Observation of a degenerate Fermi gas confined by a standing electromagnetic wave

We have observed a gas of Fermi atoms confined in the antinodes of a standing electromagnetic wave. The standing wave is formed by two counter-propagating beams with the wavelength of 10.6 μm focused on the same spot. Each antinode confines a pancake-shaped cloud of 7500 lithium-6 atoms in two equally populated spin states at the temperature T = 0.1E _F, where E _F is the Fermi energy. The system is in the regime beyond the local density approximation: Only the 3 lowest energy states of the axial motion are populated. The system may become an instrument for the study of 2D Fermi physics and 3D effects beyond the local density approximation.     

Defect-level analysis of semiconductors by a new differential evaluation ofn(1/T)-characteristics

A new method for the defect-level analysis of extrinsic semiconductors is described. Provided that the defect-level concentration is not too large and the temperature is not too low, the Fermi levelE _F is shifted with increasing temperature from a position near the conduction (or valence) band towards the middle of the forbidden gap monotonously. Thus majority carriers are emitted into the conduction (or valence) band from the defect levels successively. If for a small increment of the temperature the Fermi levelE _F is shifted by ΔE _F and the concentration of free majority carriers is increased by Δn, then the ratio Δn/ΔE _F is a measure of the defect-level concentration within ΔE _F . Furthermore we discuss how this analysis is influenced by additional defect levels outside the range over which the Fermi energy can be shifted by variation of the temperature.     

Spin susceptibility in small Fermi energy systems: effects of nonmagnetic impurities

In small Fermi energy metals, disorder can deeply modify superconducting state properties leading to a strong suppression of the critical temperature T_c. In this paper, we show that also normal state properties can be seriously influenced by disorder when the Fermi energy E _F is sufficiently small. We calculate the normal state spin susceptibility χ for a narrow band electron-phonon coupled metal as a function of the non-magnetic impurity scattering rate . We find that as soon as is comparable to E _F, χ is strongly reduced with respect to its value in the clean limit. The effects of the electron-phonon interaction including the nonadiabatic corrections are discussed. Our results strongly suggest that the recent finding on irradiated MgB₂ samples can be naturally explained in terms of small E _F values associated with the σ-bands of the boron plane, sustaining therefore the hypothesis that MgB₂ is a nonadiabatic metal. Received 31 July 2002 / Received in final form 21 September 2002 Published online 31 December 2002     

Study of the NMR spectra of hydrogen isotopic analogs and estimation of the magnetic moment of the triton

Nuclear magnetic resonance spectra of the hydrogen isotopic analogs TH and TD on the triton resonance and HT, HD, and H₂ on the proton resonance in the gas phase have been studied. The triton and proton spectra from a single sample have been recorded simultaneously by using the inductivity of a common receiver LC circuit. The energies of the interaction between the magnetic moments of the nuclei of the hydrogen isotopic analogs have been determined: J _tp = 299.3(1) Hz, J _td = 45.5(1) Hz, and J _pd = 43.3(1) Hz. The ratio of the resonance frequencies of the HT molecule nuclei: F _t (TH)/F _p (HT) = 1.066693898(2), which is equal to the ratio of the magnetic moments of the nuclei in the bound state, has been obtained. If the value Δσ(TH) = 2.04 × 10⁻⁸ calculated previously is used for the difference in the screening of nuclei in the HT molecule, then the ratio of the magnetic moment of the triton to the magnetic moment of the proton is μ _t /μ _p = 1.066693920(2), where the statistical standard deviation of the data is given in the parentheses in the units of the last digit.     

Zero-Temperature Dynamics of ±J Spin Glasses¶and Related Models

We study zero-temperature, stochastic Ising models σ ^t on Z ^d with (disordered) nearest-neighbor couplings independently chosen from a distribution μ on R and an initial spin configuration chosen uniformly at random. Given d, call μ type ℐ (resp., type ℱ) if, for every x in Z ^d , σ _x ^t flips infinitely (resp., only finitely) many times as t→∞ (with probability one) – or else mixed type ℳ. Models of type ℒ and ℳ exhibit a zero-temperature version of “local non-equilibration”. For d=1, all types occur and the type of any μ is easy to determine. The main result of this paper is a proof that for d=2, ±J models (where μ=αδ _J +(1-α)δ_{- J} ) are type ℳ, unlike homogeneous models (type ℐ) or continuous (finite mean) μ's (type ℳ). We also prove that all other noncontinuous disordered systems are type ℳ for any d≥ 2. The ±J proof is noteworthy in that it is much less “local” than the other (simpler) proof. Homogeneous and ±J models for d≥ 3 remain an open problem. Received: 3 November 1999 / Accepted: 10 April 2000     

Excited state absorption and energy transfer losses in thulium doped fluoroindogallate glass

Thulium doped fluoroindogallate glass was characterized by means of excited state absorption experiment in the 0.95 to 1.55 μm spectral range. The three bands corresponding to the electronic transitions ³ F ₄→³ F ₂ (at 1.05 μm), ³ F ₄→³ F ₃ (at 1.125 μm), and ³ F ₄→³ H ₄ (at 1.45 μm) were observed. The energy transfer microscopic parameters for the reverse cross relaxation process ³ F ₄, ³ F ₄→³ H ₆, ³ H ₄ were calculated for different multipolar interaction mechanisms using the Kushida model, and it was verified that the probability of this process is 100 times lower than that of the direct ³ H ₄, ³ H ₆→³ F ₄, ³ F ₄ cross relaxation, responsible for the 1.8 μm emission pumping. PACS 78.20.-e; 78.55.Qr; 42.70.Hj; 42.55.Wd     

The Precise Measurement of the μ+ Lifetime

The lifetime of the positive muon (τ_μ ⁺) can be directly associated with the Fermi Coupling Constant (G _F ), which is one of the most basic parameters of the Standard Model. However, the current experimental accuracy of the τ_μ ⁺ is ∼30 ppm and it has not been improved for more than 15 years. We propose a new experiment for a pulsed muon facility such as RIKEN-RAL to measure the muon lifetime with multi-decay per one time window method. The advantage of our setup, no time window limitation, enables us to test the exponential decay law (EDL) in the long decay time region at the same time. The preliminary analysis set a new upperlimit for the EDL deviation in the muon decay. We accumulated ∼10¹⁰ muon decays and analysis is in progress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Synchrotron and annihilation channels for axion production in an external electromagnetic field

We consider axion formation processes in the synchrotron (e ⁻→e ⁻ a) and annihilation (e ⁻ e ⁺→a) channels in a constant crossed field F _μν F^μν=Fμν*F ^μν =0, which approximates constant fields of other configurations in the ultrarelativistic asymptotic limit. The probability and intensity of axion emission are obtained, and we analyze the energy and field asymptotics. A comparison with the characteristic neutrino channel yields the constraints on the axion mass and the energy scale for Peccei-Quinn symmetry breaking. Possible astrophysical applications are discussed. Zh. éksp. Teor. Fiz. 112, 25–31 (July 1997)     

Damping of zero sound in Luttinger liquids

We calculate the damping γ_q of collective density oscillations (zero sound) in a one-dimensional Fermi gas with dimensionless forward scattering interaction F and quadratic energy dispersion k² / 2 m at zero temperature. Using standard many-body perturbation theory, we obtain γ_q from the expansion of the inverse irreducible polarization to first order in the effective screened (RPA) interaction. For wave-vectors | q| /k_F ≪F (where k_F = m v_F is the Fermi wave-vector) we find to leading order γ_q ∝| q |³ /(v_F m²). On the other hand, for F ≪| q| /k_F most of the spectral weight is carried by the particle-hole continuum, which is distributed over a frequency interval of the order of q²/m. We also show that zero sound damping leads to a finite maximum proportional to |k - k_F | ^{-2 + 2 η} of the charge peak in the single-particle spectral function, where η is the anomalous dimension. Our prediction agrees with photoemission data for the blue bronze K_0.3MoO₃. We comment on other recent calculations of γ_q.     

Paramagnetic susceptibility of Bi-Mn alloys in the liquid state

The magnetic susceptibility of Bi_{100 − x} Mn _x (x = 5, 7.5, 10, 12.5, 15, 17.5, 20, 25) alloys is experimentally studied by the Faraday method in the temperature range T = 300−1200°C and the magnetic-field range B = 0.6−1.3 T. To calculate the electronic characteristics of the Bi-Mn alloys, the experimental results are approximated by the generalized Curie-Weiss law. The calculated parameters of the electronic structure of the alloys demonstrate that manganese is present in the melt in an ionic state with an effective magnetic moment μ_eff ≈ 5μ_B, all Bi-Mn alloys have negative paramagnetic temperatures (which indicate the antiferromagnetic character of the exchange between transition 3d element atoms), and the density of states near the Fermi level n(E _F) is low. Therefore, the Fermi level is outside the d band of manganese and its position is controlled by the sp band of bismuth.     

Pressure dependence of the superconducting state parameters of metallic glass superconductor Mg70Zn30

Explicit expressions have been derived for the volume dependence of electron-phonon coupling strength (λ) and the Coulomb pseudopotential (μ^*) considering the variation of Fermi momentum (κ _F) and Debye temperature (θ _D) with volume. Ashcroft’s model pseudopotential and RPA form of dielectric screening have been used for obtaining pressure dependence of transition temperature (T _C) and the logarithmic volume derivative (Φ) of the effective interaction strength (N ₀ V) for metallic glass superconductor Mg₇₀Zn₃₀. It has been observed that T _C of the metallic glass Mg₇₀Zn₃₀ decreases rapidly with increase of pressure and the superconducting phase disappears at about 30% decrease of volume, for which the μ^* curve shows a minimum and an elbow is formed in the Φ graph.     

Self-pulsing dynamics of ytterbium-doped fiber laser with pump-bypassed cavity

In addition to efficient continuous wave laser action at 1.06 μm (⁴ F _3/2→⁴ I _11/2 transition), with a slope efficiency 30% and 30 mW pump power threshold, this work presents a throughout study of the spectroscopic, and thermo-optical properties of a 2.0 mol % Nd³⁺ doped fluoroindogallate glass. The characterization of the sample using the pump probe experimental technique indicated the presence of two broad excited state absorption bands from 0.94 to 1.05 μm (⁴ F _3/2→² D _3/2, ² G _9/2, ⁴ G _11/2, ² K _15/2) and from 1.16 to 1.42 μm (⁴ F _3/2→⁴ G _9/2, ⁴ G _7/2, ² K _13/2), found not to interfere with the stimulated emission at 1.06 μm, with a peak cross section value σ_SE = 2.55×10^-21 cm² and a full width at half maximum of 22 nm. Thermal lens measurements were performed to quantify the Auger upconversion parameter (γ=(1.26±0.09)×10^-16 cm³/s) and evaluate the thermal loading in the sample in comparison to other commercial glasses. PACS 181.05.Kf; 78.45.+h; 42.70.Hj     

Electronic properties of orthorhombic LiGaS<Subscript>2</Subscript> and LiGaSe<Subscript>2</Subscript>

We report theoretical calculations of the band structure and density of states for orthorhombic LiGaS₂ (LGS) and LiGaSe₂ (LGSe). These calculations are based on the full potential linear augmented plane wave (FP-LAPW) method within a framework of density functional theory. Our calculations show that these crystals have similar band structures. The valence band maximum (VBM) and the conduction band minimum (CBM) are located at Γ, resulting in a direct energy band gap. The VBM is dominated by S/Se-p and Li-p states, while the CBM is dominated by Ga-s, S/Se-p and small contributions of Li-p and Ga-p. From the partial density of states we find that Li-p hybridizes with Li-s below the Fermi energy (E _F), while Li-s/p hybridizes with Ga-p below and above E _F. Also, we note that S/Se-p hybridizes with Ga-s below and above E _F.     

Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ≈ 3G₀ to G ≈ 1G₀ (G₀ = 2e/h²). Formation of a true one-atom point contact, with G ≈ 1G₀ and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 ≤ E-E_F ≤ 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear “gold molecule" between the electrodes, and a weak coupling of this “molecule" to the tips.     

<Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> suppression and <Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> spectra in RHIC and LHC energy collisions

In a hydrodynamic model, we have studied J/ψ production in Au+Au/Cu+Cu collisions at RHIC energy, GeV. At the initial time, J/ψ’s are randomly distributed in the fluid. As the fluid evolves in time, the free streaming J/ψ’s are dissolved if the local fluid temperature exceeds a threshold temperature T _J/ψ . Sequential melting of charmonium states (χ _c , ψ ^′ and J/ψ), with melting temperatures , T _J/ψ ≈2T _c and feed-down fraction F≈0.3, explains the PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions. J/ψ p _T spectra and the nuclear modification factor in Au+Au collisions are also well explained in the model. The model however overpredicts the centrality dependence of J/ψ suppression in Cu+Cu collisions by 20–30%. The J/ψ p _T spectra are underpredicted by 20–30%. The model predicts that in central Pb+Pb collisions at LHC energy,  GeV, J/ψ’s are suppressed by a factor of ∼10. The model predicted a J/ψ p _T distribution in Pb+Pb collisions at LHC is similar to that in Au+Au collisions at RHIC.     

Break junctions of the heavy-fermion antiferromagnet CeAl2

Mechanical-controllable break junctions of CeAl₂ were investigated down to 50 mK. The size of the contacts turns out to be the most important parameter. We analyze the junctions by applying Wexler's resistance formula, and derive the contact radius by comparing the T³-power law of the zero-bias resistance below T_N with that of the electrical resistivity. On the average the local residual resistivity ρ₀≈15 μΩcm amounts to about 5 times the bulk value, while the effective Fermi wave number k_F≈2 nm⁻¹ is considerably smaller than expected.     

Excitation cross sections of thulium atom transitions terminating at 4f13(2Fo)6s6p(3Po (7/2, J2) levels with J2 = 0, 1

The excitation of odd levels of the thulium atom, spontaneous transitions from which terminate at the 4f ¹³(² F ^o)6s6p(³ P ^o) (7/2, J ₂) levels with J ₂ = 0, 1, is studied by the method of extended crossing beams. Fifty four excitation cross sections are measured at an excitation electron energy of 50 eV. Seven optical excitation functions are recorded in the electron energy range of 0–200 eV.     

Sub-doppler laser cooling of thulium atoms in a magneto-optical trap

Sub-Doppler laser cooling in a magneto-optical trap for thulium atoms at a wavelength of 410.6 nm has been experimentally studied. Without any dedicated molasses period of sub-Doppler cooling, the cloud of 3 × 10⁶ atoms at a temperature of 25(5) μK was observed. The measured temperature is significantly lower than the Doppler limit of 240 μK for the cooling transition at 410.6 nm. The high efficiency of the sub-Doppler cooling process is due to a near-degeneracy of the Landé g-factors of the lower 4f ¹³6s ² (J = 7/2) and the upper 4f ¹²5d _3/26s ² (J = 9/2) cooling levels.     

The effect of Cu addition on the sintering process and superconductive properties of?μm-SiC-doped MgB2 bulks

Based on the phase identification and microstructural observation, it is found that the Cu addition in the μm-SiC-doped samples could depress the reaction between Mg and SiC and thus decrease the amount of both C substitution for B and the Mg₂Si nanoparticles. As a result, compared to the only μm-SiC-doped MgB₂ sample, the T _c of the Cu and μm-SiC multi-doped MgB₂ sample is improved and the J _c in high fields is deteriorated.     

Mean square number fluctuation for a fermion source and its dependence on neutrino mass for the universal cosmic neutrino background

Using the general formulation for obtaining chemical potentialμ of an ideal Fermi gas of particles at temperature T, with particle rest mass m₀ and average density 〈N〉/V, the dependence of the mean square number fluctuation 〈ΔN ²〉/V on the particle mass m₀ has been calculated explicitly. The numerical calculations are exact in all cases whether rest mass energym ₀c² is very large (non-relativistic case), very small (ultra-relativistic case) or of the same order as the thermal energy k_BT. Application of our results to the detection of the universal very low energy cosmic neutrino background (CNB), from any of the three species of neutrinos, shows that it is possible to estimate the neutrino mass of these species if from approximate experimental measurements of their momentum distribution one can extract, someday, not only the density 〈N _v〉/V but also the mean square fluctuation 〈Δ _v ² 〉/V. If at the present epoch, the universe is expanding much faster than thermalization rate for CNB, it is shown that our analysis leads to a scaled neutrino massm _v instead of the actual massm _0v .