Armen N. KOCHARIAN
Our main interest has been the electronic structure calculations and physical properties of solids, surfaces and interfaces that provide us information pertaining to the ground state electronic and magnetic structures, surface and interface induced changes etc. We use to study the bewildering variety of phases in condensed-matter physics. We emphasize the importance of the breaking of symmetries, and develop the idea of an order parameter through several examples. We discuss elementary excitations and incommensurate phases to find out analogy between the mean field Hamiltonians with attractive and repulsive electron-electron interactions. We have employed these basis approaches in the past for studies of the electronic or electron-hole instabilities and phase transitions of the first and second order in different anomalous rare-earth and transition metal compounds from classical critical phenomena to quantum phase transitions driven by pressure, temperature and composition of material. Also, there are fundamental problems that have to be addressed and made computationally efficient. As an example, although the mean field (MF) approach has been quite successful with respect to the ground state properties, improvements beyond MF such as better treatments of correlations and computationally efficient ways of handling the single electron spectrum and electronic structure problem are necessary, especially when studying correlated electron systems at finite temperatures, magnetic field or (real) materials with nonlocal interactions. These calculations use a basis set to expand a one electron wave function and are based on the unrestricted mean field calculations and local (spin) density approximation (L(S)DA) by including the effects of random phase approximation.
Exact and Self-consistent studies
We explore the question whether the self-consistent field approach, which commonly applied to higher dimensions, can properly describe the single particle spectrum and ground state properties of 1d system, such as a Luttinger or Fermi liquid. We test the developed generalized self-consistent field (GSCF) approach for attractive and repulsive Hubbard model by comparison of its predictions with the Bethe-ansatz. We also analyze the GSCF spectrum and the relation between the incommensurate parameter q and corresponding exact momentum within the Luttinger theorem and asymptotic of the correlation function. Our studies show that there is no principle differences for the self-consistent treatment of both U>0 and U<0 models with electron-electron and electron-hole instabilities as a signature for the presence or absence of a normal Fermi liquid behavior. It is difficult to assess the reliability of the resulting approximations in the perturbation theory for general interaction strength. However in the GSCF approach for all n in the extreme cases of small and large U/t limits becomes exact (results coincide with the Bethe-ansatz one) and therefore corresponding fluctuations around the GSCF solution are small. The calculated second order perturbation correction to the ground state energy near the GSCF solution converges to the exact solution everywhere. We developed dynamical self-consistent perturbation theory around the self-consistent solution is similar to that derived within the free Fermi gas approach in the weak interaction limit. Obtained results in entire range of U/t, n and h show significant improvements over the standard perturbation theory and the GSCF results. This approach gives also insight into a single-particle picture within 1d Hubbard model.
Research is dedicated to a wide range of subjects in the general area of strongly correlated electron systems, quantum fluids and solids. Characteristics of materials been investigated in extreme conditions of high temperature, pressure and strong magnetic field. Systems currently being explored include high temperature superconductors, various unusual magnetic materials, strongly correlated f-electron materials, materials in confined geometric configurations such as one and quasi-one-dimensional systems, the two dimensional planar surfaces and nanocrystals, interplay of quantum disorder and thermal fluctuations, classical and quantum magnetism, superfluity. Some basic problems in condensed matter physics that we have addressed using the above mentioned techniques include; chemisorption on transition metals, surface magnetism, phase stabilities and related energetics of (3d, 4d, 5d) transition metals and their compounds, diffusion in simple metals from the simple models of the bulk and surfaces electronic structure of transition metals, rare-earth and actinides, spin and orbital magnetism, electronic phase transitions, mixed valence states on surfaces and interfaces, and related issues in hard magnets.
Spin and Orbital Exchange
We consider the superexchange in european monochalcogenides EuO, EuS, EuSe, EuTe, transition metal oxides. We have explored spin, charge, and orbitally ordered states in generalized Hubbard-like two component models using model Hartree-Fock calculations on d-p-type lattice models. We also discuss why in some cases these materials may still behave as an orbital quantum liquid in anisotropic Heisenberg-like model, while in other there is a classical behavior based on limiting Ising model. Several charge and orbitally modulated states are found to be stable and almost degenerate in energy with a homogeneous and spatially ordered orbital magnetism consistent with the experiment observation of mixed valence states inCeAl2O3, SmS, SmSe. We also have studied the magnetic saturation of iron in metallic environments.
One way of making the transition between the quasi-long range order in a chain of S=1/2 spins coupled antiferromagnetically and the true long range order that occurs in a plane, is by assembling chains to make ladders of increasing width. Surprisingly this crossover between one and two dimensions is not at all smooth. Ladders with an even number of legs have purely short range magnetic order and a finite energy gap to all magnetic excitations. Predictions of this novel groundstate for the strongly correlated electrons in finite size lattices have now been verified for the the finite size closed chains with even and odd numbers of atoms placed in strong magnetic field. The research integrates understanding the physics of the novel phenomena being studied and the materials exhibiting the phenomena of superconductivity, discovery and characterization of several novel Kondo materials, observation of superconducting and magnetic crossover from the itinerant BCS behavior into the localized Bose condensation regime in 1D and 2D metals, giant magnetoresistance in several types of ferromagnetic materials, and studies of magnetization reversal in individual sub-micron particles. My research interest ranges from materials science to low-dimensional physics, mainly on nanoscale materials and composites. My current research efforts are focused on gaining atomic-scale understanding and control of the surface-adsorbate and adsorbate-adsorbate interactions in terms of bonding, electronic structure, and dynamics.
BCS -- Bose condensation crossover
I am interested in the qualitative and quantitative understanding of the macroscopic and collective properties of condensed matter systems, and on the relation between this and the microscopic physics at the single-electron level. I have been particularly interested in exploring the spectacular consequences of strong correlation effects in electronic materials where the low energy properties are qualitatively different from those of a non-interacting electron gas. Prime examples of this on which I have focused my attentions are the one and two dimensional electron on a lattice placed in a strong magnetic field, which exhibits phenomena associated with the induced ferromagnetic effect and on the amazing electronic properties of the cuprate perofskites, including high temperature superconductivity, quantum anti-ferromagnetism, BCS--Bose condensation crossover and a delicate interplay between superconducting, charge-density-wave, and spin-density wave ordering. Recently, I have also developed an approach to understanding of the magnetic crossover from itinerant into localized magnetism. It is my feeling that understanding of these physically important problems is vital to obtain exact, and well controlled approximate solutions of simplified model problems which properly caricature the important physics, and this has dictated the use of more accurate self-consistent methods based on incorporation of the second order perturbation term into this scheme. These techniques have been applied to the Hubbard model in order to successfully predict the ground state properties by comparison with the exact solution in 1d case without using significant experimental input. In my opinion, the LDA has worked better than it should have, even in describing certain ground state properties of materials. However, there are clear problems in the existing mean field that have to be addressed in order to have better self-consistent predictions of complex materials.
Nucleon on a lattices
Thermal properties of nucleon matter are investigated assuming a simple form of the nucleon-nucleon interaction. The nucleons are dynamically interacting through a spin-dependent force on a three-dimensional lattice in a way similar to the extended attractive Hubbard model with the intersite interaction. A mean field calculation suggests that the super-fluid ground state generated by strong nucleon parings undergoes a second-order phase transition to a normal state at a higher temperature.
There are several projects currently underway. Magnetic interactions in rare-earth/transition metal compounds are being studied in order to understand various phase diagrams and observed enhancements in magnetic properties of these materials.
Fundamental aspects of electronic structure are being examined in order to go beyond various approximations necessary in the self-consistent studies.
Currently, we are also interested in studying the coexistence of the magnetism and superconductivity. Dynamical self-consistent studies in the presence of magnetic field would be able to guide engineers and materials scientists towards improving the critical parameters of high temperature superconducting materials. Based on present calculations of the phase diagrams there is also the exciting possibility of designing completely new materials with desirable properties. This is an important example of how advances in computational and materials physics can be applied to very practical problems.
We have access to several IBM and SUN work stations and powerful PCs in US and Taiwan.
Most of the projects mentioned above have been collaborative efforts, with the condensed matter theory group at Tamkang University in Taiwan, Brookhaven National Laboratory, other groups at University of Connecticut, and other universities. These collaborations have been very fruitful, and we intend to continue these during the years ahead.
Last Updated: August 11, 2002