Euclidian Crystals in Many-Body Systems: Breakdown of Goldstone’s Theorem

It is proven via modification of the Nambu–Goldstone theorem, based on the Ward–Takahashi identity that Goldstone’s bosons of the quantum order could be massive. The quantum order parameter (QOP), introduced previously as the instantonic crystal in Euclidian space (Mukhin, J. Supercond. Nov. Magn. 24:1165–1171, 2011), breaks Matsubara’s time translational invariance while possessing zero classical expectation value and vanishing scattering cross-section (“hidden order”). The amplitude of the mass-gap can be calculated from the eigenmodes spectrum of the effective Euclidian action for a particular self-consistent QOP solution in the fermionic repulsive Hubbard model. Theoretical results are discussed in relation with possible linking together of the two phenomena that previously were considered separately, i.e., “hidden order” fingerprints in ARPES measurements and the “neutron resonance” feature in the magnetic spectrum in lightly hole-doped copper oxides.     

Dielectric Design of Current Lead Parts for a 154 kV Superconducting Apparatus

Currently, studies on the development of high voltage superconducting machines are being conducted actively in many research institutes all over the world (Kang et al. in IEEE Trans. Appl. Supercond. 17:1493–1496, 2007). In this paper, a sub-cooled liquid nitrogen cooling method is considered to design current lead parts for a 154 kV superconducting apparatus because of its excellent dielectric and mechanical characteristics (Kang et al. in IEEE Trans. Appl. Supercond. 17:1493–1496, 2007). Insulation gases such as SF₆, CF₄, and N₂ are considered as suitable pressurization gases for enhancing the dielectric characteristics of a superconducting system in the Republic of Korea (Kang and Ko in IEEE Trans. Appl. Supercond. 21:1332–1335, 2011). Dielectric experiments are conducted on GFRP (glass fiber reinforced plastic) and insulation gases (SF₆, CF₄, and N₂) with various pressures to establish experimental criteria for designing current lead parts for superconducting applications. It is found that the criteria for calculating electrical breakdown voltage at sparkover of GFRP and insulation gases conform to the mean electric field intensity and the maximum electric field intensity, respectively. Also, the criteria for calculating the electrical breakdown voltage at sparkover according to various conditions are derived. Finally, the conceptual dielectric design of current leads for a 154 kV superconducting apparatus by applying the presented criteria is performed considering various safety factors.     

Transition into the Supersolid (SS) State, Supersolid Density �� ss and?the Critical Velocity V c to Destroy the?SS State

We have reported the onset of the vortex fluid (VF) state in hcp solid ⁴He below an onset temperature T _o (Penzev et al. in Phys. Rev. Lett. 101:065301, 2008). We have also shown the transition from the VF state into the supersolid (SS) state, which is characterized by a supersolid density and macroscopic coherence, below a transition (Kubota et al. in J. Low Temp. Phys. 158:572, 2010; Shimizu et al. in arXiv:0903.1326, 2009; Yagi et al. in J. Low Temp. Phys., QFS2010, 2011) at T _c , as well as a critical velocity for the destruction of the SS state. Here we describe further confirming evidence of the phase transition into the SS state, namely, we report here some other observations than the hysteretic behavior. We find some other features which indicate a clear change of the properties in the hcp ⁴He below the same T _c as hysteretic behavior, reported in Kubota et al. (J. Low Temp. Phys. 158:572, 2010), Shimizu et al. (arXiv:0903.1326, 2009), Yagi et al. (J. Low. Temp. Phys., QFS2010, 2011). It was discovered by detailed study of the torsional oscillator (TO) drive velocity V _ac dependence of the nonlinear rotational susceptibility (NLRS) of the VF state. We discuss the features of the VF to SS transition. The large ratio between T _o and T _c may indicate large fluctuations of the system and it may give a basis for the vortex state (Fisher et al. in Phys. Rev. B 43(1):130, 1991) appearance in the hcp ⁴He, as seen in the new type of superconductors. We discuss a question related to the observation of the VF phase with specific dynamic features (Nemirovskii et al. in arXiv:0907.0330, 2009), coexisting with the SS phase and a possible new kind of phase transition coexisting with the SS phase and a possible new kind of phase transition for the VF to SS transition.     

Enhancing T c by Oxygen Isotope Substitution in Cuprates

The phase diagram of doped cuprates is generic. For low dopings a pseudogap is present at temperatures above the superconducting phase. The pseudogap phase is heterogeneous containing superconducting clusters which become phase coherent at T _c (Müller in J. Phys. Cond. Matter 19:251002, 2007). Most recent propositions suggest to increase the cluster size and therefore to enhance T _c . In the present note it is recalled that over a decade ago a giant oxygen isotope effect in low doped LSCO was reported. Thus, if indeed increasing the cluster sizes becomes possible, enhancing T _c towards the onset of the pseudogap phase, then it may be even further enhanced by ¹⁶O to ¹⁸O substitution.     

Quantized Vortex State in hcp Solid 4He

The quantized vortex state appearing in the recently discovered new states in hcp ⁴He since their discovery (Kim and Chan, Nature, 427:225–227, 2004; Science, 305:1941, 2004) is discussed. Special attention is given to evidence for the vortex state as the vortex fluid (VF) state (Anderson, Nat. Phys., 3:160–162, 2007; Phys. Rev. Lett., 100:215301, 2008; Penzev et al., Phys. Rev. Lett., 101:065301, 2008; Nemirovskii et al., arXiv:0907.0330, 2009) and its transition into the supersolid (SS) state (Shimizu et al., arXiv:0903.1326, 2009; Kubota et al., J. Low Temp. Phys., 158:572–577, 2010; J. Low Temp. Phys., 162:483–491, 2011). Its features are described. The historical explanations (Reatto and Chester, Phys. Rev., 155(1):88–100, 1967; Chester, Phys. Rev. A, 2(1):256–258, 1970; Andreev and Lifshitz, JETP Lett., 29:1107–1113, 1969; Leggett, Phys. Rev. Lett., 25(22), 1543–1546, 1970; Matsuda and Tsuneto, Prog. Theor. Phys., 46:411–436, 1970) for the SS state in quantum solids such as solid ⁴He were based on the idea of Bose Einstein Condensation (BEC) of the imperfections such as vacancies, interstitials and other possible excitations in the quantum solids which are expected because of the large zero-point motions. The SS state was proposed as a new state of matter in which real space ordering of the lattice structure of the solid coexists with the momentum space ordering of superfluidity. A new type of superconductors, since the discovery of the cuprate high T _c superconductors, HTSCs (Bednorz and Mueller, Z. Phys., 64:189, 1986), has been shown to share a feature with the vortex state, involving the VF and vortex solid states. The high T _c s of these materials are being discussed in connection to the large fluctuations associated with some other phase transitions like the antiferromagnetic transition in addition to that of the low dimensionality. The supersolidity in the hcp solid ⁴He, in contrast to the new superconductors which have multiple degrees of freedom of the Cooper pairs with spin as well as angular momentum freedom, has a unique feature of possessing possibly only the momentum fluctuations and vortex ring excitations associated with the possible low dimensional fluctuations of the subsystem(s). The high onset temperature of the VF state can be understood by considering thermally excited low D quantized vortices and it may be necessary to seek low dimensional sub-systems in hcp He which are hosts for vortices.     

High-Tc Superconductivity Driven by Overscreening of Coulomb Interaction

The recent neutron scattering experiments in high-T _c superconductors show that the frequencies of the longitudinal optical (LO) phonon of a stretching mode is strongly softened and the frequencies of the LO and transverse optical (TO) phonons are inverted in certain wave number regions. This fact suggests that the electronic dielectric function becomes negative because of some correlation effect in these regions, and that the Coulomb interaction in the LO phonon is overscreened. In this case, theory suggests that the attractive interaction mediated by optical phonons is highly enhanced. This fact will be a capable of mechanism of high-T _c superconductivity. There the flat bands appear just below the Fermi level along the K _x and K _y axes as observed by angle-resolved photoemissions. A calculation taking account of the above effects shows that the symmetry of the order parameter is of the $d_{x^2 - y^2 } $ wave and the superconducting transition temperature can exceed 200 K.     

Optical study of charge dynamics along thec-axis in high-T c superconductors

Comparing the optical spectra withE∥c for typical high-T _c superconducting cuprates, we discuss the charge dynamics along thec-axis. The plasma energy or the mass anisotropic factorm _* ^c //m _* ^b is one of the key parameters determining the spectrum forE∥c. In Bi₂Sr₂CaCu₂O₈ and La_2?x Sr _x CuO₄ with a largem _* ^c /m _* ^b , the plasma energy is smaller than the superconducting gap energy, and thus the supercurrent along thec-axis is a kind of Josephson current flowing through insulating layers such as the BiO layers. On the other hand, in YBa₂Cu₃O₇ with a smallm _* ^c /m _* ^a , it seems that the coherent supercurrent flows along thec-axis. Although the spectrum forE∥c strongly depends on samples, presumably due to the difference in the hole concentration, a clear anisotropy between theab- and thec-directions is observed in the characteristics energy scales such as the plasma energy as well as the reflectivity knee energy.     

Probable Observation of Quantized Vortex Lines Through Solid He Under DC Rotation

We report how one can detect quantized vortices in superfluids contained in cylindrical vessels in well-designed torsional oscillator (TO) experiments under DC rotation. We show the case of an artificial 3D superfluid (Fukuda et al. in Phys. Rev. B 71:212502, 2005) which is made of Kosterlits-Thouless 2D He film condensed on a porous glass substrate with a 3D connected surface of well-controlled pore size. We understand the TO experimental results with an extra energy dissipation peak under DC rotation by considering the circular quantized superflow around each of the vortex lines and interaction with thermally excited 2D vortices as discussed in Fukuda et al. (Phys. Rev. B 71:212502, 2005) and in Nemirovskii and Sonin (Phys. Rev. B 76: 224507, 2007). We discuss here the case of hcp solid ⁴He (see for ex. Balibar and Caupin in J. Phys., Condens. Matter 20:173201-1-19, 2008) and show the evidence of observation of the vortex lines penetration below a supersolid transition temperature (Kubota et al. in J. Low Temp. Phys. 158:572, 2010), T _c , where macroscopic phase coherence is realized. It is found at exactly the same temperature T _c , below which the hysteresis occurrs (Shimizu et ail. in arXiv:0903.1326, 2009). For hcp solid ⁴He we have reported the vortex fluid (VF) state onset temperature (Penzev et al. in Phys. Rev. Lett. 101:065301, 2008) T _o =??500 mK, by detailed drive velocity, V _ac dependence study using TO technique. The TO response of the hcp ⁴He is characterized by the energy dissipation peaked at T _p near 100 mK similar to the behavior in a KT transition. The real supersolid (SS) state occurs at T _c below T _p and much lower than T _o . Our observation of the evidence of vortex lines penetration just below T _c together with the VF state properties gives support for the idea that hcp ⁴He shares some features with the ??new type of superconductors??, where the vortex state, involving the VF as well as various vortex solid states, has been commonly discussed (Fisher et al. in Phys. Rev. B 43(1):130, 1991; Leggett in Quantum Liquids, Oxford University Press, London, 2006).     

The Unique Properties of Superconductivity in Cuprates

Copper oxides are the only materials that have transition temperatures, T _c, well above the boiling point of liquid nitrogen, with a maximum \(T_{\mathrm {c}}^{\mathrm {m}}\) of 162 K under pressure. Their structure is layered, with one to several CuO₂ planes, and upon hole doping, their transition temperature follows a dome-shaped curve with a maximum of \(T_{\mathrm {c}}^{\mathrm {m}}\) . In the underdoped regime, i.e., below \(T_{\mathrm {c}}^{\mathrm {m}}\) , a pseudogap Δ* ∝ T* is found, with T* always being larger than T _c, a property unique to the copper oxides. In the superconducting state, Cooper pairs (two holes with antiparallel spins) are formed that exhibit coherence lengths on the order of a lattice distance in the CuO₂ plane and one order of magnitude less perpendicular to it. Their macroscopic wave function is parallel to the CuO₂ plane near 100 % d at their surface, but only 75 % d and 25 % s in the bulk, and near 100 % s perpendicular to the plane in yttrium barium copper oxide (YBCO) [1]. There are two gaps with the same T _c [2]. As function of doping, the oxygen isotope effect is novel and can be quantitatively accounted for by a vibronic theory or by the presence of bipolarons [2, 3]. These cuprates are intrinsically heterogeneous in a dynamic way. In terms of quasiparticles, bipolarons are present at low doping and aggregate upon cooling [2] so that probably ramified clusters and/or stripes are formed, leading over to a more Fermi liquid-type behavior at large carrier concentrations.     

Enhancement of Magnetization in Liquid 3He at Aerogel Interface

A novel feature of condensate state in liquid ³He is predicted theoretically, which consists of spin triplet s-wave Cooper pairs (Higashitani et al. in J. Low. Temp. Phys. 155:83–97, 2009). Such a spin triplet s-wave state will appear inside aerogel near the surface boundary contacting with superfluid ³He-B, and the enhancement of magnetization due to s-wave state is theoretically expected (Nagato et al. in J. Phys. Soc. Jpn. 78:123603, 2009; Higashitani et al. in Phys. Rev. B 85:024524, 2012). In order to detect this proximity effect, we made the interface in columnar glass tube which coated with 2.5 layer ⁴He, and set a saddle shape NMR coil very near the interface. At 7 bar, we found that superfluidity in liquid ³He inside aerogel never occurred, even at considerably low temperatures. At 24 bar below T/T _c =0.392, we observed no decrease of magnetization with decreasing temperatures. This phenomenon might be due to spin triplet s-wave Cooper pairs.     

Superconducting transition temperature of aluminum,indium, and lead fine particles

Aluminum, indium, and lead fine particles with free surfaces were made by evaporation in helium gas to study the size dependence of the superconducting transition temperatureT _c in very small particles. The transition temperature was determined from the change of diamagnetic susceptibility with temperature. The present experiment shows thatT _c rises with decrease of the average diameter \(\bar d\) for aluminum and indium, while it remains nearly constant for lead. The enhancement factorsT _c /T _c ^b (T _c ^b isT _c for the bulk material) in the case of \(\bar d = 100 {\AA}\) are determined as 1.45, 1.15, and 1.0 for aluminum, indium, and lead, respectively. The effect of the soft phonon mode arising from the increasing fraction of surface satisfactorily explains the experiment with respect to both the size dependence and the difference among elements by a rather simple model.     

The Magnetically-Tuned Transition-Edge Sensor

We present the first measurements on the proposed magnetically-tuned superconducting transition-edge sensor and compare the modified resistive transition with the theoretical prediction (Sadleir et al., IEEE Trans App Supercond 23:2101405, 2013). A TES’s resistive transition is customarily characterized in terms of the unitless device parameters \(\alpha \) and \(\beta \) corresponding to the resistive response to changes in temperature and current respectively. We present a new relationship between measured IV quantities (sensor current \(I\) and voltage \(V\) ) and the parameters \(\alpha \) and \(\beta \) and use these relations to confirm we have stably biased a TES with negative \(\beta \) parameter with magnetic tuning. Motivated by access to this new unexplored parameter space, we investigate the conditions for bias stability of a TES taking into account both self and externally applied magnetic fields.     

Angular Dependence of the Fluctuation Magnetization Vector Above the Superconducting Transition of a Highly Anisotropic High-T c Cuprate

In highly anisotropic cuprate superconductors it is generally accepted that the reversible magnetization vector, $\vec{M}$ , is essentially perpendicular to the superconducting CuO₂ layers in a wide range of crystal orientations with respect to the applied magnetic field, $\vec{H}$ . In a recent work (Mosqueira et al. in Phys. Rev. B 84:134504, 2011) it is shown, however, that the dependence of $\vec{M}$ on the $\vec{H}$ orientation in the reversible mixed state of a high-quality Tl-based cuprate presents a notable deviation from this behavior. Here we extend these measurements to the fluctuation region above T _c , in order to check whether the above-mentioned effect is also present.     

Describing How the Superconducting Transition in La2CuO4+y Is Related to the i-O Phase Separation

The phase separation of oxygen interstitial in the spacer layers separating the CuO₂ planes in the high critical temperature superconductor La₂CuO_4+y was studied by Poccia et?al. (Nat. Mater. 10:733, 2011). We model this transition by the electronic charge in the planes that can be described by the Cahn?CHilliard theory, with the formation of clusters of low and high densities separated by potential barriers. The charge diffusion into bound regions minimizes the energy and favors the formation of Cooper pairs. Self-consistent calculations yield local dependent superconducting amplitudes ?? _sc(r), and the resistivity transition at T _c is driven by Josephson coupling among these nanoscale clusters.     

Paramagnetic Meissner Transitions in Pb Films and the?Vortex Compression Model

The magnetic flux exclusion known as Meissner effect resulting from a perfectly diamagnetic susceptibility ??=?1 is one of the defining features of type-I superconductivity. More recently some materials have been reported to undergo a transition to a paramagnetic state as they are cooled through their superconducting transition temperature (Svedlindh et?al. in Physica?C, pp.?162?C164, 1989; Li in Phys. Rep. 376:133?C223, 2003). This is known as the Paramagnetic Meissner Effect (PME). Here we report the observation of a transition from the Meissner state to a PME state in thin Pb films. We go on to show how simple modifications to the vortex compression model developed by Koshelev and Larkin (Phys. Rev.?B 52:13559?C13562, 1995) yield magnetization-vs-temperature curves in good qualitative agreement with the magnetization curves reported in the experiment.     

Physical Picture of High-T c Superconductivity in Cuprates

A simple physical picture of high-T _c superconductivity of CuO₂ planes is proposed. It possesses all characteristic features of HTS, such as a high superconducting transition temperature, the $d_{x^{2}-y^{2}}$ symmetry of order parameter, and the coexistence of a single-electron Fermi surface and a pseudogap in the normal state. Values of pseudogap are calculated for different doping levels.     

The Study of Magnetically Soft Fe–B–P Based Nanostructures

Rapidly quenched Fe–B–P alloys with addition Cu exhibit high saturation induction, high permeability, low coercivity, and excellent mechanical properties (?vec et al. in IEEE Trans. Magn. 46:408, 2010; Makino et al. in IEEE Trans. Magn. 45(10):4302–4305, 2009; Makino et al. in Ser. Mater. 48:869, 2003). The systems based on Fe–B–P–Cu have been prepared by planar flow casting in form of thin ribbons. Selected magnetic properties in the as-cast state and after controlled annealing targeted to produce fine-grain structure of bcc-Fe in amorphous matrix were determined. The effect of the additions of P and Cu on structure and soft magnetic properties in the nanocrystalline state was investigated. Using the direct magnetostriction measurement method, the magnetostrictions as functions of external magnetic field in parallel and perpendicular directions were determined and the values of saturation magnetostriction λ _s were calculated. The Curie temperature was determined by thermogravimetry analysis (TGA). X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations at selected temperatures were performed to obtain information about the structure, morphology, size, and distribution of grains in an amorphous matrix, and to correlate microstructure and selected physical properties of the investigated system.     

Superconductivity at 25 K under Hydrostatic Pressure for FeTe 0 . 5 Se 0 . 5 Superconductor

We report the impact of hydrostatic pressure on the superconductivity and normal-state resistivity of FeTe_0.5Se_0.5 superconductor. At the ambient pressure, the FeTe_0.5Se_0.5 compound shows the superconducting transition temperature \(T_{\mathrm {c}}^{\text {onset}} \) at above 13 K and \(T_{\mathrm {c}}^{\rho =0} \) at 11.5 K. We measure pressure-dependent resistivity from 250 to 5 K, which shows that the normal-state resistivity increases initially for the applied pressures of up to 0.55 GPa, and then the same is decreased monotonically with increasing pressure of up to 1.97 GPa. On the other hand, the superconducting transition temperatures ( \(T_{\mathrm {c}}^{\text {onset}} \) and \(T_{\mathrm {c}}^{\rho =0} )\) increase monotonically with increasing pressure. Namely the \(T_{\mathrm {c}}^{\text {onset}} \) increases from 13 to 25 K and \(T_{\mathrm {c}}^{\rho =0} \) from 11.5 to 20 K for the pressure range of 0–1.97 GPa. Our results suggest that superconductivity in this class of Fe-based compounds is very sensitive to pressure as the estimated pressure coefficient d T _c(onset)/dP is ～5.8 K/GPa. It may be suggested that the FeTe_0.5Se_0.5 superconductor is a strong electron-correlated system. The enhancement of T _c with applying pressure is mainly attributed to an increase of charge carriers at the Fermi surface.     

Homological models for semidirect products of finitely generated Abelian groups

Let G be a semidirect product of finitely generated Abelian groups. We provide a method for constructing an explicit contraction (special homotopy equivalence) from the reduced bar construction of the group ring of G, ${\overline{B}(\mathsf{\textstyle Z\kern-0.4em Z}[G])}$ , to a much smaller DGA-module hG. Such a contraction is called a homological model for G and is used as the input datum in the methods described in álvarez et?al. (J Symb Comput 44:558–570, 2009; 2012) for calculating a generating set for representative 2-cocycles and n-cocycles over G, respectively. These computations have led to the finding of new cocyclic Hadamard matrices (álvarez et al. in 2006).