Study of the heavy-fermion compound CeRu<Subscript>2</Subscript>Si<Subscript>2</Subscript> by the small-angle neutron scattering method

In order to directly observe neutron scattering by heavy fermion quasiparticles at low temperatures, a CeRu₂Si₂ single crystal has been studied by the small-angle neutron scattering method. In the experiment, neutron scattering is observed at T = 0.85 K for momentum transfers q ≤ 0.04 Å^?1, which is treated as the orbital component of magnetic scattering by heavy fermion quasiparticles. It has been found that the application of a magnetic field H = 1 T leads to both an increase in the observed scattering and its anisotropy with respect to the field direction. Moreover, measurements in the magnetic field reveal additional scattering for q > 0.04 Å^?1, which is well described by a Lorentzian and is interpreted as neutron magnetic scattering by spin-density fluctuations with a correlation radius R_c ≈ 30 Å.     

Energy scales and the non-Fermi liquid behavior in YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript>

Multiple energy scales are detected in measurements of the thermodynamic and transport properties in heavy fermion metals. We demonstrate that the experimental data on the energy scales can be well described by the scaling behavior of the effective mass at the fermion condensation quantum phase transition, and show that the dependence of the effective mass on temperature and applied magnetic fields gives rise to the non-Fermi liquid behavior. Our analysis is placed in the context of recent salient experimental results. Our calculations of the non-Fermi liquid behavior, of the scales and thermodynamic and transport properties are in good agreement with the heat capacity, magnetization, longitudinal magnetoresistance and magnetic entropy obtained in remarkable measurements on the heavy fermion metal YbRh₂Si₂.     

Double-stage continuous-discontinuous superconducting phase transition in the Pauli paramagnetic limit of a 3D superconductor: the URu<Subscript>2</Subscript>Si<Subscript>2</Subscript> case

The sharp suppression of the de-Haas van-Alphen oscillations observed in the mixed superconducting (SC) state of the heavy fermion compound URu₂Si₂ is shown to confirm a theoretical prediction of a narrow double-stage SC phase transition, smeared by fluctuations, in a 3D paramagnetically-limited superconductor. The predicted scenario of a second order transition to a nonuniform (FFLO) state followed by a first order transition to a uniform SC state, obtained by using a non-perturbative approach, is also found to be consistent with recent thermal conductivity measurements performed on this material.     

Quasi-one-dimensional quantum spin liquid in the Cu(C<Subscript>4</Subscript>H<Subscript>4</Subscript>N<Subscript>2</Subscript>)(NO<Subscript>3</Subscript>)<Subscript>2</Subscript> insulator

We analyze measurements of the magnetization, differential susceptibility and specific heat of quasi-onedimensional insulator Cu(C₄H₄N₂)(NO₃)₂ (CuPzN) subjected to magnetic fields. We show that the thermodynamic properties are defined by quantum spin liquid formed with spinons, with the magnetic field tuning the insulator CuPzN towards quantum critical point related to fermion condensation quantum phase transition (FCQPT) at which the spinon effective mass diverges kinematically. We show that the FCQPT concept permits to reveal and explain the scaling behavior of thermodynamic characteristics. For the first time, we construct the schematic T–H (temperature-magnetic field) phase diagram of CuPzN that contains Landau–Fermi-liquid, crossover and non-Fermi liquid parts, thus resembling that of heavy-fermion compounds.     

High pressure study of the organic compound (TMTTF)<Subscript>2</Subscript>BF<Subscript>4</Subscript>

High pressure resistivity measurements of the organic compound (TMTTF)₂BF₄ have been performed in a newly developped Bridgman cell providing good pressure conditions on a wide pressure range. For the first time in this compound a zero resistance superconducting state is observed between 3 and 4 GPa. At temperatures above the superconducting transition, the resistivities of the two high quality samples show a different behavior. One sample, provides indications for a magnetic quantum critical point at the maximum of T_c, whereas in the other antiferromagnetic spin-fluctuations are present above T_c.     

Intrinsic superconducting transport properties of ultra-thin Fe<Subscript>1+<Emphasis Type="Italic">y</Emphasis></Subscript>Te<Subscript>0.6</Subscript>Se<Subscript>0.4</Subscript> microbridges

We investigated the superconducting properties of Fe_1+y Te_0.6Se_0.4 single-crystalline microbridges with a width of 4 μm and thicknesses ranging from 20.8 to 136.2 nm. The temperature-dependent in-plane resistance of the bridges exhibited a type of metal-insulator transition in the normal state. The critical current density (J _c) of the microbridge with a thickness of 136.2 nm was 82.3 kA/cm² at 3K and reached 105 kA/cm² after extrapolation to T = 0 K. The current versus voltage characteristics of the microbridges showed a Josephson-like behavior with an obvious hysteresis. These results demonstrate the potential application of ultra-thin Fe-based microbridges in superconducting electronic devices such as bolometric detectors.     

Vibrational and electronic properties of a Cu<Subscript>90</Subscript>Nb<Subscript>10</Subscript> nanocrystalline composite

The vibrational, electronic, and superconducting properties of a Cu₉₀Nb₁₀ nanocrystalline composite undergoing a transition from a coarse-grained state to a nanocrystalline state are investigated using neutron scattering and low-temperature heat capacity measurements. It is found that, compared to a coarse-grained sample, the nanocomposite is characterized by a higher density of low-frequency excitations and a decrease both in the density of states and in the superconducting transition temperature due to the size effect.     

Effect of hybridization and dispersion of quasiparticles on the coexistent state of superconductivity and antiferromagnetism in <Emphasis Type="Italic">R</Emphasis>Ni<Subscript>2</Subscript>B<Subscript>2</Subscript>C

The effect of hybridization of conduction electrons and f-level on superconductivity (SC) and antiferromagnetism (AFM) in the coexistent phase of rare-earth nickel borocarbide superconductors (RNi₂B₂C) is reported. The Hamiltonian of the system is a mean field one and has been solved by writing equations of motion for the single-particle Green functions. It is assumed that superconductivity arises due to BCS pairing mechanism in the presence of antiferromagnetism in nickel lattices of Ni₂B₂ plane. The expressions for superconducting and antiferromagnetic order parameters are derived using double time electron Green functions. The quasiparticle energy bands are plotted and the nature of band dispersion of the quasiparticles is studied.     

Phase diagram of unconventional superconductor UPt<Subscript>3</Subscript> in weak crystal field model

We study the phase diagram of heavy fermion superconductor UPt₃ within a phenomenological model with two nearly degenerate order parameters corresponding to theA ₁ andE ₁ irreducible representations. A weak effect on superconductivity from the crystal lattice is assumed. This assumption explains a closeness of two critical temperatures and yields parameters of the Ginzburg-Landau functional which lead to a single superconducting transition at pressures above critical: TheH-T diagram with a tetracritical point is recovered in theAE model. Experimental consequences which can distinguish this model from others are also discussed.     

HgO-added YB<Subscript>a2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>−δ superconductors

The HgO-added YB_a2Cu₃O_7−δ (YBCO) superconductor has been studied for its structural and superconducting properties. Polycrystalline YBCO samples were synthesized through solid-state reaction method by adding HgO in different concentrations without using oxygen annealing. All the samples showed a sharp superconducting transition temperature around 90 K. The X-ray diffraction patterns of all the samples revealed monophasic Y-123 nature. The structural studies were carried out by neutron scattering and Rietveld analysis. The neutron scattering revealed that Hg is not incorporated in the Y-123 system and has shown optimum oxygen concentration. The significant role played by the HgO is to provide oxygen ambient through its decomposition, thus changing the oxygen balance in favour of high Cu-valence state.     

Features of the superconducting state and structural disorder in ultrafine particles of the HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

We study the reasons for which the compound YBa₂Cu₃O _y begins to lose its superconducting ability with a decrease in particle sizes to values of ∼0.1 μm. Our analysis of the results of structural and magnetic studies has allowed us to reveal changes in the parameters of the crystal structure and of the superconducting state in small particles of an HTSC and to show that the main cause of these changes is a special kind of a structural disorder peculiar only to small particles of YBa₂Cu₃O _y and is realized as a consequence of the need for nonequilibrium conditions during their synthesis.     

Observation of Bose condensation in (Pb<Subscript>0.4</Subscript>Sn<Subscript>0.6</Subscript>)<Subscript>0.86</Subscript>In<Subscript>0.14</Subscript>Te semiconductor solid solutions using Mössbauer spectroscopy

⁷³As(⁷³Ge) Mössbauer emission spectroscopy is used to ascertain that the transition of the (Pb_0.4Sn_0.6)_0.86In_0.14Te solid solution to a superconducting state is accompanied by a change in electronic density at the cation sites, and spatial inhomogeneities in the Bose condensate of Cooper pairs are revealed.     

Magnetic susceptibility of fine crystalline high-temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> superconductors. The role of nanoscale structural disordering

Static magnetic susceptibility χ(T) in the normal state (T_c ≤ T ≤ 400 K) and specific heat C(T) near temperature T_c of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa₂Cu₃O_y superconductor, having y and T_c close to optimal but differing in the degree of nanoscale structural disordering. It is shown that under the influence of structural disordering, there is enhancement of anomalous pseudogap behavior of the studied characteristics and a significant increase in the width of the pseudogap.     

Magnetic resonance of Cu and of Gd in insulating GdSr<Subscript>2</Subscript>Cu<Subscript>2</Subscript>NbO<Subscript>8</Subscript> and in superconducting GdSr<Subscript>2</Subscript>Cu<Subscript>2</Subscript>RuO<Subscript>8</Subscript>

Weak-ferromagnetic (or antiferromagnetic) resonance of Cu and electron spin resonance (ESR) of Gd are observed both in insulating GdSr₂Cu₂NbO₈ and in superconducting GdSr₂Cu₂RuO₈. The Cu resonance implies that the CuO₂ planes are magnetic and indicates that the superconducting layer of GdSr₂Cu₂RuO₈ is SrO (not CuO₂), as in its related superconducting compound without cuprate planes, doped Sr₂YRuO₆.     

A pseudopotential approach to the superconducting state properties of Cu<Subscript>C</Subscript>Zr<Subscript>100-C</Subscript> binary metallic glasses

The theoretical investigation of the superconducting state properties viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _C, isotope effect exponent α and effective interaction strength N ₀ V of ten binary Cu_CZr_100-C (C = 25–60 at%) metallic glasses is performed, using Ashcroft’s empty core model potential. Five local-field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used to study the screening influence on the aforesaid properties. It is shown that the electronphonon coupling strength λ and the transition temperature T _C are quite sensitive to the selection of the local-field correction functions, whereas the Coulomb pseudopotential μ*, isotope effect exponent α and effective interaction strength N ₀ V show a weak dependence on the local-field correction functions. The values of T _C obtained from the H-local-field correction function are found to be in qualitative agreement with available theoretical or experimental data and show almost linear behavior with respect to the concentration C of Cu. The present results are shown to be in good agreement with other available theoretical or experimental data. The obtained results confirm the existence of the superconducting phase in the metallic glasses.     

General properties of a magnetic-field-induced landau Fermi liquid in high-temperature superconductors and heavy fermion metals

It has been shown that the magnetic-field-induced transition from a non-Fermi-liquid state to a Fermi liquid state in the Tl₂Ba₂CuO_{6 + x} high-temperature superconductor is similar to a transition observed in heavy fermion metals. This behavior is explained in the theory of the Fermi condensate quantum-phase transition implying the existence of Landau quasiparticles. The Fermi condensate quantum-phase transition can be considered as a universal cause of the strongly correlated behavior observed in various metals and liquids such as high-temperature superconductors, heavy fermion metals, and two-dimensional Fermi systems.     

Improvement of the intergranular pinning energy in uniaxially compacting (Bi-Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+δ</Subscript> ceramic samples

Measurements of the electrical resistivity as a function of temperature, ρ(T), for different values of applied magnetic field, B_a (0 ≤ B_a ≤ 50 mT), were performed in polycrystalline samples of Bi_1.65Pb_0.35Sr₂Ca₂ Cu₃O_10+δ subjected to different uniaxial compacting pressure (UCP). We have found appreciable differences in the grain orientation between samples by using X-ray diffractometry. From the X-ray diffraction patterns performed, in powder and pellet samples, we have estimated the Lotgering factor along the (00l) direction, F_(00l). The results indicate that F_(00l) increases ~23% with increasing UCP suggesting that grains of these samples are preferentially aligned along the c-axis, which is parallel to the compacting direction. The resistive transition of the samples have been interpreted in terms of the thermally activated flux-creep model. In addition, the effective intergranular pinning energy, U₀, have been determined for different applied magnetic field. The magnetic field dependence of U₀, for B_a > 8 mT, was found to follow a H^{- α} dependence with α = 0.5 for all samples. The analysis of the experimental data strongly suggested that increasing UCP results in appreciable changes in both the grain alignment and the grain connectivity of the samples. We have successfully interpreted the data by considering the existence of three different superconducting levels within the samples: the superconducting grains, the weak-links, and the superconducting clusters.     

Spin-polaron nature of fermion quasiparticles and their <Emphasis Type="Italic">d</Emphasis>-wave pairing in cuprate superconductors

In the framework of the spin-fermion model, to which the Emery model is reduced in the limit of strong electron correlations, it is shown that the fermion quasiparticles in cuprate high-T _c superconductors (HTSCs) arise under a strong effect of exchange coupling between oxygen holes and spins of copper ions. This underlies the spin-polaron nature of fermion quasiparticles in cuprate HTSCs. The Cooper instability with respect to the d-wave symmetry of the order parameter is revealed for an ensemble of such quasiparticles. For the normal phase, the spin-polaron concept allows us to reproduce the fine details in the evolution of the Fermi surface with the changes in the doping level x observed in experiment for La_2-xSr_xCuO₄. The calculated T–x phase diagram correlates well with the available experimental data for cuprate HTSCs.     

Peculiarities in the low-temperature specific heat related to nanoscale structural inhomogeneity in fine-crystalline YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors

A comparative study of the low-temperature specific heat for two types of YBa₂Cu₃O _y high-T _c superconductor samples is performed within the temperature range of 2?10 K. The samples of the first type are fine-crystalline optimally doped ones with different degrees of nanoscale structural inhomogeneity. The second type includes coarse-crystalline equilibrium samples with different hole doping levels. A similarity in the behavior of different contributions to the specific heat for structurally inhomogeneous and underdoped samples is revealed. The samples of both types exhibit a metal-like contribution linear in temperature to the specific heat ~γT, which is not characteristic of the superconducting phase. It is found that this contribution moderately grows with the decrease in the oxygen content, whereas with the increase in the structural inhomogeneity, such growth of the linear contribution (γT) becomes anomalously large. This leads to the conclusion about the coexistence of metallic and superconducting states in the bulk of the samples under study. Such common feature of electron systems could be related to the formation of the pseudogap regime. It is demonstrated that this regime suppresses just the superconducting states, leaving intact the metallic ones.     

On the character of chiral symmetry breaking and fermion vacuum structure in QED<Subscript>3</Subscript>

Equation for the Bethe-Salpeter wave function of the Goldstone boson in QED₃ is considered in the ladder approximation with the use of the Landau gauge for the photon propagator. With the help of standard simplifications, the existence of nonzero solutions for this equation is demonstrated, which testifies to the production of the above-described boson in the process of chiral symmetry breaking. At the same time, it is demonstrated that only one of the entire set of solutions describing the Goldstone boson corresponds to the stable ground state; this solution has the greatest fermion mass. In the remaining cases, the compound boson state with zero mass is excited, and all other states having smaller energies appear tachyon states and hence are unstable. The fermion condensate is calculated; it is demonstrated that in the examined case, it is finite. Based on the foregoing, conclusions are drawn about spontaneous rather than dynamic character of chiral symmetry breaking in QED₃, complex structure of fermion vacuum for the examined model, and at the same time, simple structure of the massive phase vacuum.