Quantum spin dynamics of mode-squeezed Luttinger liquids in two-component atomic gases

We report on the observation of many-body spin dynamics of interacting, one-dimensional (1D) ultracold bosonic gases with two spin states. By controlling the nonlinear atomic interactions close to a Feshbach resonance we are able to induce a phase diffusive many-body spin dynamics of the relative phase between the two components. We monitor this dynamical evolution by Ramsey interferometry, supplemented by a novel, many-body echo technique, which unveils the role of quantum fluctuations in 1D. We find that the time evolution of the system is well described by a Luttinger liquid initially prepared in a multimode squeezed state. Our approach allows us to probe the nonequilibrium evolution of one-dimensional many-body quantum systems.     

Platinum nanoclusters on graphite substrates: a molecular dynamics study

Molecular dynamics simulations are used to investigate the shape and structure evolution of single platinum clusters of cubic and spherical shape containing 256 and 260 atoms, respectively, deposited on a static graphite substrate. The evolution is monitored at variable temperature, and as a function of metal-substrate interactions at constant temperature. The Pt-Pt interactions are modelled with the many-body Sutton-Chen potential, whereas a Lennard-Jones potential is used to describe the Pt-C interactions. Heating and cooling curves calculated between 200 K and 1800 K are used to determine solid-solid and solid-liquid transitions. Structural changes are detected through analyses of density profiles and diffusion coefficients. A clear analogy is observed between temperature-induced wetting phenomena and those resulting from enhancement of the metal-substrate interactions.     

Spectroscopy of soft modes and quantum phase transitions in coupled electron bilayers

Strongly correlated two-dimensional electrons in coupled semiconductor bilayers display remarkable broken symmetry many-body states under accessible and controllable experimental conditions. In the case of continuous quantum phase transitions (QPTs), soft collective modes drive the transformations that link distinct ground states of the electron double layers. In this paper we consider results showing that resonant inelastic light scattering methods detect soft collective modes of the double layers and probe their evolution with temperature and magnetic field. The light scattering experiments offer venues of research of fundamental interactions and continuous QPTs in low-dimensional electron liquids.     

A Remark on the Mean-Field Dynamics of Many-Body Bosonic Systems with Random Interactions and in a Random Potential

The mean-field limit for the dynamics of bosons with random two-body interactions and in the presence of a random external potential is rigorously studied, both for the Hartree dynamics and the Gross–Pitaevskii dynamics. First, it is shown that, for interactions and potentials that are almost surely bounded, the many-body quantum evolution can be replaced in the mean-field limit by a single particle nonlinear evolution that is described by the Hartree equation. This is an Egorov-type theorem for many-body quantum systems with random interactions. The analysis is then extended to derive the Gross–Pitaevskii equation with random interactions.     

Coherent ultrafast core-hole correlation spectroscopy: x-ray analogues of multidimensional NMR

We propose two-dimensional x-ray coherent correlation spectroscopy for the study of interactions between core-electron and valence transitions. This technique may find experimental applications in the future when very high intensity x-ray sources become available. Spectra obtained by varying two delay periods between pulses show off-diagonal crosspeaks induced by coupling of core transitions of two different types. Calculations of the N1s and O1s signals of aminophenol isomers illustrate how novel information about many-body effects in electronic structure and excitations of molecules can be extracted from these spectra.     

Spectral theory of Schrödinger operators of many-body systems with permutation and rotation symmetries

We investigate the spectra of many-body Schrödinger operators with permutation and rotation symmetry on subspaces of a given symmetry. For relatively compact two-body interactions the essential spectrum is a half line starting at the lowest two-body threshold with compatible symmetry. For dilatation analytic interactions the singular continuous spectrum is empty, and the eigenvalues accumulate at most at thresholds with compatible symmetries. For systems of atomic type there is an infinite sequence of discrete eigenvalues of each symmetry type.     

Exchange-enhanced spin-splitting in a two-dimensional electron gas in the presence of the Rashba spin–orbit interaction

We present a theoretical study on how many-body effects can affect the spin-splitting of a two-dimensional electron gas in the presence of the Rashba spin–orbit interaction. The standard Hartree–Fock approximation and Green's function approach are employed to calculate the energy spectrum and density of states of a spin-split two-dimensional electron gas (2DEG). We find that the presence of the exchange interaction can enhance significantly the spin-splitting of a 2DEG on top of the Rashba effect. The physical reasons behind this important phenomenon are discussed.     

Many-body effects in the Si(111)7×7 reconstructed surface

A two-dimensional Hubbard Hamiltonian is introduced in order to analyse the many-body effects associated with a dangling bond Si(111) 7×7 surface state located at the Fermi energy. Our results indicate that many-body effects can offer an explanation of recently reported experimental data.     

Double-resonance spectroscopy of interacting Rydberg-atom systems

The energy level spectrum of a many-body system containing two shared, collective Rydberg excitations is measured using cold atoms in an optical dipole trap. Two pairs of independently tunable laser pulses are employed to spectroscopically probe the spectrum in a double-resonance excitation scheme. Depending on the magnitude of an applied electric field, the Rydberg-atom interactions can vary from resonant dipole-dipole to attractive or repulsive van der Waals, leading to characteristic signatures in the measured spectra. Our results agree with theoretical estimates of the magnitude and sign of the interactions.     

Optical conductivity of a two-dimensional electron liquid with spin-orbit interaction

The interplay of electron-electron interactions and spin-orbit coupling leads to a new contribution to the homogeneous optical conductivity of the electron liquid. The latter is known to be insensitive to many-body effects for a conventional electron system with parabolic dispersion. The parabolic spectrum has its origin in the Galilean invariance which is broken by spin-orbit coupling. This opens up a possibility for the optical conductivity to probe electron-electron interactions. We analyze the interplay of interactions and spin-orbit coupling and obtain optical conductivity beyond RPA.     

Quantum dynamics of atomic coherence in a spin-1 condensate: Mean-field versus many-body simulation

We analyse and numerically simulate the full many-body quantum dynamics of a spin-1 condensate in the single spatial mode approximation. Initially, the condensate is in a “ferromagnetic” state with all spins aligned along the y axis and the magnetic field pointing along the z axis. In the course of evolution the spinor condensate undergoes a characteristic change of symmetry, which in a real experiment could be a signature of spin-mixing many-body interactions. The results of our simulations are conveniently visualised within the picture of irreducible tensor operators.     

Self-diffusion of spheres in a concentrated suspension

We calculate the concentration-dependence of the short-time self-diffusion coefficient D_s for spherical particles in suspension. Our analysis is valid up to high densities and fully takes into account the many-body hydrodynamic interactions between an arbitrary number of spheres. The importance of these many-body interactions can be inferred from our calculation of the second virial coefficient of D_s.     

Magnetization of a strongly interacting two-dimensional electron system in perpendicular magnetic fields

We measure the thermodynamic magnetization of a low-disordered, strongly correlated two-dimensional electron system in silicon in perpendicular magnetic fields. A new, parameter-free method is used to directly determine the spectrum characteristics (Landé g factor and the cyclotron mass) when the Fermi level lies outside the spectral gaps and the interlevel interactions between quasiparticles are avoided. Intralevel interactions are found to strongly modify the magnetization, without affecting the determined g* and m*.     

Efficient simulation of one-dimensional quantum many-body systems

We present a numerical method to simulate the time evolution, according to a generic Hamiltonian made of local interactions, of quantum spin chains and systems alike. The efficiency of the scheme depends on the amount of entanglement involved in the simulated evolution. Numerical analysis indicates that this method can be used, for instance, to efficiently compute time-dependent properties of low-energy dynamics in sufficiently regular but otherwise arbitrary one-dimensional quantum many-body systems. As by-products, we describe two alternatives to the density matrix renormalization group method.     

The effect of 3-body forces and anharmonicity on the phonon spectrum of solid argon

The phonon spectra of solidified argon have been computed by a phenomenological rigid-atom-model. This model, which takes the constituent atoms as rigid-hard spheres, assumes that the potential energy of the solid is the sum of central and non-central interactions, and derives the same from the Buckingham-Corner potential function together with the Axilrod-Teller interaction term. The zero-point quantum and anharmonic effects, have been included. The effect of many-body forces as well as anharmonicity on the frequency spectrum and the lattice heat capacities of the solid is seen to be appreciable. The agreement between theoretical and the experimental results is not very satisfactory.     

Single-particle excitation spectrum of the Hubbard model with magnetic frustration at finite temperature

The single-particle excitation spectrum of the Hubbard model with magnetic frustration at finite temperature is examined using numerical exact diagonalization techniques. The magnetic frustration is introduced by a proper choice of the Hamiltonian parameters, which lead to rich low-energy spin excitation behavior, resembling those observed in heavy fermion systems. At finite temperature, the low-lying excited states become thermally populated with significant weight. As a result, the calculated spectrum shows interesting temperature dependent evolution. The calculated results are presented and discussed in a many-body picture to gain insight into the photoelectron spectroscopy of strongly correlated electron systems.     

ARPES measurements of SnAs electronic band structure

A change in the time dependence of the second moment of the distribution of intensities of coherences with various orders in the spectrum of multiple-quantum NMR in a solid at the inclusion of an inhomogeneous magnetic field in the effective interaction is studied. Both the secular dipole–dipole and nonspecular twoquantum interactions are considered as nucleus–nucleus interactions, which correspond to traditional experimental realizations. It is shown that, with an increase in the magnitude of the inhomogeneous field, an exponential increase in the second moment of multiple-quantum NMR with time changes to a power-law increase. The results obtained in this work indicate that this second moment, which determines the average number of dynamically correlated spins, can be used as a convenient characteristic for studying a transition to a many-body localized state.     

Melting a Hubbard dimer: benchmarks of ‘ALDA’ for quantum thermodynamics

The competition between evolution time, interaction strength, and temperature challenges our understanding of many-body quantum systems out-of-equilibrium. Here, we consider a benchmark system, the Hubbard dimer, which allows us to explore all the relevant regimes and calculate exactly the related average quantum work. At difference with previous studies, we focus on the effect of increasing temperature, and show how this can turn the competition between many-body interactions and driving field into synergy. We then turn to use recently proposed protocols inspired by density functional theory to explore if these effects could be reproduced by using simple approximations. We find that, up to and including intermediate temperatures, a method which borrows from ground-state adiabatic local density approximation improves dramatically the estimate for the average quantum work, including, in the adiabatic regime, when correlations are strong. However at high temperature and at least when based on the pseudo-LDA, this method fails to capture the counterintuitive qualitative dependence of the quantum work with interaction strength, albeit getting the quantitative estimates relatively close to the exact results.     

Gigantic exchange enhancement of spin g-factor for two-dimensional electron gas in GaAs

Gigantic, oscillatory values of the spin g-factor for two-dimensional electron gas in GaAs have been measured, using the Shubnikov-deHaas effect in GaAs - Al_0.3Ga_0.7As heterojunctions. The effect has been attributed to the exchange interaction and compared with the theory of Ando and Uemura. The observed g-factor enhancement is, together with the fractional quantum Hall effect, the strongest evidence for the decisive role of many-body interactions in the 2D electron gas.