d-Mott phases in one and two dimensions

We use exact diagonalization to determine the spectrum of reduced Hamiltonians based on renormalization group flows to strong coupling. For the half-filled two-leg Hubbard ladder we reproduce the known insulating d-Mott ground state with spin and charge gaps. For the saddle point regions of the two-dimensional Hubbard model near half filling we find a crossover to a similar strong coupling state, which truncates the Fermi surface near the saddle points. At lower scales d-wave superconductivity appears on the remaining Fermi surface.     

Transitions in glasses at low temperatures

Length-temperature measurements from ?180°C to the glass transition temperature have been made on a poly(vinyl alkyl ether) series, poly(ethylene terephthalate), and a tetrafluoroethylene-trifluoropropylene copolymer series.

Consistent with low frequency dynamic measurements, the poly(vinyl alkyl ethers) exhibit a glass-glass transition? Tgg [1] between Tg and Tg ?70°C with evidence of another at about ?160 to ?180°C. With increasing side-chain length, the expansion coefficient below Tg- and the magnitude of the g-g transition increase, and the value of the Simha-Boyer free-volume product decreases, suggesting retention of excess free volume by the flexible pendant group. This behavior is analogous to that observed for poly(alkyl methacrylates) except that the latter system exhibited two g-g transitions, the lower of which coincided with Tgg [1] observed here.

Poly(ethylene terephthalate) samples, both amorphous and crystalline, exhibit two T > Tg transitions at about ?85 and ?40°C. Dynamic results resolve only one relaxation in this temperature range.

The glass temperatures of the tetrafluoroethylene-trifluoropropylene copolymer series, as determined by both dilatometry and differential scanning calorimetry, extrapolate to a Tg, for 100% amorphous PTFE of 11-16°C. Two g-g transitions, present in each homo-polymer, persist in the copolymers, indicating that only a small number of molecular units are involved. The magnitude of these relaxations, however, as measured by the change in expansion coefficient, is dependent on copolymer composition.     

Transport in two dimensions. III self-diffusion coefficient at low densities

The low density form of the generalized frequency (s) and wavevector (k) dependent self-diffusion coefficient D(k, s) is calculated, from which the low density forms of related quantities, e.g. the velocity autocorrelation function, are derived. Agreement is obtained with the low density kinetic theory results. A closed form expression for D(k, s) valid over a wide range of densities and times is also given, showing consistency between the asymptotic long time results, obtained previously, and the low density kinetic theory results.     

Leading temperature corrections to Fermi-liquid theory in two dimensions

We calculate the basic parameters of the Fermi liquid: the scattering vertex, the Landau interaction function, the effective mass, and physical susceptibilities for a model of two-dimensional (2D) fermions with a short-ranged interaction at nonzero temperature. The leading temperature dependences of the spin components of the scattering vertex, the Landau function, and the spin susceptibility are found to be linear. T-linear terms in the effective mass and in the "charge-sector" quantities are found to cancel to second order in the interaction, but the cancellation is argued not to be generic. The connection with previous studies of the 2D Fermi-liquid parameters is discussed.     

New crystalline phases of an equiatomic K-Cs alloy at low temperature

Solid equiatomic K-Cs alloys have been investigated by X-ray diffraction throughout the temperature range 300-100K. The results indicate that a phase separation occurs below 185K accompanied by the appearance of an ordered phase in this range. This phase has a hexagonal lattice with parameters: a = 9.32(1) Å and c = 11.80(2) Å (at 170K). Evidence from our other studies [7] indicates that its composition is K₂Cs. Another phase transformation in this ordered crystal is observed below 120K. There is no change of lattice symmetry but the unit cell constants shrink to the values: a = 9.11(1) Å and c = 10.86(2) Å (at 100K). The transformation can be ascribed to a rearrangement of the electronic structure of Cs.     

Photon sector of quantum electrodynamics at high temperature in two spatial dimensions

The photon sector of quantum electrodynamics (QED) in two spatial dimensions is analyzed at high temperature to all orders of perturbation theory. Imaginary-time formalism is used. The photon self-energy and propagator at finite temperature with vanishing frequency is calculated to the second order of perturbation theory. Based upon the latter, an improved perturbation theory which incorporates Debye screening is formulated. By virtue of the latter and gauge invariance, infrared finitness holds. The temperature dependence of any contribution to the connected Green's functions in the improved perturbation theory is analyzed systematically. At very high temperature, the photon sector becomes equivalent to a very massive scalar boson field plus a massless electromagnetic field and both become decoupled: all connected Green's functions containing, at least, one closed fermion loop with four or more vertices are shown to tend to zero.     

Masterfields and the phases of quantum chromodynamics in two dimensions with fermions

The phase structure of two-dimensional quantum chromodynamics is analyzed in the large-N limit. Using a variational approximation, we show that a first-order phase transition occurs as the quark bare mass squared m₀² is made less than $\text{g}{}^2\text{N}\text{π}$. This novel phase structure goes beyond summing the perturbative large-N planar graphs.     

Dense loops,supersymmetry, and Goldstone phases in two dimensions

Loop models in two dimensions can be related to O(N) models. The low-temperature dense-loops phase of such a model, or of its reformulation using a supergroup as symmetry, can have a Goldstone broken-symmetry phase for N<2. We argue that this phase is generic for -2相似文献   

Compositeness condition at finite temperature in four dimensions

The effect of finite temperature on the compositeness condition for gΨΨσ field theory is studied in four space-time dimensions. It is shown that this condition remains satisfied at low temperature, the compositeness condition is completely broken, contrary to the results of two and three dimensions.     

Zero frequency modes of low temperature magnetic phases in YbIG

The stability of low temperature magnetic phases of YbIG in dependence on field and temperature is investigated using the model with six Yb sublattices and one saturated Fe sublattice. The conditions for existence of zero-frequency modes are deduced in the analytical form for magnetic fields lying in {110} planes. General results are applied to the case of magnetic field parallel to [111] direction and the existence of a metastability region is shown, where both the colinear as well as the canted phase may occur.     

Critical temperature of interacting Bose gases in two and three dimensions

We calculate the superfluid transition temperature of homogeneous interacting Bose gases in three and two spatial dimensions using large-scale path integral Monte Carlo simulations (with up to N=10;{5} particles). In 3D we investigate the limits of the universal critical behavior in terms of the scattering length alone by using different models for the interatomic potential. We find that this type of universality sets in at small values of the gas parameter na3 < or approximately 10(-4). This value is different from the estimate na3 < or approximately 10(-6) for the validity of the asymptotic expansion in the limit of vanishing na3. In 2D we study the Berezinskii-Kosterlitz-Thouless transition of a gas with hard-core interactions. For this system we find good agreement with the classical lattice |psi|4 model up to very large densities. We also explain the origin of the existing discrepancy between previous studies of the same problem.     

Johnson noise at low temperature

It has long been accepted that Johnson noise in metals is proportional to absolute temperature, and is materials-independent. This is a semi-classical result, based on the equipartition of energy. Johnson noise in copper below 10 mK suggests a departure from this relation. The data have been fitted empirically to T_J = T_o coth $\text{T}{}_{\mathrm{<mtext>o</mtext>}}\text{T}$, where T_J is the “Johnson noise” temperature, T is measured by CMN, and T_o is chosen for best fit and is about 2 mK. T_o is now identifiable with a zero-point frequency of electronic charge imbalance, which moves with average Fermi velocity ν from end to end of the resistor of length l. $\text{T}{}_{\mathrm{<mtext>o</mtext><mtext>\; =\; </mtext><mtext>h\nu </mtext><mtext>2kl\; \approx \; 2\; </mtext><mtext>mK</mtext>}}$, and so is materials- and geometry-dependent.     

Approach to two dimensions in then-vector model

The specific heat of then-vector model to lowest order in 1/n is analyzed when the dimensionality varies continuously betweend=3 andd=2. The changeover from three-dimensional to two-dimensional behavior is related to the location of the bare transition temperature with respect to the critical region.     

Specific heat and magnetic susceptibility at low temperature of two conductive TCNQ salts

The specific heat measurements between 1.4 and 4.4 K of acridinium (TCNQ)₂ and quinolinium (TCNQ)₂ salts show up a linear component; moreover, in presence of a strong magnetic field (H = 40kG), an artificial Schottky anomaly is revealed. Magnetic susceptibility experiments confirm the simultaneous existence of charge carriers, in a partially filled energy band, and localized paramagnetic centers. A standard energy band model is proposed to interpret these two properties.     

Dynamical instability of a boson-fermion mixture at low dimensions

We examine theoretically the dynamical response of a homogeneous mixture of condensed bosons and spin-polarized fermions confined inside a quasi-two-dimensional or a quasi-one-dimensional geometry, considering quasi-three-dimensional boson-boson and boson-fermion interactions. We focus on the effects of low dimensions on the density response functions in the crossover from weak to strong boson-fermion coupling up to the onset of instability. The dynamical condition is found to be in agreement with a linear stability analysis at equilibrium.     

Electron-damage studies in GaP at low temperature

The defects introduced in p-type Zn doped GaP by beta particle irradiation, of 1 MeV average energy, at low temperatures are monitored by bridge measurements of capacitance and delectric losses of a capacitor. The results of measurements, between 15 and 65 K, are attributed, in two different temperature regimes, to a GaP capacitor with losses arising from (a) an a.c hopping conduction process, below 45 K, and (b) a band conduction, above 45 K. Electron irradiation affects both hopping and band conduction as well as the transition temperature.