Exchange and correlation in finite-temperature TDDFT

We discuss the finite-temperature generalization of time-dependent density functional theory (TDDFT). The theory is directly analogous to that at temperature T = 0. For example, the finite-T TDDFT exchange-correlation kernel f_xc(T, n) in the local density approximation can again be expressed as a density derivative of the exchange correlation potential f_xc(T, n) = [?v_xc(T, n)∕?n]δ(r ? r′), where n = N∕V is the electron number density. An approximation for the kernel f_xc(T, n) is obtained from the finite-T generalization of the retarded cumulant expansion applied to the homogeneous electron gas. Results for f_xc and the loss function are presented for a wide range of temperatures and densities including the warm dense matter regime, where T ≈ T_F, the electron degeneracy temperature. The theory also permits a physical interpretation of the exchange and correlation contributions to the theory.     

The anomalous Knight shift in beryllium

We calculate the Knight shift (K) in beryllium and magnesium by using the pseudopotential formalism and degenerate perturbation theory. We have included the effects of electron-electron interaction and spin-orbit interaction. We show that K_s, the spin contribution to K, is not linearly related to the spin susceptibility (χ_s) when spin-orbit effects are included. This non-linearity between K_s and χ_s is the origin of the negative Knight shift in beryllium.     

Many body theory of magnetic susceptibility of bloch electrons

We present a theory of the total susceptibility (χ) of solids using a finite temperature Green's function formalism. The many body effects on orbital (χ_o), spin (χ_s) and spin-orbit (χ_s-o) contributions to χ are calculated. Our expression for χ_s is equivalent to the earlier results but with the g-factor replaced by the effective g-factor. An important aspect of our work is the analysis of many body effects on χ_s?o.     

Pseudopotential theory of spin susceptibility of metals

We derive an expression for the exchange enhanced spin susceptibility (χ_s) of metals by using the pseudopotential formalism and degenerate perturbation theory. The advantage of our theory over the recent spin-density-functional formalism is that we have included spin-orbit effects. Our results for χ_s of alkali metals agree with the experimental results.     

Higher order QCD corrections to an exclusive two-photon process

We calculate the next-to-leading order QCD corrections in the $\text{MS}$ scheme to the coefficient functions in an operator product expansion of the amplitude T(q², p²) for the process $\text{γ}{}^1\text{(q) + γ}{}^1\text{(p) →}\text{helicity-zero}$, flavour non-singlet meson in which ?q² is large and ?p² ? 0. For an asymptotic wave function the complete O(α_s) correction for a pseudoscalar meson is about 16% for p² = 0 and α_s = 0.3; most of this correction can be removed by using a modified evolution equation for the wave function, leaving a correction of about 7%. For large p² the complete O(α_s) correction for a pseudoscalar meson is about 10%. We discuss how our results can be combined with similar calculations for the pion form factor F_π(Q²) to give a prediction: F_π(Q²) = Aα_s(Q²)T_π²(q², 0)(1 + Bα_s) that is independent of the as yet unknown two-loop anomalous dimension matrix.     

Optical pumping of short lived Β-radioactive isotopes and the magnetic moment of37K

²¹Na (T _1/2=23 sec) and³⁷K(T _1/2=1.25 sec) have been produced in gas targets by (d, n) reactions and polarized by means of optical pumping or spin exchange scattering with optically pumped⁸⁷Rb. An asymmetry up to 3% in theΒ-decay of the polarized nuclei was found, which served to detect rf transitions between hfs levels of the atomic ground states of²¹Na and³⁷K.δF=0 andδF=1 resonances have been recorded, yielding spin, hfs separation and magnetic moment, especiallyI(³⁷K)=3/2,δW(³⁷K)=240.266 (3) Mc/sec andΜ _I(³⁷K)=0.02033(6) nm (diamagnetically corrected). For²¹Na earlier rf-spectroscopic data have been confirmed.     

Manifestation of vortex depinning transition in nonlinear current-voltage characteristics of polycrystalline superconductor Y1−xPrxBa2Cu3O7−δ

We present our recent results on the temperature dependence of current-voltage characteristics for polycrystalline Y_1−xPr_xBa₂Cu₃O_7−δ superconductors with x=0.0, 0.1 and 0.3. The experimental results are found to be reasonably well fitted for all samples by a power like law of the form V=R(I−Ic)^a(T). Here, we assume that a(T)=1+Φ₀IC(T)/2πkBT and IC(T)=IC(0)(1−T/TC)^3/2 for the temperature dependences of the power exponent and critical current, respectively. According to the theoretical interpretation of the obtained results, nonlinear deviation of our current-voltage characteristics curves from Ohmic behavior (with a(TC)=1) below TC is attributed to the manifestation of dissipation processes. They have a characteristic temperature Tp defined via the power exponent as a(Tp)=2 and are related to the current induced depinning of Abrikosov vortices. Both TC(x) and Tp(x) are found to decrease with an increase of Pr concentration x reflecting deterioration of the superconducting properties of the doped samples.     

Low field63Cu NMR study of indirect nuclear spin-spin coupling in YBa2Cu3O6.9. A new approach to probe strong correlation effects in high-T c superconductors

The authors carried out a low field (6.3 kOe) NMR measurement of the Gaussian component of⁶³Cu nuclear spin-spin relaxation rate 1/T _2G at the planar Cu site in a high-T _c superconductor YBa₂Cu₃O_6.9 (T _c=92 K). They demonstrate that the results provide quantitative information concerning the static spin susceptibility χ′(q) at non-zero wave vector q. The detailed analysis of the data assuming a model susceptibility peaked at the corner of first Brillouin zone q=Q (Q=(π/a, θ/a),a: lattice constant) shows that χ′(q) satisfies a Curie-Weiss law around q=Q. Stoner enhancement over the calculated Lindhard function at q=Q is estimated to be of the order of ～10–20. They also demonstrate that combined analysis of 1/T ₁ and 1/T _2G allows one to judge whether anomalous shift of the low frequency spectral weight of χ″(Q,ω) to higher frequencies (i.e.pseudo gap) exists or not. Application of the method to YBa₂Cu₃O_6.9 revealed no appreciable pseudo gap in the material.     

Thermal destruction of antiferromagnetic Fe-Fe exchange bonds and mechanism of the transition to the spin-glass state in (Fe0.65Ni0.35)1−x Mnx systems with antiferromagnetic competing exchange interactions

The thermal evolution of the competition between the ferro-and antiferromagnetic exchange interactions in (Fe_0.65Ni_0.35)_1?x Mn_x alloys, which display different magnetic properties, depending on composition and temperature, is investigated. The distribution functions of the magnetic hyperfine fields P(B _hf) for ⁵⁷Fe are determined by Mössbauer spectroscopy in the temperature range 5–300 K for the alloys with x=0, 0.024, 0.082, 0.136, 0.195, and 0.252. The temperature dependence of the integrated intensity I _s (T) is analyzed for the low-and high-field portions of P(B _hf). The features found in the behavior of I _s (T) are interpreted as results of variation of the ratio between the competing exchange interactions of different signs as a result of the thermal destruction of antiferromagnetic Fe-Fe exchange bonds. It is shown that the changes in the spin structure in the low-temperature range are due to the thermal destruction of Fe-Fe exchange bonds. One of the consequences of this destruction is “reentrance” (an increase in the hyperfine field with increasing temperature for some of the Fe atoms). The relationship between the thermal destruction of Fe-Fe exchange bonds and the magnetic transitions of the Fe-Ni-Mn system to the spin-glass state is considered.     

Evolution of spin susceptibility from the Pauli to Curie-Weiss type in strongly correlated Fermi systems

The magnetic properties of strongly correlated Fermi systems are studied within the framework of the fermioncondensation model—phase transition associated with the rearrangement of the Landau quasiparticle distribution, resulting in the appearance of a plateau at T=0 exactly in the Fermi surface of the single-particle excitation spectrum. It is shown that the Curie-Weiss term ～T^?1 appears in the expression for the spin susceptibility χ_ac(T) of the system after the transition point at finite temperatures. The behavior of χ_ac(T, H) as a function of temperature and static magnetic field H in the region where the critical fermion-condensation temperature T _f is close to zero is discussed. The results are compared with the available experimental data.     

Isotopic shifts and the hyperfine structure of the samarium spectral lines at 672 and 686 nm

Spectra of saturated absorption of Sm atomic vapor from the ground state ⁷ F ₀ and the first even metastable level ⁷ F ₁, ε′=292.58 cm^?1 of the 4f ⁶6s ² configuration to the odd level 4_f ⁶(⁷ F)6s6p(³ P ^o)⁹ F ₁ ^o , ε,=14863.85 cm^?1 were recorded. The lines of the isotopic series were identified, and the hyperfine structure of lines in the spectra of isotopes with a nonzero nuclear spin was determined. The relative isotopic shifts and the hyperfine splitting of the even level 4f ⁶6s ²(⁷ F ₁) were determined.     

Relaxation phenomena due tos—d exchange interaction of dilute iron in molybdenum

It is demonstrated that the localized magnetic moments of Fe in Mo which undergo spin compensation at low temperatures vias—d exchange interaction (Kondo-effect) also exhibit, at temperaturesT>T _k , relaxation phenomena in the Mössbauer spectra due to the same exchange interaction. Utilizing the Korringa relation, the product of the exchange parameterJ and the density of statesp(? _F is estimated to be ¦Jp(? _F )¦=0.008. At low temperatures, however, deviations from this relation have been observed. The relaxation rate becomes temperature independent and decreases with increasing external magnetic field.     

On the susceptibility exponent of random anisotropy magnets

The temperature dependence of the non-linear susceptibility ≈₂(T) of random anisotropy magnets in the Ising limit (speromagnets) is calculated for temperatures above the freezing temperature T_f within the framework of the correlated molecular field theory. For the effective susceptibility exponent λ_s(T) = (T?T_f)≈₂d^-1_≈2/dT a non-monotonic temperature dependence is found as for the case of spin glasses. This must be taken into account in order to obtain reliable values for the critical susceptibility exponent from experimental data.     

Electron-electron interaction and instabilities in one-dimensional metals

The possible instabilities of a 1-dimensional itinerant electron gas are discussed, assuming electron-electron interaction to play the dominant role. As is well known, in the RPA, a 1-dimensional metal is prone to spin density wave (SDW), charge density wave (CDW) and Cooper pair (CP) instabilities. The spin channel decomposition of the irreducible scattering amplitude I is made and the spin channel projections are evaluated in terms of the matrix elements of bare electron-electron interactionV(x) for momenta of interest. It is found that if the bare electron interactionV(x) is repulsive and decreases monotonically with separation, only the SDW instability will occur. If the small separation (x?(2k _F )^?1) part of the interaction is greatly reduced or is made attractive,V(x) is non-monotonic,V _q (q?2k _F ) is negative, and a CDW instability is preferred. A CP instability is possible if the electron interaction is attractive,i.e., if [V _q (0<q<k _F )+V _q (q?2k _F )]<0. The above RPA results serve only as rough indicators, since in general there are important two-electron configurations with two-electron momentum close to zero and with electron hole momentum close to 2k _F , an example being the near Fermi energy configurationk ₁?k _F ,k ₂??k _F ,k ₃??k _F k ₄?k _F . Therefore as pointed out first by Bychkov, Gorkov and Dzhyaloshinskii (BGD), cross channel coupling is especially significant. It is shown that the cross channel coupling is constructive is some cases,eg., exchange of CD fluctuations leads to an effective electron-electron spin singlet attraction and vice-versa. A formalism for studying such effects is set up, and the particular example mentioned above is discussed. An RPA-like approximation is made for the form of the reducible singlet electron hole scattering amplitudeγ _s ^d and the resulting induced Cooper pair attraction is calculated to be $$\begin{gathered} [I_s ^e ]_{ind.} \rho _{{}^\varepsilon F} = [ln(\lambda \beta \omega _c )]^{ - 1} ln\{ [1 + 2\pi ^{ - 1} ln(\lambda \beta \omega _c )^2 ]/ \hfill \\ 1 + [8\pi ^{ - 1} \gamma _s ^d (q = 2k_F )^{ - 1} )^2 ]\} \hfill \\ \end{gathered} $$ where λ=1.14,β=(k _B T)^?1 andω ₀ is an electronic energy cut-off ～ε _F . The induced electron hole attraction due to the exchange of virtual Cooper pairs has a similar expression, but with a factor of (1/4) and withγ _s ^e (q=0) replacingγ _s ^d (q=2k _F ). The induced Cooper pair attraction is seen to be quite large over a broad range of temperatures close to but aboveT _CDW [i.e., aboveT such thatγ _s ^d (q=2k _F )^?1=0]. There is no requirement thatγ _s ^d (q=2k _F ) andγ _s ^e (q=0) become singular at the same temperature, as found by BGD. The BGD prediction is seen to arise from the neglect of real particle hole and particle-particle excitations while calculatingγ _s ^d andγ _s ^e . The effect of impurities, of electron-phonon coupling, of interchain coupling and of interaction between thermal order parameter fluctuations is discussed. The results are then applied to a discussion of the properties of TTF-TCNQ, where it is suggested that a CDW instability occurs becauseV _q (q=2k _F )<0,i.e., because the small separation electron repulsion is strongly reduced by the highly polarizable TTF. Because of substantial interchain coupling, the bulk CDW instability occurs close to the RPA instability temperature. The giant conductivity observed by Colemanet al is attributed to superconductive fluctuations in a 1-dimensional system with large mean field superconductive transition temperatureT _CP ^MF of order 300°K. Such a largeT _CP ^MF is shown to result from the induced Cooper pair attraction due to CD fluctuation exchange.     

On the anomaly in the magnetic susceptibility of iron monosilicide

The temperature variation of the paramagnetic spin susceptibility χ(T) of FeSi is calculated by taking into account the effect of spin fluctuations in the gaussian approximation. It is suggested that the anomalous increase in χ(T) may be related to a discontinuous change in χ(T) from a low fluctuating spin state to a high fluctuating spin state.     

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

DSC and complex impedance studies of the protonic conductor (NH₄)₄H₂(SeO₄)₃, which undergoes a superionic phase transition of first order at T_s = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔC_p of the crystal are proportional: d(lg σ)/dT = XΔC_p/RT_s + const, as found for MAg₄I₅ crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at T_s, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH₄)₄H₂(SeO₄)₃ crystal above T_s.     

Non-equilibrium structure factor for the disordering of adsorbed monolayers

We consider the non-equilibrium, time-dependent elastic-scattering structure factor S(q,t), for the disordering of an ordered overlayer, initially in equilibrium at temperature T_I and characterized by the structure factor S(q,0)=x(q,T_I, upon a sudden increase in temperature T_I→T_F at constant coverage, such that the adsorbates equilibrate at T_F in a disordered phase. The initial decay of a peak in x(q,T_I) proceeds exponentially in time, $\text{exp}\text{(−}\text{t}\text{τ}{}_{\mathrm{q}}\text{)}$, where τ_q is a wavevector-dependent lifetime, before it crosses over to a power-law, t⁻¹ decay. When x(q,T_I) is peaked at the boundaries of the Brillouin zone (BZ), the peak approximately maintains its shape in q-space as it decays exponentially. Except near the center of the BZ, after the peak has decayed sufficiently, the dependence of S(q,t) on q is as though the spins quasi-equilibrate to the equilibrium structure factor associated with T_F, x(q,T_F), in that the ratio $\text{S(q,t)}\text{x(q,T}{}_{\mathrm{<mtext>F</mtext>}}\text{)}$ is independent of q, is dependent on time, approaching unity as t⁻¹ for large t. For systems exhibiting an initial peak for q ≈ 0, the peak decays exponentially but does not preserve its shape, since τ_q strongly depends on q, diverging as q⁻² for q→0. For these systems too, away from the center of the BZ, $\text{S}\text{(q,t)}\text{x(q,T}{}_{\mathrm{<mtext>F</mtext>}}\text{)}$ rapidly evolves to a slowly decaying function of $\text{t}\text{t}{}_{\mathrm{w}}$, independent of q. In this case, however, the characteristic time scale, t_w, is anomalously long, proportional to ξ², where ξ is the correlation length associated with the initial state. This behavior of t_w can be related to the random walk of domain boundaries.     

Observation of the strongly anomalous temperature and composition dependence of (δp/δT)s in liquid Li/Pb alloys

(δp/δT)_s which is proportional to the specific heat has been measured between the liquidus temperature and 1250 K for the whole concentration range of liquid Li/Pb alloys. For all concentrations it shows a positive deviation from the values expected for an ideal solution and it exhibits a narrow peak close to the composition Li₄Pb. The variation of T(δp/δT)_s with T is linear only for pure Pb. At 0.2 < x_Li < 0.6 a fall-off after melting is followed by a broad maximum at about 1000 K.