Change in the chemical potential of niobium at transition to the superconducting state

A temperature-modulation technique was applied to investigate change in the chemical potential of niobium film at transition to the superconducting state. The first derivative of the chemical potential is continuous at T _c whereas the second derivative shows a jump. Received 10 April 2000     

Twisting of α-quartz tetrahedra at pressures near the transition to the amorphous state

It is found that the main contribution to the distortion of α-quartz tetrahedra at high pressures is their twisting. It is shown that torsional vibrational modes lead to an instability that gives rise to amorphization of the structure. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 431–435 (25 March 1999)     

Two-dimensional superconductive state near the intermediate plane

The superconductive layer near a plane surface arises due to the electronic states (Tamm levels) localized on the surface at temperature T₀, higher than the bulk critical temperature.     

Disorder-induced inhomogeneities of the superconducting state close to the superconductor-insulator transition

Scanning tunneling spectroscopy at very low temperatures on homogeneously disordered superconducting titanium nitride thin films reveals strong spatial inhomogeneities of the superconducting gap Delta in the density of states. Upon increasing disorder, we observe suppression of the superconducting critical temperature Tc towards zero, enhancement of spatial fluctuations in Delta, and growth of the Delta/Tc ratio. These findings suggest that local superconductivity survives across the disorder-driven superconductor-insulator transition.     

Studies of the surface magnetic state of the Sr-M hexagonal ferrites near the phase transition at the Curie temperature

A study is reported of the temperature dependences of the hyperfine (HF) interaction parameters in a ～200-nm thick surface layer and in the bulk of macroscopic hexagonal ferrite crystals of the Sr-M type (SrFe₁₂O₁₉ and SrFe_10.2Al_1.8O₁₉). The method used for the measurements is Mössbauer spectroscopy with simultaneous detection of gamma quanta, characteristic x-ray emission, and electrons, which permits direct comparison of the HF parameters in the bulk and the near-surface layers of a sample. As follows from the experimentally determined temperature dependences of the effective magnetic fields, the fields at the nuclei of the iron ions located in a ～200-nm thick near-surface layer decrease with increasing temperature faster than those of the ions in the bulk. The transition to paramagnetic state in a ～200-nm thick surface layer was found to occur 3° below the bulk Curie temperature. This offers the first experimental evidence for the transition to paramagnetic state in a surface layer of macroscopic ferromagnets to take place below the Curie temperature T _c for the bulk of the crystal. It has been established that the transition temperature T _c (L) of a thin layer at a depth L from the surface of a crystal increases as one moves away from the surface to reach T _c at the inner boundary of the surface layer called critical. In the vicinity of T _c one observes a nonuniform state, with the crystal being magnetically ordered in the bulk but disordered on the surface. The experimental data obtained were used to construct a phase diagram of surface and bulk states for macroscopic magnets near the Curie (or Néel) temperature.     

Fate of the superconducting ground state on the Moyal plane

It is known that Berry curvature of the band structure of certain crystals can lead to effective noncommutativity between spatial coordinates. Using the techniques of twisted quantum field theory, we investigate the question of the formation of a paired state of twisted fermions in such a system. We find that to leading order in the noncommutativity parameter, the gap between the non-interacting ground state and the paired state is smaller compared to its commutative counterpart. This suggests that BCS type superconductivity, if present in such systems, is more fragile and easier to disrupt.     

The superconducting transition and the behavior of the ac susceptibility

The ac susceptibility behavior of superconductors close to the superconducting transition temperature is explained on the basis of the BCS theory. Previous approaches to this problem were based on the average conductivity model, the London and the two-fluid models. It is shown that they are not able to explain the observed χPrime;-peak at T_c and are basically, not adequate for the treatment of such a problem.     

Superconductivity of the twinning plane

Lately superconductivity of a new object, twinning plane (TP) of metal crystals, has been discovered and investigated (Shaikin and Khlustikov 1981; Khlustikov and Khaikin 1982; Buzdin and Khlustikov 1984; Khlustikov and Moskvin 1985). The only one monolayer of atoms of the samples is in crystallographically shaped position, which proves TP to be unique. The monolayer of atoms generating TP is a two-dimensional crystal. The two-dimensional crystal is supposed to have its own two-dimensional electrons and phonons (Khaikin and Khlustikov 1981). Superconductivity of twinning plane (STP) is the first effect discovered which demonstrates unusual properties in such two-dimensional systems. STP is essentially different from three-dimensional superconductivity and is observed in a number of metals, such as In, Nb, Re, Sn and Tl. The fact that STP emerges at higher temperatures than three-dimensional superconductivity is a case of special interest to this problem. Superconductors of first type (Sn) and of second type (Nb) have been studied in detail. The measurements of phase (H, T) diagram STP in these metals have been carried out. STP is shown to appear in Sn by the first type phase transition (Khlustikov and Khaikin 1982; Buzdin and Khlustikov 1984), and in Nb, most likely, by the second type phase transition. Topological phase transition of Berezinsky-Kosterlitz-Thouless (Khlustikov and Moskvin 1985) transition type was discovered in twins of Nb.     

Scaling function near a transition to an insulator state

Analysis of experimental results yields a scaling function of the form β(g)=121/g near the metal-insulator transition in three-dimensional systems. In two-dimensional electronic systems demonstrating a transition to an insulator state, the same relation holds for the function νβ, where ν is the critical exponent characterizing the divergence of the correlation length. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 11, 807–811 (10 December 1998)     

Energy spectrum of transition metals in the superconducting state

It is shown that in pure transition metals even a small probability of interband Cooper pairing leads to foundation of an energy spectrum consisting of two branches, each of which possesses several maxima and minima.Translated from Izvestiya VUZ. Fizika, No. 6, pp. 20–24, June, 1973.     

First-order superconducting transition near a ferromagnetic quantum critical point

We address the issue of how triplet superconductivity emerges in an electronic system near a ferromagnetic quantum critical point (FQCP). Previous studies found that the superconducting transition is of second order, and T(c) is strongly reduced near the FQCP due to pair-breaking effects from thermal spin fluctuations. In contrast, we demonstrate that near the FQCP, the system avoids pair-breaking effects by undergoing a first order transition at a much larger T(c). A second order superconducting transition emerges only at some distance from the FQCP.     

Phenomenological approach to the non-spin-wave behavior of cuprate magnetic susceptibility in the superconducting state

Experimental data on the non-spin-wave behavior of the cuprate magnetic susceptibility χ(q, ω) in the superconducting state are reconstructed on the basis of a spherically symmetric self-consistent approach within the framework of the frustrated Heisenberg model. For the Green’s function of the spin-wave excitations, a polarization operator resonant with the superconducting state is introduced phenomenologically. This gives rise to an additional (compared to the normal state) branch of the incommensurate peaks and to a pronounced non-spin-wave behavior of the susceptibility χ(q, ω) in the frequency range near 80 meV (the so-called “dark region”). The renormalization of the real part of the polarization operator plays a crucial role in the theory.     

Analysis of bolometer operation near the superconducting transition edge using microwave readout

The results of pioneering experiments on using a new method of microwave recording the bolometric response from a metal film near its superconducting transition edge are analyzed. Experiments are carried out at 4.5 K using a thin-film niobium absorber with a critical temperature of 6.7 K. The validity of the adopted electrodynamic model is confirmed. The chip contains a miniature antenna tuned to approximately 600 GHz and integrated into a planar resonator with a frequency of 6 GHz, which is weakly coupled with a pump line. Based on the experimental and model data, the presence of electrothermal feedback is shown, stability criteria are found, and I–V characteristics at microwaves are obtained. Bulk experimental samples with an absorber measuring 5 μm × 2.5 μm × 15 nm are fabricated with optical photolithography. At an amplifier channel noise of 3 K, the optical sensitivity of a prototype receiver can be expected at a level of 10^?15 W/Hz^1/2.     

Eigenfunction fractality and pseudogap state near the superconductor-insulator transition

We develop a theory of a pseudogap state appearing near the superconductor-insulator (SI) transition in strongly disordered metals with an attractive interaction. We show that such an interaction combined with the fractal nature of the single-particle wave functions near the mobility edge leads to an anomalously large single-particle gap in the superconducting state near SI transition that persists and even increases in the insulating state long after the superconductivity is destroyed. We give analytic expressions for the value of the pseudogap in terms of the inverse participation ratio of the corresponding localization problem.