In the anisotropic limit with zero perpendicular hoppings, the finite temperature transition is a Kosterlitz-Thouless transition driven by proliferation of hybrid half vortices, and the zero temperature quantum phase transition is split into a bond order transition and a 3D XY transition, which can be driven by the condensation of either single vortices or half vortices. After the condensation of half vortices the resultant state is a Mott Insulator of paired bosons. A small perpendicular hopping $J_b$ leads to a 2D Ising transition at low temperature and a 2+1d quantum Ising transition with a small charging energy at zero temperature. Global phase diagrams for both classical and quantum phase transitions are drawn. The analytical results obtained in this work are expected to be checked both numerically and experimentally.

cond-mat.str-el↗
arXiv2007

Two-particle binding energy of interacting Bose gases

The pole of the two-particle T-matrix including the influence of the surrounding medium is analyzed for an interacting Bose gas. The phase diagram of the Bose -Einstein condensation (BEC) depending on the temperature, density, scattering length, and momentum is derived from this pole. The critical momentum for the occurrence of superfluidity is obtained in this way. As a new observation a two- particle binding energy is reported intimately connected with the occurrence of the BEC. It is suggested that this might have cosmological consequences on the dark energy problem.

cond-mat.other↗
arXiv2007

Hilbert++ Manual

We present here an installation guide, a hand-on mini-tutorial through examples, and the theoretical foundations of the Hilbert++ code.

cs.OH↗
arXiv2007

Gutzwiller-RVB Theory of High Temperature Superconductivity: Results from Renormalised Mean Field Theory and Variational Monte Carlo Calculations

We review the Resonating Valence Bond (RVB) theory of high temperatur e superconductivity using Gutzwiller projected wave functions that incorporate strong correlations. After a general overview of the phenomenon of high temperature superconductivity, we discuss Anderson's RVB picture and its implementation by renormalised mean field theory (RMFT) and variational Monte Carlo (VMC) techniques. We review RMFT and VMC results with an emphasis on recent development s in extending VMC and RMFT techniques to excited states. We compare results obtained from these methods with angle resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM). We conclude by summarising recent successes of this approach and discuss open problems that need to be solved for a consistent and complete description of high temperature superconductivity using Gutzwiller projected wave functions.

cond-mat.str-el↗