A functional method for condensed systems

Schwingers functional method is applied to the Green functions of the many body problem. By including the “anomalous” Green functions introduced by Gorkov, a formalism results which permits in a natural way the treatment of systems showing “superfluid” properties (“condensed” systems).     

Measuring entanglement in condensed matter systems

We show how entanglement may be quantified in spin and cold atom many-body systems using standard experimental techniques only. The scheme requires no assumptions on the state in the laboratory, and a lower bound to the entanglement can be read off directly from the scattering cross section of neutrons deflected from solid state samples or the time-of-flight distribution of cold atoms in optical lattices, respectively. This removes a major obstacle which so far has prevented the direct and quantitative experimental study of genuine quantum correlations in many-body systems: The need for a full characterization of the state to quantify the entanglement contained in it. Instead, the scheme presented here relies solely on global measurements that are routinely performed and is versatile enough to accommodate systems and measurements different from the ones we exemplify in this work.     

Phase operator properties in quantum optics

Eigenstate properties of the phase variables of quantum optics fields are investigated. Arbitrary states are expanded in them. The statistics of these states are determined. The normalformof the corresponding phase operator is found.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 44–48, December, 1972.     

凝聚态材料中的拓扑相与拓扑相变——2016年诺贝尔物理学奖解读

凝聚态物理中拓扑相变和拓扑物态的发现,获得了2016年度诺贝尔物理学奖。文章系统介绍了凝聚态物理中拓扑性的起源,并简要介绍了目前凝聚态物理中发现的主要几类拓扑态：拓扑绝缘体、量子反常霍尔效应、拓扑晶体绝缘体和拓扑半金属。     

Next generation interatomic potentials for condensed systems

The computer simulation of condensed systems is a challenging task. While electronic structure methods like density-functional theory (DFT) usually provide a good compromise between accuracy and efficiency, they are computationally very demanding and thus applicable only to systems containing up to a few hundred atoms. Unfortunately, many interesting problems require simulations to be performed on much larger systems involving thousands of atoms or more. Consequently, more efficient methods are urgently needed, and a lot of effort has been spent on the development of a large variety of potentials enabling simulations with significantly extended time and length scales. Most commonly, these potentials are based on physically motivated functional forms and thus perform very well for the applications they have been designed for. On the other hand, they are often highly system-specific and thus cannot easily be transferred from one system to another. Moreover, their numerical accuracy is restricted by the intrinsic limitations of the imposed functional forms. In recent years, several novel types of potentials have emerged, which are not based on physical considerations. Instead, they aim to reproduce a set of reference electronic structure data as accurately as possible by using very general and flexible functional forms. In this review we will survey a number of these methods. While they differ in the choice of the employed mathematical functions, they all have in common that they provide high-quality potential-energy surfaces, while the efficiency is comparable to conventional empirical potentials. It has been demonstrated that in many cases these potentials now offer a very interesting new approach to study complex systems with hitherto unreached accuracy.     

Structural-phase transformations in low-stability condensed state systems

The conditions of formation of low-stability condensed state systems, their behavior and structure are investigated. The objects under study are alloys and compounds undergoing structural-phase transitions of the second type or close to it. The low-stability (pre-transitional) state is treated here as the state of a system near its structural-phase transformations, in which its structure and properties exhibit anomalies. An attempt is made to interpret the system from the physical standpoint relying on a new insight into its state, in which the traditionally accepted phase-transition point is represented by a range of values of the parameter controlling the transition. The material state within this range of values is structurally weakly stable in terms of slight variations in the controlling parameter. It is shown that the thermodynamics of structural-phase transformations of the material in this transient state is significantly affected by the interaction of structure defects.     

Combustion of volatile condensed systems during pressure decay

The combustion and extinction of volatile condensed systems during pressure decay are studied. In contrast to the existing theories of this phenomenon, where only the thermal inertia of the condensed phase (t _c -approximation) is considered, an analysis of the time-dependent behavior of the gas phase is also included. Extinction curves, i.e., dependence between the pressure decay depth and the pressure decay rate at which the burning of the propellant ceases are calculated. The analysis is performed within the framework of the Belyaev model. A comparison of the results with calculations based on the t _c -approximation shows that, at high pressures, the thermal inertia of the gas phase is of considerable importance.     

On the low-temperature behavior of condensed Bose systems

A microscopic derivation of theT ²-law for the low-temperature behavior of the condensate depletion in an interacting Bose liquid is given using the knownT=0 low-lying single-particle excitations. TheT ²-term gives no contribution to the orderT ³ in the microscopic proof of Landau'sT ³-law for the specific heat of an interacting Bose liquid.     

On the existence of condensed phases in low-dimensional systems

It is shown that the threshold interaction constant (TIC) α_D for the existence of a condensed phase increases with the dimension D of a Bose system (α₃ < α₂ < α₁). TICs are evaluated exactly for a wide class of two-parameter potentials (the latter may approximate real physical interactions). The results are applied to spin-polarized systems. The possibility of an electric field induced liquid-gas transition is discussed. The TIC for N-particle clusters diminishes with N.     

Physicochemical characteristics of the components of energetic condensed systems

The physicochemical characteristics of nanosized energetic materials (aluminum, ammonium nitrate, RDX) are studied and the burning rate of energetic condensed systems containing nanosized components is determined. The morphology, chemical purity, and thermal properties of nanosized powders produced by vacuum and plasma recondensation were examined. The materials obtained are identical to the precursors and are characterized by an increased reactivity compared to their microsized counterparts. At a pressure of 100 atm, a monopropellant prepared from nanosized RDX particles is found to have twice as high the burning velocity as a microsized RDX propellant. For nanoaluminum-ammonium perchlorate compositions, the burning velocity is demonstrated to be an order of magnitude higher than that for similar compositions with microsized aluminum. An important feature of nanoaluminum-based formulations is a decrease in the degree of agglomeration of metal fuel products and, hence, in two-phase losses in solid-propellant rocket engines.     

Dynamics in condensed molecular systems studied by incoherent light

Applications of incoherent light techniques to time-resolved studies in condensed matter are reviewed. Vibrational dephasing of-carotene and a heptamethine dye was studied by interferometric coherent anti-Stokes Raman scattering in solution revealing dephasing times between 300 fs and 1.1 ps. This technique gives results that are analogos to spectral methods, but vibrational frequencies can be determined more precisely. Electronic dephasing was studied for a variety of dyes. Forced light scattering is used to measure the initial evolution of the lineshape functiong(t) up to about 50 fs. In the lowest order approximation,g(t) = ² t ² /2,, the dephasing is characterized by a modulation strength = 25 to 150 THz depending on the dye, the solvent and its temperature, and the pump laser wavelength. Interestingly, the modulation decreases for heptamethine in the long-wavelength tail of the absorption band. Brillouin scattering gives rise to finite dephasing times of neat solvents in forced light scattering.     

Phase diagram of the Gross-Neveu model: exact results and condensed matter precursors

Recently the revised phase diagram of the (large N) Gross-Neveu model in 1 + 1 dimensions with discrete chiral symmetry has been determined numerically. It features three phases, a massless and a massive Fermi gas and a kink-antikink crystal. Here we investigate the phase diagram by analytical means, mapping the Dirac-Hartree-Fock equation onto the non-relativistic Schrödinger equation with the (single gap) Lamé potential. It is pointed out that mathematically identical phase diagrams appeared in the condensed matter literature some time ago in the context of the Peierls-Fröhlich model and ferromagnetic superconductors.     

First-principles calculations of quasiparticle excitations of open-shell condensed matter systems

We develop a Green's function approach to quasiparticle excitations of open-shell systems within the GW approximation. It is shown that accurate calculations of the characteristic multiplet structure require a precise knowledge of the self-energy and, in particular, its poles. We achieve this by constructing the self-energy from appropriately chosen mean-field theories on a fine frequency grid. We apply our method to a two-site Hubbard model, several molecules, and the negatively charged nitrogen-vacancy defect in diamond and obtain good agreement with experiment and other high-level theories.     

Mössbauer effect and molecular motion in condensed systems with diffusion

Mössbauer effect in solutions of FeSO₄, 7H₂O and Na₂[Fe(CN)₅NO], 2H₂O in CH₂OH-CH₂-CH₂OH has been investigated. Areas, line widths, isomer shifts and quadrupole splittings are determined as functions of temperature in both the supercooled liquid state and the polycrystalline state. Phase transformations show up in all four parameters. We discuss the customary interpretations of areas in terms of 〈x ²〉 _T for rapid oscillatory motion and line widths in terms of the diffusion coefficientD. An unexpected linear dependence ofD on 〈x ²〉 _T found experimentally is shown to be in accordance with a model expression presented for the velocity autocorrelation function.     

Simulations: A tool for studying quantum condensed matter systems

Quantum Monte Carlo techniques provide a new method for studying the properties of condensed matter systems. A review of this approach and the type of information which it can provide is given. Talk presented at the Frontiers of Monte Carlo Meeting, Los Alamos National Laboratory, September, 1985.