On the nature of the phase transition in the KDP systems

Using the Landau theory of phase transitions it has been shown that for a second order phase transition Ω/k_BT_c ? 0.01. and its isomorphs 4Ω/J¹ ? 1 and for a first order transition Ω/k_BT_c ? 0.01.     

Liquid Madelung energy and Schottky defect energy related to liquid structure and melting temperature for alkali halides

Motivated by the work of Reiss et al in which the melting temperature T_m of alkali halides is correlated with Coulomb energy, we consider the cohesive energy W of ionic melts and Schottky defect energy E_s in the hot crystal, relative to the thermal energy k_BT_m It is shown here that W is accurately approximated by the liquid Madelung energy and hence that Wk_BT_m relates to the charge-charge direct correlation function c_QQ(r) at r = 0 The existence of a “Madelung constant” for the liquid at T_m is thereby demonstrated through the alkali halide series An estimate of the ratio E_s/k_BT_m then considered, the basic additional ingredient being argued to be the static dielectric constant of the solid The BarrDawson-Lidiard empirical correlation between E_s, and k_BT_m can be understood in this way.     

Photoemission study of electronic structure evolution across the metal-insulator transition of heavily B-doped diamond

We studied the electronic structure evolution of heavily B-doped diamond films across the metal-insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (E_F) up to ∼5k_BT can be observed (k_B is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near E_F. Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of E_F, can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT.     

Negative-ion formation from surface scattering at finite temperatures

Temperature effects on negative-ion formation in positive-ion-surface scattering are studied within the framework of the time-dependent Anderson-Newns model. It is shown that the negative-ion formation is significantly enhanced at finite temperature T, provided k_BT is not less than the Anderson correlation energy U, where k_B is the Boltzmann constant. In the transient region (femtosecond timescale), temperature effects are, however, masked by large energy fluctuations.     

A nonlinear transport device with no intrinsic threshold

A ballistic rectifier, based on guidance of carriers by a triangular antidot, is shown to be both experimentally and theoretically capable of operating for weak signals. At T = 4.2 K, we find that the ballistic rectifier works when the input voltage is as low as 0.5 mV, the same order ofk_BT. Based on an extended Landauer–Büttiker formula for nonlinear transport, we show that even when the input signal is much smaller thank_BT, temperature has no obvious influence on the rectification effect.     

Thermodynamic properties of small extended Hubbard rings

We have obtained the exact numerical results of the specific heat, the susceptibility and the correlation functions L₀(T) and L₁(T) for finite extended Hubbard rings at the large U limit. It is shown that the nearest neighbor interactions favor the ferromagnetic ordering. When the number of electrons is less than the number of sites, the electron hopping results in itinerant magnetism and washes out the high temperature peak (around T = U/k_B) in the specific heat. In all cases, the behaviors of L₀(T) and L₁(T) are consistent with the characteristic features of the specific heat and the susceptibility. Thd exact results are used to test the accuracy of the Roth's decoupling scheme.     

Self-assemblies of magnetic nanoparticles and di-block copolymers: Magnetic micelles and vesicles

Magnetic nanocomposites are obtained by the self-assembly in water of polypeptide-based di-block copolymers polybutadiene-b-poly(glutamic acid) combined with hydrophobic γ-Fe₂O₃ nanoparticles. These hybrid supramolecular objects are either—(3D) spherical micelles filled with a hydrophobic ferrofluid at a concentration as high as 45 vol% or—hollow vesicles with a (2D) magnetic membrane. In this last case, the organic amphiphile copolymers are able to confine the hydrophobic nanoparticles within the thin layer of polybutadiene blocks. We probe these objects by atomic force microscopy, by small-angle neutron scattering (SANS) and by light scattering. Furthermore, anisotropic SANS data bring the experimental evidence of the capability to modify the shape of the mineralized membranes in response to a magnetic field intensity as low as 290 G.     

Temperature dependence of EPR linewidth in one-dimensional magnets: A quasi-classical approach

An analytical expression for the EPR linewidth of an exchange-coupled 1/2-spin chain originating from uniform Dzyaloshinskii-Moriya interaction is derived using a quasi-classical approach. The expression successfully reproduces the results obtained by numerical quantum mechanical calculations based on Green’s function method at T > 2J/k _B.     

New exact approach to models of diffusion in one-dimensional random systems and optimal network kinetics in spin glass models

It is shown that in one-dimensional stochastic models with gaussian random energy levels along a quantum reaction coordinate the dominant, rate-determining time-scale does not follow the conventional Arrhenius law, but rather has a much stronger temperature dependence, of the form τ ～ exp[(B/k_BT)²], where B is proportional to the width of the energy distribution. The new activation law can be ascribed to the large number of energy barriers of varying heights which exist in the random structure, as distinct from the conventional case of a single barrier, leading to the Arrhenius form τ ～ T^p × exp(A/k_BT). In systems with random structure and configuration space which are not strictly one-dimensional it is discussed if the thermal energy bias of detailed balance may lead to a kinetics that is essentially restricted to an energetically optimal network at low temperatures, thus leading to an essentially one-dimensional diffusion. Several recent studies of spin glass models appear to support the relevance of this principle, and include the observation of the new activation law in Monte Carlo experiments.     

Hadron structure in the ladder model (II)

The (flavor non-singlet) probability Φ(k) to find a far-off-shell quark in a hadron is obtained in the renormalization group improved ladder model for QCD in the space-like axial gauge in the region k_T²??2k·P, extending an earlier result for the region k_T²≈?2k·P. The resulting Drell-Yan cross section at measured Q_T agrees in the appropriate limit with that given by Parisi and Petronzio (and disagrees with the DDT form). By using a soft photon method in an abelian gauge theory, I argue that ladder diagrams with strong ordering of gluon q· P's in fact dominate Φ(k) in the high-energy limit considered.     

Accounting for the energies and entropies of kinesin’s catalytic cycle

When the processive motor protein kinesin walks along the biopolymer microtubule it can occasionally make a backward step. Recent single molecule experiments on moving kinesin have revealed that the forward-to-backward step ratio decreases exponentially with the load force. Carter and Cross (Nature 435, 308-312, 2005) found that this ratio tightly followed 802 × exp[−0.95F], where F is the load force in piconewtons. A straightforward analysis of a Brownian step leads to L/(2k _B T) as the factor in front of the load force, where L is the 8 nm stepsize, k _B is the Boltzmann constant, and T is the temperature. The factor L/(2k _B T) does indeed equal 0.95 pN⁻¹. The same analysis shows how the 802 prefactor derives from the power stroke energy G as exp[G/(2k _B T)]. There are indications that the power stroke derives from the entropically driven coiling of the 30 amino acid neck linker that connects the two kinesin heads. This idea is examined and consequences are deduced.     

Valence inhomogeneities in intermediate-valent Eu compounds

In each of the intermediate valent compounds EuNi₂P₂, EuPd₂Si₂, and EuPd₆B₄, the line width of the ¹⁵¹Eu-Mössbauer resonance attains a maximum when its isomer shift varies most strongly with temperature. We associate this anomalous line broadening with a distribution of the local Eu valence, which may be described by an inhomogeneous width, Δ, of the interconfigurational excitation energy, E_exc, in addition to the homogeneous fluctuation width, k_BT_f. Applying this model to the analysis of isomer shift, line width, and magnetic susceptibility, the model parameters Δ, E_exc, and k_BT_f are obtained. Δ is found to attain maximal values at temperatures where E_exc varies most steeply with temperature.     

Decay of a metastable state: Comparison between the transition state theory and the generalized Kramers model

The activation decay of a metastable state in a dissipative system is considered in the frame of the multidimensional transition state theory. This theory is shown to be equivalent to the problem of the escape of a brownian particle over a potential barrier, when the influence of the medium is described by the nonwhite gaussian random process (the generalized Kramers model). The generalization of the transition state theory is given for the case of high temperatures if k_BT > ?Ω'₀/2π, where Ω'₀ is the “frequency” of the potential top.     

Classical modelization of symmetry effects in the dense high-temperature electron gas

A Kramerslike classical pseudopotential which includes Diffraction and Symmetry effects is worked out for k_BT?1 Ry.     

Theoretical study of the susceptibility of nearly one-dimensional VF2

The paramagnetic susceptibility of a nearly one-dimensional helical magnet VF₂, of the tetragonal body-centered lattice, is calculated on the assumption of classical spins which form linear chains along the c-axis, with strong intra-chain and weak inter-chain exchange coupling. The partition function and the susceptibility are expanded in powers of J′/k_BT, J′ being the inter-chain coupling constant. The coefficients of the expansion are expressible in terms of spin correlation functions for independent linear chains. The susceptibility up to the 3rd power of independent linear chains. The susceptibility up to the 3rd power of J′/k_BT shows, as a function of temperature, a pronounced broad maximum as observed. The specific heat calculated with classical spins fails to reproduce the observation.     

Hexagonal spin structure of A-phase in MnSi: Densely packed skyrmion quasiparticles or two-dimensionally modulated spin superlattice?

We have studied in detail the A-phase region in the field-temperature (H-T) phase diagram of the cubic heli-magnet MnSi using small angle neutron diffraction. The A-phase revealed itself as a two-dimensional hexagonal pattern of Bragg spots with k _{h(1, 2, 3)} ⊥ H. The directions and magnitudes of the wave vectors k _{h(1, 2, 3)} are well preserved over the whole crystal of the size of 100 mm³, but in the small room of the (H-T) phase diagram just below T _c = 29 K. The droplets of the orientationally disordered, presumably hexagonal, spin structure with k _h ⊥ H are observed in the wide range beyond the A-phase boundaries in the field range from B _T1 ≈ 0.1 T to B _T2 ≈ 0.25 T at temperatures down to 15 K. No melting of these droplets into individual randomly located skyrmions is observed for all temperatures and magnetic fields. The wave vector of two-dimensional modulations k _h is equal to the wave vector of the cone phase k _c . We conclude that observable is a two dimensionally modulated hexagonal spin superlattice built on the same competition of interactions (ferromagnetic exchange and Dzyaloshinskii-Moriya interactions) similar to a case of one-dimensionally modulated simple spin spiral.     

The superconducting energy gap measured by tunneling into quench-condensed germanium-gold

Ge_0.5Au_0.5 films, quenched on a substrate below 2K are superconducting with T_c = 3.10 ± 0.12K, and 2Δ(0)k_BT_c= 3.75 ± 0.1. No phonon structure is visible in the second derivative of the tunnel current.     

Investigation of LiFeAs by means of “break-junction” technique

In our tunneling investigation using Andreev superconductor-normal metal-superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap ??_L = (2.5?C3.4) meV and the small gap ??_S = (0.9?C1) meV at T = 4.2 K for the T _C ^local ?? (10.5?C14) K (the contact area critical temperature which deviation causes the variation of ??_L). The BCS-ratio is found to be 2??_L/k _B T _C = 4.6?C5.6, whereas 2??_S/k _B T _C ? 3.52 results from induced superconductivity in the bands with the small gap.     

Strong short range magnetic order in ferromagnetic transition metals above Tc: A theoretical explanation

A theory for the existence of strong short range magnetic order in itinerant magnetism above T_c is presented. We derive a characteristic lengthscale for short range order, given by the square root of bandwidth over k_BT_c. This is of order 20 Å in the ferromagnetic transition metals.