arXiv subjects
Publications and source records attributed to Peter B. Littlewood.
Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this work, we develop and carry out state of the art linear scaling DFT calculations refined with non-local dynamical mean-field theory. We identify a complex mechanism, a Peierls-assisted orbital selection Mott instability, which is responsible for the insulating M$_1$ phase, and furthermore survives a moderate degree of disorder.
Quantum state preparation through external control is fundamental to established methods in quantum information processing and in studies of dynamics. In this respect, excitons in semiconductor quantum dots (QDs) are of particular interest since their coupling to light allows them to be driven into a specified state using the coherent interaction with a tuned optical field such as an external laser pulse. We propose a protocol, based on adiabatic rapid passage, for the creation of entangled states in an ensemble of pairwise coupled two-level systems, such as an ensemble of QD molecules. We show by quantitative analysis using realistic parameters for semiconductor QDs that this method is feasible where other approaches are unavailable. Furthermore, this scheme can be generically transferred to some other physical systems including circuit QED, nuclear and electron spins in solid-state environments, and photonic coupled cavities.
A new NMR anvil cell design is used for measuring the influence of high pressure on the electronic properties of the high-temperature superconductor YBa$_2$Cu$_4$O$_8$ above the superconducting transition temperature $T_{\rm c}$. It is found that pressure increases the spin shift at all temperatures in such a way that the pseudo-gap feature has almost disappeared at 63 kbar. This change of the temperature dependent spin susceptibility can be explained by a pressure induced proportional decrease (factor of two) of a temperature dependent component, and an increase (factor of 9) of a temperature independent component, contrary to the effects of increasing doping. The results demonstrate that one can use anvil cell NMR to investigate the tuning of the electronic properties of correlated electronic materials with pressure.
We consider an electron-hole bilayer in the limit of extreme density imbalance, where we have a single particle in one layer interacting attractively with a Fermi liquid in the other parallel layer. Using an appropriate variational wave function for the dressed exciton, we provide strong evidence for the existence of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in electron-hole bilayers with a large density imbalance. Furthermore, within this unusual limit of FFLO, we find that a dilute gas of minority particles forms excitons that condense into a two-dimensional "supersolid".
We derive an effective Hamiltonian for the one-dimensional Hubbard-Holstein model, valid in a regime of both strong electron-electron (e-e) and electron-phonon (e-ph) interactions and in the non-adiabatic limit ($t/ω_0 \leq 1$), by using a non-perturbative approach. We obtain the phase diagram at quarter-filling by employing a modified Lanczos method and studying various density-density correlations. The spin-spin AF (antiferromagnetic) interactions and nearest-neighbor repulsion, resulting from the e-e and the e-ph interactions respectively, are the dominant terms (compared to hopping) and compete to determine the various correlated phases. As e-e interaction $(U/t)$ is increased, the system transits from an AF cluster to a correlated singlet phase through a discontinuous transition at all strong e-ph couplings $2 \leq g \leq 3$ considered. At higher values of $U/t$ and moderately strong e-ph interactions ($2 \leq g \leq 2.6$), the singlets break up to form an AF order and then to a paramagnetic order all in a single sublattice; whereas at larger values of $g$ ($> 2.6$), the system jumps directly to the spin disordered charge-density-wave (CDW) phase.
Solid state quantum condensates often differ from previous examples of condensates (such as Helium, ultra-cold atomic gases, and superconductors) in that the quasiparticles condensing have relatively short lifetimes, and so as for lasers, external pumping is required to maintain a steady state. On the other hand, compared to lasers, the quasiparticles are generally more strongly interacting, and therefore better able to thermalise. This leads to questions of how to describe such non-equilibrium condensates, and their relation to equilibrium condensates and lasers. This chapter discusses in detail how the non-equilibrium Green's function approach can be applied to the description of such a non-equilibrium condensate, in particular, a system of microcavity polaritons, driven out of equilibrium by coupling to multiple baths. By considering the steady states, and fluctuations about them, it is possible to provide a description that relates both to equilibrium condensation and to lasing, while at the same time, making clear the differences from simple lasers.
We develop a theory of spin noise spectroscopy of itinerant, noninteracting, spin-carrying fermions in different regimes of temperature and disorder. We use kinetic equations for the density matrix in spin variables. We find a general result with a clear physical interpretation, and discuss its dependence on temperature, the size of the system, and applied magnetic field. We consider two classes of experimental probes: 1. electron-spin-resonance (ESR)-type measurements, in which the probe response to a uniform magnetization increases linearly with the volume sampled, and 2. optical Kerr/Faraday rotation-type measurements, in which the probe response to a uniform magnetization increases linearly with the length of the light propagation in the sample, but is independent of the cross section of the light beam. Our theory provides a framework for interpreting recent experiments on atomic gases and conduction electrons in semiconductors and provides a baseline for identifying the effects of interactions on spin noise spectroscopy.
Time-of-flight neutron powder diffraction and specific heat measurements were used to study the nature of thermal expansion in rhenium trioxide, an electrically conducting oxide with cubic symmetry. The temperature evolution of the lattice parameters show that ReO3 can exhibit negative thermal expansion at low temperatures and that the transition from negative to positive thermal expansion depends on sample preparation; the single crystal sample demonstrated the highest transition temperature, 300 K, and largest negative value for the coefficient of thermal expansion, alpha = -1.1(1)x 10^-6 K^-1. For the oxygen atoms, the atomic displacement parameters are strongly anisotropic even at 15 K, indicative of a large contribution of static disorder to the displacement parameters. Further inspection of the temperature evolution of the oxygen displacement parameters for different samples reveals that the static disorder contribution is greater for the samples with diminished NTE behavior. In addition, specific heat measurements show that ReO3 lacks the low energy Einstein-type modes seen in other negative thermal expansion oxides such as ZrW2O8.
The dielectric response in a magnetic field is routinely used to probe the existence of coupled magnetic and elastic order in the multiferroics. However, here we demonstrate that magnetism is not necessary to produce a magnetocapacitance when the material is inhomogeneous. By considering a two-dimensional, two-component composite medium, we find a characteristic dielectric resonance that depends on magnetic field. We propose this as a possible signature of inhomogeneities and we argue that this behavior has already been observed in nanoporous silicon and some manganites.
The superfluid to Mott insulator transition in cavity polariton arrays is analyzed using the variational cluster approach, taking into account quantum fluctuations exactly on finite length scales. Phase diagrams in one and two dimensions exhibit important non-mean-field features. Single-particle excitation spectra in the Mott phase are dominated by particle and hole bands separated by a Mott gap. In contrast to Bose-Hubbard models, detuning allows for changing the nature of the bosonic particles from quasi-localized excitons to polaritons to weakly interacting photons. The Mott state with density one exists up to temperatures $T/g\gtrsim0.03$, implying experimentally accessible temperatures for realistic cavity couplings $g$.
Exciton-polaron formation in one-dimensional lattice models with short or long-range carrier-phonon interaction is studied by quantum Monte Carlo simulations. Depending on the relative sign of electron and hole-phonon coupling, the exciton-polaron size increases or decreases with increasing interaction strength. Quantum phonon fluctuations determine the (exciton-)polaron size and yield translation-invariant states at all finite couplings.
Spin electronics (spintronics) exploits the magnetic nature of the electron, and is commercially exploited in the spin valves of disc-drive read heads. There is currently widespread interest in using industrially relevant semiconductors in new types of spintronic devices based on the manipulation of spins injected into a semiconducting channel between a spin-polarized source and drain. However, the transformation of spin information into large electrical signals is limited by spin relaxation such that the magnetoresistive signals are below 1%. We overcome this long standing problem in spintronics by demonstrating large magnetoresistance effects of 61% at 5 K in devices where the non-magnetic channel is a multiwall carbon nanotube that spans a 1.5 micron gap between epitaxial electrodes of the highly spin polarized manganite La0.7Sr0.3MnO3. This improvement arises because the spin lifetime in nanotubes is long due the small spin-orbit coupling of carbon, because the high nanotube Fermi velocity permits the carrier dwell time to not significantly exceed this spin lifetime, because the manganite remains highly spin polarized up to the manganite-nanotube interface, and because the interfacial barrier is of an appropriate height. We support these latter statements regarding the interface using density functional theory calculations. The success of our experiments with such chemically and geometrically different materials should inspire adventure in materials selection for some future spintronics
We present a model of magnetotransport of inhomogeneous conductors based on an array of coupled four-terminal elements. We show that this model generically yields non-saturating magnetoresistance at large fields. We also discuss how this approach simplifies finite-element analysis of bulk inhomogeneous semiconductors in complex geometries. We argue that this is an explanation of the observed non-saturating magnetoresistance in silver chalcogenides and potentially in other disordered conductors. Our method may be used to design the magnetoresistive response of a microfabricated array.
We present a general derivation of the electron spin noise power spectrum in alkali gases as measured by optical Faraday rotation, which applies to both classical gases at high temperatures as well as ultracold quantum gases. We show that the spin-noise power spectrum is determined by an electron spin-spin correlation function, and we find that measurements of the spin-noise power spectra for a classical gas of $^{41}$K atoms are in good agreement with the predicted values. Experimental and theoretical spin noise spectra are directly and quantitatively compared in both longitudinal and transverse magnetic fields up to the high magnetic field regime (where Zeeman energies exceed the intrinsic hyperfine energy splitting of the $^{41}$K ground state).
Ultracold alkali atoms provide experimentally accessible model systems for probing quantum states that manifest themselves at the macroscopic scale. Recent experimental realizations of superfluidity in dilute gases of ultracold fermionic (half-integer spin) atoms offer exciting opportunities to directly test theoretical models of related many-body fermion systems that are inaccessible to experimental manipulation, such as neutron stars and quark-gluon plasmas. However, the microscopic interactions between fermions are potentially quite complex, and experiments in ultracold gases to date cannot clearly distinguish between the qualitatively different microscopic models that have been proposed. Here, we theoretically demonstrate that optical measurements of electron spin noise -- the intrinsic, random fluctuations of spin -- can probe the entangled quantum states of ultracold fermionic atomic gases and unambiguously reveal the detailed nature of the interatomic interactions. We show that different models predict different sets of resonances in the noise spectrum, and once the correct effective interatomic interaction model is identified, the line-shapes of the spin noise can be used to constrain this model. Further, experimental measurements of spin noise in classical (Boltzmann) alkali vapors are used to estimate the expected signal magnitudes for spin noise measurements in ultracold atom systems and to show that these measurements are feasible.
We propose a new method of detecting the onset of superfluidity in a two-component ultracold fermionic gas of atoms governed by an attractive short-range interaction. By studying the two-body correlation functions we find that a measurement of the momentum distribution of the density and spin response functions allows one to access separately the normal and anomalous densities. The change in sign at low momentum transfer of the density response function signals the transition between a BEC and a BCS regimes, characterized by small and large pairs, respectively. This change in sign of the density response function represents an unambiguous signature of the BEC to BCS crossover. Also, we predict spin rotational symmetry-breaking in this system.
Bilayer electron-hole systems, where the electrons and holes are created via doping and confined to separate layers, undergo excitonic condensation when the distance between the layers is smaller than typical distance between particles within a layer. We argue that the excitonic condensate is a novel dipolar superfluid in which the phase of the condensate couples to the {\it gradient} of the vector potential. We predict the existence of dipolar supercurrent which can be tuned by an in-plane magnetic field and detected by independent contacts to the layers. Thus the dipolar superfluid offers an example of excitonic condensate in which the {\it composite} nature of its constituent excitons is manifest in the macroscopic superfluid state. We also discuss various properties of this superfluid including the role of vortices.
We study the crossover from BEC to BCS pairing for dilute systems but with a realistic finite-range interaction. We exhibit the changes in the excitation spectrum that provide a clean qualitative distinction between the two limits. We also study how the dilute system converges to the results from a zero-range pseudo-potential derived by Leggett.