Pairing, Magnetic Spin Fluctuations, and Superconductivity Near a Quantum Critical Point

The properties of a wide variety of intermetallic compounds exhibiting magnetic localized spin and superconducting fluctuations near a quantum critical point (QCP) are reviewed. They show highly anomalous critical indices (anomalously small). Laws of corresponding are observed in these materials and a theory is presented which gives a fully quantitative explanation of these laws. The theory employs a gauge transformation which rotates the electron spin quantization axis into the direction of the instantaneous staggered localized spin direction , where is the localized spin array wave vector. Many properties of these materials are worked out on the basis of this theory. The technological promise of these substances is truly immense, including energy generation, storage and transmission, MRI magnets, industrial and scientific magnets, maglev, cellular communications, -wave electronics, etc.     

Normal State Properties and Upper Critical Magnetic Field in Three-dimensional Polycrystalline Niobium Films

Journal of Superconductivity and Novel Magnetism - Three-dimensional (3D) niobium (Nb) films have been deposited on quartz substrates by changing substrate temperature or film thickness. The...     

Fractal Structure Favoring Superconductivity at High Temperatures in a Stack of Membranes Near a Strain Quantum Critical Point

The statistical physics of the 3D ordered oxygen interstitials has been measured in La₂CuO_4+y using an advanced tool, scanning x-ray diffraction with focused synchrotron radiation. The observed fractal scale invariant distribution is found in a cuprate in the proximity to a stripes critical point in the 3D Aeppli-Bianconi phase diagram of cuprates, where T _c is function of both hole doping and superlattice misfit strain. Therefore high-temperature superconductivity is favored by complex fractal systems while on the contrary standard low temperature superconductivity is favored in simple periodic crystals. This work shows that the fractal structural distribution in a stack of membranes favors the macroscopic quantum coherent condensate at high temperature. This result opens new perspectives for the understanding the relationship between emergent scale-free distribution in living matter and possible quantum coherent phenomena able to resist to the attacks of temperature decoherence effects.     

Further Analysis of the Quantum Critical Point of Ce1-x La x Ru2Si2

New data on the spin dynamics and the magnetic order of Ce_1-xLa_xRu₂Si₂ are presented. The importance of the Kondo effect at the quantum critical point of this system is emphasized from the behavior of the relaxation rate at high temperature and from the variation of the ordered moment with respect to the one of the Néel temperature for various x.     

Heat Transport in a Pure Fluid Near the Critical Point: Steady State and Relaxation Dynamics

The steady-state conditions and relaxation dynamics of a fluid layer during heat transport experiments near the liquid-vapor critical point are studied theoretically. The application is to ³ He along the critical isochore and under normal gravity—where experimental data are available—as well as zero gravity. First, the steady-state density stratification from gravity and from heat flow is calculated. The resulting observable thermal conductivity is obtained as a function of the temperature difference T (or the heat current Q) across the fluid layer and becomes a strong non-linear function of T as T_c is approached. The calculated is compared with that from experiments both above and below T_c. Second, the spatial profiles of temperature and density and their temporal evolutions are calculated above T_c as T is established after Q is switched on, or conversely as T decays to zero after Q has been switched off. From the calculations, done both from a closed-form expression and from simulations, the observable and asymptotic relaxation times for reaching the steady state are calculated as a function of Q, and compared with the experiments above T_c.     

High-Tc Superconductivity with Bipolarons and Normal State Gap in Cuprates

The BCS theory is extended to a strong-coupling regime, λ≥1 with smallpolarons and bipolarons. The normal state gap is found in this regime whichis half of the bipolaron binding energy. In the framework of the bipolarontheory we explain the magnitude of the carrier specific heat andsusceptibility as well as their universal scaling with temperature in a widerange of doped cuprates. PACS numbers: 74.20.?z,74.65.+n,74.60.Mj

     

Anisotropic Upper Critical Field in Single Crystal MgB2

Recent experiments on MgB₂ single crystals appear to indicate that superconductivity in this material is described not only by two superconducting order parameters attached to the band and the band, respectively, but also these two order parameters have different anisotropy. More explicitly the anisotropy of H _c2(t) requires an oblate order parameter in momentum space attached to the a band while H _c1(t) is described by a prolate order parameter, attached to the band. Therefore the vortex state in MgB₂ should be described by an interplay of these two superconducting order parameters.     

Quantum Critical Point in the Quasi 2D Conductor, (Me-DH-TTP)2AsF6

A new TTP donor, Me-DH-TTP (2-methyl-5-(1,3-dithiolan-2-yliden)-1,3,4,6-tetrathiapentalene), was designed to realize a system with large on-site Coulomb repulsion as compared with the previously known bis-fused type TTP donors. Probably as a consequence, (Me-DH-TTP)₂AsF₆ exhibits semiconducting behavior from room temperature to liquid helium temperature. By increasing pressure, metallic behavior appears below 300 K, and with distinct metal-insulator (M-I) transition up to 2.2 GPa. This M-I transition suddenly disappears beyond 2.5 GPa, and metallic state is stabilized down to 2.6 K. We discuss the possibility of quantum critical point around 2.4 GPa.     

Effect of Molecular Structure on the Thermodynamics of Nitromethane+Butanol Isomers Near the Upper Critical Point

Densities, isentropic compressibilities, and isobaric molar heat capacities were determined over the whole composition range for nitromethane+(2-butanol or isobutanol) at atmospheric pressure and at the temperatures 288.15, 293.15, 298.15, and 308.15 K. These results allowed us to obtain isobaric thermal expansivities, isothermal compressibilities, and isochoric molar heat capacities at the temperature 298.15 K. The excess quantities for the given properties were obtained. In addition, liquid–liquid phase separation temperatures were also determined, locating upper critical solution temperatures near the experimental temperatures. The variation of the properties among isomers is discussed. Also, the effect of the nonrandomness of the mixtures expected near the critical point is discussed.     

Nucleation and Boiling Near the Gas–Liquid Critical Point

Nucleation near the gas–liquid critical point depends sensitively on whether the pressure or the volume is fixed. We consider near-critical fluids close to the coexistence curve. (i) Upon decompression to a constant pressure with a fixed boundary temperature, bulk nucleation can well be induced from a gas state, whereas from a liquid state boiling is easily triggered in the thermal diffusion layer near the boundary. In this case, bulk nucleation in a metastable gas is described by the classical Lifshitz–Slyozov theory. (ii) Upon cooling of the boundary temperature under the fixed-volume condition, bulk nucleation can be realized in a liquid and a modified Lifshitz–Slyozov theory follows. However, if a gas is cooled from the boundary at a fixed volume, liquid droplets readily appear in the thermal diffusion layer, apparently suggesting no metastability in a gas in agreement with previous experiments. (iii) On the other hand, if a liquid is heated at the boundary wall, boiling readily occurs both at a fixed volume and at a fixed pressure.     

Resolving the Yang-Yang Dilemma in 3He Near the Critical Point

A challenging problem is to experimentally resolve the Yang-Yang dilemma for fluids-whether the critical singularity of the isochoric heat capacity is shared between the second temperature derivatives of pressure and chemical potentials. In the lattice-gas model, which serves as a well-established guidance to study fluid criticality, the divergence of the second derivative of pressure is solely responsible for the heat capacity anomaly. If it is proven that the heat capacity singularity is shared between the pressure and chemical potential derivatives, a revision of the conventional scaling theory for fluids based on the analogy between fluids and lattice-gas model will be required. Thus far, experiments on real fluids have been inconclusive, in particular, because of possible impurity effects and the intrinsic vapor-liquid asymmetry. A study of the ³He isotope gives a unique opportunity to resolve the Yang-Yang dilemma. In ³He all other impurities except ⁴He are frozen out. The almost perfect purity of the available ³He samples (typically less than 1 ppm of ⁴He in ³He) enables the elimination or quantitative study of impurity effects. Also the well-known symmetry of the ³He coexistence curve implies an absence of confluent singularities caused by liquid-gas asymmetry. In this paper we reanalyze early experimental data of near-critical ³He and show that they are internally not completely consistent. While the second derivative of chemical potential obtained from the density dependence of the two-phase isochoric heat capacity does not show a noticeable anomaly, the analysis of the same property obtained by combining the PVT and heat capacity implies the existence of the Yang-Yang anomaly. An unambiguous test of the Yang-Yang dilemma will require more accurate heat-capacity measurements closer to the critical point. To thoroughly distinguish between these alternatives one may ultimately require a microgravity experiment that will allow measurements very close to the critical point.     

Inhomogeneous Superconducting State and Intrinsic T c : Near Room Temperature Superconductivity in the Cuprates

Doped cuprates are inhomogeneous superconductors. The concept of an intrinsic critical temperature, T_c^intr. o T_c^*T_{c}^{\mathrm{intr.}} \equiv T_{c}^{*}, whose value greatly exceeds that for the resistive T_c^res. o T_cT_{c}^{\mathrm{res.}}\equiv T_{c}, is supported by a number of experimental studies, including those performed recently. These data are discussed in this review. The anomalous diamagnetism observed at T_c^res. < T < T_c^*T_{c}^{\mathrm{res.}} 相似文献   

High-Tc Superconductivity with Bipolarons and Normal State Gap in Cuprates

The BCS theory is extended to a strong-coupling regime, 1 with smallpolarons and bipolarons. The normal state gap is found in this regime whichis half of the bipolaron binding energy. In the framework of the bipolarontheory we explain the magnitude of the carrier specific heat andsusceptibility as well as their universal scaling with temperature in a widerange of doped cuprates. PACS numbers: 74.20.–z,74.65.+n,74.60.Mj     

Nonasymptotic Transport Properties in Fluids and Mixtures Near a Critical Point

We review the critical dynamics of fluids and mixtures. Special attention in the comparison with experiment is paid to nonasymptotic effects. Our theoretical results are based on the complete model H of Siggia, Halperin, and Hohenberg including the sound mode variables. Using the dynamic renormalization group theory, we calculate the temperature dependence of the transport coefficients as well as the frequency-dependent sound velocity and sound attenuation. In mixtures a time ratio between the Onsager coefficients related to the diffusive modes, which is directly related to the critical enhancement of the thermal conductivity near a consolute point, has to be taken into account. The sound mode contains, besides the dynamic parameters, a static coupling related to the logarithmic derivative of the weakly diverging specific heat. The deviation from the asymptotic value of this coupling at finite frequencies and temperature distance from T _c leads to additional nonasymptotic effects. Our theory, which derives the phenomenological ansatz of Ferrell and Bhattacharjee for pure fluids and mixtures near a consolute point, is also applicable near a plait point.     

Identify the Nematic Superconductivity of Topological Superconductor Pd $$_x$$ x Bi $$_2$$ 2 Te $$_3$$ 3 by Angle-dependent Upper Critical Field Measurement

Journal of Superconductivity and Novel Magnetism - Odd-parity superconductivity is recognized as a key ingredient to realize topological superconductivity. In Bi2Se3-based bulk topological...     

Anomalous Hall and Nernst Effects in Normal State of SC Cuprates

It is demonstrated that scattering of mobile charge carriers by fluctuations of local spin density in the normal state of SC-cuprates results in anomalous contribution to the transport phenomena (Hall and Nernst effects are included). Depending on their sign and magnitude, they can change value and sign of the corresponding effect measured.     

Thermal Conductivity and Critical Boundary Resistance of Helium Near the Lambda Point

We report measurements of the temperature dependence of the thermal conductivity of helium in the region just above the lambda point and the critical boundary resistance on both sides of the transition. The data were obtained using a cell with a gap that was varied from 0.12 to 0.73 mm. The range of the data was from about 2 mK below to 8 mK above the transition, with a maximum resolution of about 5×10⁻⁹ K, well within the gravity rounded region. We find good agreement with the expected behavior of the thermal conductivity based on the dynamic renormalization group theory. The boundary resistance results above the transition are in fair agreement with the theoretical prediction of Frank and Dohm. Below the transition the wide range results are in approximate agreement with additional predictions and other measurements at higher powers. Closer to the transition on the low temperature side phenomena are observed which appear to be apparatus-dependent.      

Surface Adsorption and Orientation Near the Critical Point of Binary Liquid Mixtures

Critical binary liquid mixtures have proven to be extremely useful for quantitatively studying wetting and adsorption phenomena that occur at the surfaces of these systems. In this paper recent experimental developments in our understanding of adsorption gained via the study of critical mixtures are reviewed. At the noncritical liquid/vapor surface the component possessing the smallest surface tension completely saturates this surface for sufficiently large surface tension differences between the two components. This complete (or strong) adsorption is described by a universal scaling function P which depends upon the dimensionless depth z/ where is the correlation length. Weakly polar and nonpolar binary liquid mixtures have been used to define the scaling function P. If the surface energy difference between the two components is small, then the surface is no longer completely saturated by one component. In this weak adsorption egime the adsorption behavior is now described by a universal function G of both z/ and . The functional dependence of G is elucidated by examining a homologous series of critical binary AB liquid mixtures where component A is an n-alkane while component B (methyl formate) is fixed. For this system changes sign with increasing n-alkane chain length. In addition to adsorption, dipole surface orientational order at the liquid/vapor surface of highly polar + nonpolar mixtures is also discussed. This orientational order is induced by the dipole-image dipole interaction in the vicinity of a surface.