Fermi Temperature Research Articles

Interaction Quench in the Hubbard Model

Motivated by recent experiments in ultracold atomic gases that explore the nonequilibrium dynamics of interacting quantum many-body systems, we investigate the opposite limit of Landau's Fermi-liquid paradigm: We study a Hubbard model with a sudden interaction quench, that is, the interaction is switched on at time t=0. Using the flow equation method, we are able to study the real time dynamics for weak interaction U in a systematic expansion and find three clearly separated time regimes: (i) An initial buildup of correlations where the quasiparticles are formed. (ii) An intermediate quasi-steady regime resembling a zero temperature Fermi liquid with a nonequilibrium quasiparticle distribution function. (iii) The long-time limit described by a quantum Boltzmann equation leading to thermalization of the momentum distribution function with a temperature T proportional, variantU.

Target design for studies of radiative properties in warm dense matter at GSI and FAIR - the WDM collaboration

Intense beams of energetic heavy ions from SIS-18 at GSI can heat extended volumes of condensed matter to a temperature of several eV. The matter at about solid density and a temperature near or above the Fermi temperature is referred to as ‘Warm Dense Matter’ (WDM). The knowledge of radiative properties and the equation of state of WDM is particularly limited because standard perturbative theoretical approaches are not applicable in this regime. The kilojoule PHELIX laser at GSI can be used to produce X-rays of several keV energy for scattering diagnostics of ion beam heated matter. In this paper special target configurations for compression and isochoric heating of matter with heavy ions are presented. The possibilities of X-ray scattering diagnostics of such targets are discussed. The target parameters achievable with the new FAIR facility are calculated.

Temperature dependence of persistent spin currents in a spin-orbit-coupled electron gas: A density-matrix approach

We present a simple analytical method, based on the canonical density matrix, for the calculation of the equilibrium spin current as a function of temperature in a two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit coupling terms. We find that the persistent spin current is extremely robust against thermal disorder: its variation with temperature is exponentially small $(\ensuremath{\propto}{e}^{\ensuremath{-}{T}_{F}∕T})$ at temperatures much smaller than the Fermi temperature ${T}_{F}$ and changes to a power law ${T}_{F}∕T$ for $T⪢{T}_{F}$.

Quantum Degenerate Fermi–Bose Mixtures of 40K and 87Rb Atoms in a Quadrupole-Ioffe Configuration Trap

We report on the attainment of quantum degeneracy of 40K by means of efficient thermal collisions with the evaporatively cooled 87Rb atoms. In a quadrupole-Ioffe configuration trap, potassium atoms are cooled to 0.5 times the Fermi temperature. We obtain up to 7.59 × 105 degenerate fermions 40K.

Nuclear magnetic relaxation of 3He in contact with an aerogel above the Fermi temperature

The spin kinetics of 3He in an aerogel has been studied above the Fermi temperature. The magnetic relaxation times T 1 and T 2 of adsorbed, gaseous, and liquid 3He in a 95% silica aerogel at a temperature of 1.5 K have been determined as functions of frequency by means of pulse nuclear magnetic resonance. It has been found that the time T 1 is linear in frequency in all three cases, whereas T 2 is independent of frequency. To explain the observed behavior of the longitudinal relaxation rate, a theoretical model of relaxation in the adsorbed layer of 3He taking into account the filamentary structure of the aerogel is proposed.

Hopping currents in the intrinsic region of III‐N‐V quantum well structures in the tight binding approximation

AbstractWe calculate for the first time Nearest Neighbor Hopping (NNH) current densities for III‐N‐V heterostructures embedded in the intrinsic layer of p‐i‐n geometries. We start from a general formalism that relates the density of states in individual wells with the transition probabilities from a quantum well state to a neighboring one, in a superlattice‐geometry. In the process, we find and point out the fact that there is zero hopping‐current without Fermi level splitting for any two consecutive layers.). We then relate the eigen‐energies with the transition probabilities and conclude that NNH ‐currents are of the order of 0.06 mA/cm2 at the most and in room temperatures, and depend on (a) device geometry (b) temperature (c) conduction band discontinuity and (d) Fermi level splitting. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quasiparticles and anomalous temperature dependence of the low-lying states in the colossal magnetoresistant oxideLa2−2xSr1+2xMn2O7(x=0.36)from angle-resolved photoemission

After years of research into colossal magnetoresistant (CMR) manganites using bulk techniques, there has been a recent upsurge in experiments directly probing the electronic states at or near the surface of the bilayer CMR materials ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ using angle-resolved photoemission or scanning probe microscopy. Here, we report temperature-dependent, angle-resolved photoemission data from single crystals with a doping level of $x=0.36$. The first important result is that there is no sign of a pseudogap in the charge channel of this material for temperatures below the Curie temperature ${T}_{C}$. The data show unprecedented sharp spectral features, enabling the unambiguous identification of clear, resolution-limited quasiparticle features from the bilayer split $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-derived Fermi surfaces both at the zone-face and zone diagonal ${k}_{F}$ locations. The data show that these low temperature Fermi surfaces describe closed shapes in ${k}_{\ensuremath{\parallel}}$, centered at the $(\ensuremath{\pi}∕a,\ensuremath{\pi}∕a)$ points in the two dimensional Brillouin zone, and are not open and arclike in nature. The second important result concerns the temperature dependence of the electronic states. The spectra display strong incoherent intensity at high binding energies and a very strong temperature dependence, both characteristics reminiscent of polaronic systems. However, the clear and strong quasiparticle peaks at low temperatures are difficult to place within a polaronic scenario. A careful analysis of the temperature-dependent changes in the Fermi surface spectra both at the zone face and zone diagonal regions in $k$ space indicates that the coherent quasiparticle weight disappears for temperatures significantly above ${T}_{C}$ and that the $k$ dependence of the $T$-induced changes in the spectra invalidates an interpretation of these data in terms of the superposition of a ``universal'' metallic spectrum and an insulating spectrum whose relative weight changes with temperature. In this sense, our data are not compatible with a phase separation scenario.

Interaction-Controlled Transport of an Ultracold Fermi Gas

We explore the transport properties of an interacting Fermi gas in a three-dimensional optical lattice. The center of mass dynamics of the atoms after a sudden displacement of the trap minimum is monitored for different interaction strengths and lattice fillings. With increasingly strong attractive interactions the weakly damped oscillation, observed for the noninteracting case, turns into a slow relaxational drift. Tuning the interaction strength during the evolution allows us to dynamically control the transport behavior. Strong attraction between the atoms leads to the formation of local pairs with a reduced tunneling rate. The interpretation in terms of pair formation is supported by a measurement of the number of doubly occupied lattice sites. This quantity also allows us to determine the temperature of the noninteracting gas in the lattice to be as low as (27+/-2)% of the Fermi temperature.

Dust-lower-hybrid waves in quantum plasmas

The dispersion relation of the dust-lower-hybrid wave has been derived using the quantum hydrodynamic model of plasmas in an ultracold Fermi dusty plasma in the presence of a uniform external magnetic field. The dust dynamics, electron Fermi temperature effect, and the quantum corrections give rise to significant effects on the dust-lower-hybrid wave of the magnetized quantum dusty plasmas.

Debye shielding in quantum plasmas

Quantum effects through the Fermi temperature and the Bohm potential on the Debye–Hückel shielding potential have been examined in a supercold quantum plasma. The Bohm potential due to collective interaction of the quantum plasma modifies the Debye length significantly under an appropriate condition. The relevance of our results in supercold dense plasmas, viz. the microelectronic systems, laser-produced plasmas, compact astrophysical objects, etc has been pointed out.

Thermodynamic properties of two-dimensional semiconductors with spin–orbit coupling

We present a quantitative analysis on the effect of Rashba spin–orbit interaction on the thermodynamic properties of two-dimensional semiconductors. Quantities such as the Helmholtz free energy, the electron internal energy, the electron entropy and the electron heat capacity are calculated as a function of temperature and the spin–orbit coupling strength. It is shown that the free energy decreases with the spin–orbit coupling while entropy increases with the spin–orbit coupling. At the temperature of around 10% of Fermi temperature, the reduction of free energy due to Rashba coupling alone is round 15%.

Theory of Coulomb drag in graphene

We study the Coulomb drag between two single graphene sheets in intrinsic and extrinsic graphene systems with no interlayer tunneling. The general expression for the nonlinear susceptibility appropriate for single-layer graphene systems is derived using the diagrammatic perturbation theory, and the corresponding exact zero-temperature expression is obtained analytically. We find that, despite the existence of a nonzero conductivity in an intrinsic graphene layer, the Coulomb drag between intrinsic graphene layers vanishes at all temperatures. In extrinsic systems, we obtain numerical results and an approximate analytical result for the drag resistivity ${\ensuremath{\rho}}_{\mathrm{D}}$, and find that ${\ensuremath{\rho}}_{\mathrm{D}}$ goes as ${T}^{2}$ at low temperature $T$, as $1∕{d}^{4}$ for large layer separation $d$, and $1∕{n}^{3}$ for high carrier density $n$. We also discuss qualitatively the effect of plasmon-induced enhancement on the Coulomb drag, which should occur at a temperature of the order of or higher than the Fermi temperature.

Quantum Degenerate Fermi Gases of Ytterbium Atoms

We performed evaporative cooling for dilute gases of ytterbium (Yb) isotopes in a crossed optical dipole trap and successfully cooled two fermionic and two bosonic species down to quantum degenerate regime, following the previous realization of Bose-Einstein condensation (BEC) in 174Yb. The elastic collision rate of fermionic 173Yb atoms with 6 spin components was found to be large enough to carry out efficient evaporation, which enables us to cool the atoms down to 0.6 T F , where T F is the Fermi temperature. In this regime, a plunge of evaporation efficiency was observed as an effect of the Fermi degeneracy. The other fermionic isotope 171Yb was cooled down to the temperature below T F by sympathetic cooling with bosonic 174Yb atoms. The sympathetic cooling technique has also been applied to 174Yb-176Yb Bose-Bose mixture. We have observed almost pure BEC of 174Yb and the bimodal distribution of 176Yb, showing the formation of BEC-BEC mixture. Moreover, we performed evaporative cooling of 170Yb atoms and realized the BEC.

Nonplanar ion-acoustic waves in a quantum plasma

The nonlinear properties of ion-acoustic (IA) waves in an electron–ion quantum plasma with the effects of quantum corrections are studied in a nonplanar spherical geometry. For this purpose quantum hydrodynamic model (QHD) is used and a variable coefficient Kadomtsev–Petviashvili (KP) equation is derived by using the standard reductive perturbation method. The pressures for both electrons and ions are considered in two ways: one by using the complete fluid pressure equation and other by the equation of state pertaining to a three-dimensional zero-temperature Fermi gas. It is found that of the two ways, the latter gives significant results when the nondimensional quantum diffraction parameter H approaches or nearer to unity. The important quantum mechanical effects are examined numerically on the profiles of the compressive and rarefactive solitons. It is found that H plays a significant role in the formation of compressive and rarefactive solitons. A critical value of H is also found which depends on the phase velocity of the wave and the ion to electron Fermi temperature ratio, for which the soliton formation ceases to exist.

Enhanced spin-relaxation time due to electron-electron scattering in semiconductor quantum wells

We present a detailed experimental and theoretical analysis of the spin dynamics of two-dimensional electron gases (2DEGs) in a series of n-doped GaAs/AlGaAs quantum wells. Picosecond-resolution polarized pump-probe reflection techniques were applied in order to study in detail the temperature-, concentration- and quantum-well-width- dependencies of the spin relaxation rate of a small photoexcited electron population. A rapid enhancement of the spin life-time with temperature up to a maximum near the Fermi temperature of the 2DEG was demonstrated experimentally. These observations are consistent with the D'yakonov-Perel' spin relaxation mechanism controlled by electron-electron collisions. The experimental results and theoretical predictions for the spin relaxation times are in good quantitative agreement.

Linear temperature dependence of the conductivity in Si two-dimensional electrons near the apparent metal-to-insulator transition

In a high mobility two-dimensional electron system in Si, near the critical density, $n_c=0.32\times10^{11}$cm$^{-2}$, of the apparent metal-to-insulator transition, the conductivity displays a linear temperature ($T$) dependence around the Fermi temperature. When $\sigma_0$, the extrapolated T=0 conductivity from the linear T-dependence, is plotted as a function of density, two regimes with different $\sigma_0(n)$ relations are seen, suggestive of two different phases. Interestingly, a sharp transition between these two regimes coincides with $n_c$, and $\sigma_0$ of the transition is $\sim$ $e^2/h$, the quantum conductance, per square. Toward T=0, the data deviate from linear $\sigma(T)$ relation and we discuss the possible percolation type of transition in our Si sample.

Degenerate Fermi Gases of Ytterbium

Evaporative cooling was performed to cool fermionic 173Yb atoms in a crossed optical dipole trap. The large elastic collision rate leads to efficient evaporation and we have successfully cooled the atoms to 0.37+/-0.06 of the Fermi temperature, that is to say, to a quantum degenerate regime. In this regime, a plunge of evaporation efficiency was observed as a result of Fermi degeneracy.

Coupling between nonlinear Langmuir waves and electron holes in quantum plasmas

Abstract The nonlinear effects on a slow timescale, compared with the electron plasma frequency, are studied using the Wigner–Poisson system, in the plasma regimes characterized by the overlapping of the wavefunctions of individual electrons, by the presence of a large amplitude Langmuir pump wave, and whose temperature is higher than the Fermi temperature. It is shown that the electron trapping on closed orbits in phase space is strongly affected both by the classical nonlinear ponderomotive effects and by the quantum super-diffusion. A solution in the form of a quantum corrected electron hole is found in terms of a generalized energy in the Wigner equation that contains higher derivatives in velocity space. In the classical limit, the high-frequency pump hampers the electron trapping due to the unfavorable sign of the ponderomotive potential and due to deformation of their distribution function by the diffusion. Conversely, in the modulational regime the leading quantum effect is shown to be related with the effective super-diffusion in the velocity space associated with the quantum effects on the high-frequency pump, which facilitates the electron trapping and allows the creation of holes with smaller amplitudes.

White dwarfs as test objects of Lorentz violations

In the present work, the thermodynamical properties of bosonic and fermionic gases are analysed under the condition that a modified dispersion relation is present. This last condition implies a breakdown of Lorentz symmetry. The implications for the condensation temperature will be studied, as well as for other thermodynamical variables such as specific heat, entropy, etc. Moreover, it will be argued that those cases entailing a violation of time reversal symmetry of the motion equations could lead to problems with the concept of entropy. Concerning the fermionic case, it will be shown that Fermi temperature suffers a modification due to the breakdown of Lorentz symmetry. The results will be applied to white dwarfs and the consequences upon the Chandrasekhar mass–radius relation will be shown. The possibility of resorting to white dwarfs for the testing of modified dispersion relations is also addressed. It will be shown that the comparison of the current observations against the predictions of our model allows us to discard some values of one of the parameters appearing in the modifications of the dispersion relation.

Computer analysis of an influence of oxygen vacancies on the electronic properties of the SnO2 surface and near‐surface region

AbstractAn influence of the surface stoichiometric defects on the electronic and electrical properties of SnO2 layers has been studied by means of computer simulation. The surface potential and in‐depth potential profiles in the accumulation region have been rigorously computed at various temperatures (from 400 to 900 K). The behaviour of both the surface and bulk Fermi level position versus temperature has been analysed. Furthermore, in‐depth profiles of carrier concentration and the surface potential as well as the SnO2 layer conductance for different surface donor density of state have been calculated. It has been found that the surface reduction and bulk donor concentration (in the range from 3 × 1014 to 3 × 1018 cm–3) are strongly correlated with the development of near‐surface accumulation region and thus modification of the layer electrical parameters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)