Normal State Of High-temperature Superconductors Research Articles

Transition to the Normal State Induced by High Current Densities in High-Tc Superconductor Microbridges Under Applied Magnetic Fields

We report some of our experimental results about the transition to the normal state of high-temperature superconductors subjected to high current densities and, simultaneously, under external magnetic fields up to 1 T. Our data analysis, which is based on a recently published instability model, show that the quenching may be explained in terms of thermal instabilities due to self-heating favored by the nonlinear nature of the current–voltage characteristics of high- $T_{c}$ superconductors (HTS). In fact, we predict the density current $J^{\ast}$ at which the samples quench, i.e., jump abrupt to the normal state, with an accuracy around 1%. Beyond its interest from a fundamental point of view, this thermal model opens a practical way to estimate the quenching point thus avoiding the damaging or even burning up of devices in high power applications of HTS.

Calorimetric determination of the magnetic phase diagram of underdoped ortho II YBa2Cu3O6.54 single crystals.

The recent discovery of a charge order in underdoped YBa2Cu3Oy raised the question of the interplay between superconductivity and this competing phase. Understanding the normal state of high-temperature superconductors is now an essential step towards the description of the pairing mechanism in those materials and determining the upper critical field is therefore of fundamental importance. We present here a calorimetric determination of the field–temperature phase diagram in underdoped YBa2Cu3Oy single crystals. We show that the specific heat saturates in high magnetic fields. This saturation is consistent with a normal state without any significant superconducting contribution and a total Sommerfeld coefficient γN∼6.5±1.5 mJ mol−1 K−2 putting strong constraints on the theoretical models for the Fermi surface reconstruction.

Electronic nematicity above the structural and superconducting transition in BaFe2(As1−xP x )2

Electronic nematicity, a unidirectional self-organized state that breaks the rotational symmetry of the underlying lattice, has been observed in the iron pnictide and copper oxide high-temperature superconductors. Whether nematicity plays an equally important role in these two systems is highly controversial. In iron pnictides, the nematicity has usually been associated with the tetragonal-to-orthorhombic structural transition at temperature T(s). Although recent experiments have provided hints of nematicity, they were performed either in the low-temperature orthorhombic phase or in the tetragonal phase under uniaxial strain, both of which break the 90° rotational C(4) symmetry. Therefore, the question remains open whether the nematicity can exist above T(s) without an external driving force. Here we report magnetic torque measurements of the isovalent-doping system BaFe(2)(As(1-x)P(x))(2), showing that the nematicity develops well above T(s) and, moreover, persists to the non-magnetic superconducting regime, resulting in a phase diagram similar to the pseudogap phase diagram of the copper oxides. By combining these results with synchrotron X-ray measurements, we identify two distinct temperatures-one at T*, signifying a true nematic transition, and the other at T(s) (<T*), which we show not to be a true phase transition, but rather what we refer to as a 'meta-nematic transition', in analogy to the well-known meta-magnetic transition in the theory of magnetism.

Spin-glass state of individual magnetic vortices inYBa2Cu3OyandLa2−xSrxCuO4below the metal-to-insulator crossover

Highly disordered magnetism confined to individual weakly interacting vortices is detected by muon spin rotation in two different families of high-transition-temperature superconductors, but only in samples on the low-doping side of the low-temperature normal state metal-to-insulator crossover (MIC). The results support an extended quantum phase transition (QPT) theory of competing magnetic and superconducting orders that incorporates the coupling between CuO2 planes. Contrary to what has been inferred from previous experiments, the static magnetism that coexists with superconductivity near the field-induced QPT is not ordered. Our findings unravel the mystery of the MIC and establish that the normal state of high-temperature superconductors is ubiquitously governed by a magnetic quantum critical point in the superconducting phase.

The effect of quasinesting and a magnon mode on the antiferromagnetic vector for the optical conductivity of a doped two-dimensional antiferromagnet

The optical spectrum of the normal state of a doped two-dimensional antiferromagnet is analyzed in a Kondo lattice model with regard to the complex structure of a spin polaron. The optical properties are determined by sharply anisotropic scattering of spin-polaron excitations by antiferromagnetic fluctuations of a system of localized spins. It is shown that the relaxation of carriers in the infrared range is mainly attributed to the strong coupling between these carriers and the mode of low-frequency spin excitations with the quasimomentum close to the antiferromagnetic vector Q=(π,π). The latter coupling is associated with the fact that the regions of the Fermi surface of the lower polaron band are close to the boundary of the antiferromagnetic Brillouin zone. The calculated optical characteristics are in qualitative agreement with experimental data for the normal state of high-temperature superconductors (HTSCs).

The mysterious spin gap in high-temperature superconductors: New NMR/NQR studies

We review work, done mainly at the author’s laboratory, on the spin gap which is the NMR manifestation of the pseudogap observed in the normal state of high-temperature superconductors. The relation of the spin gap to an electronic crossover in YBa2Cu4O8 is discussed. A possible explanation of both effects by assuming a charge density wave transition is presented. This suggestion is supported by measuring the isotope dependence of the spin gap in YBa2Cu4O8.

Persistence of pseudogap formation in quasi-2D systems with arbitrary carrier density

The existence of a pseudogap above the critical temperature has been widely used to explain the anomalous behaviour of the normal state of high-temperature superconductors. In two dimensions the existence of a pseudogap phase has already been demonstrated in a simple model. It can now be shown that the pseudogap phase persists even for the more realistic case where coherent interlayer tunneling is taken into account. The effective anisotropy is surprisingly large and even increases with increasing carrier density.

Pressure dependence of the resistivity and magnetoresistance in single-crystal

We have measured the resistivity and magnetoresistance of single-crystal as a function of hydrostatic pressure up to 12 kbar. The pressure dependence of the resistivity at temperatures near the metal - insulator transition exceeds , greater than that observed in the normal state of high-temperature superconductors. The extreme pressure sensitivity of the resistivity below the transition temperature provides evidence that the low-temperature phase is not a simple metal. The metal - insulator transition temperature decreases by , in accord with an expectation for greater pressure sensitivity in lower-doped materials with lower . The peak magnetoresistance defined as decreases with increasing pressure as the metal - insulator transition becomes less abrupt. The influence of thermal expansion upon the resistivity is expected to be large.

Marginal flux Fermi liquid in high temperature superconductors

The microscopic origin of the coherent behavior of the Fermi liquid in the normal state of high-temperature superconductors is considered beyond the frames of the perturbation theory. It is shown that the marginal behavior is the direct consequence of the confinement phenomenon that pins the chemical potential, and of hidden supersymmetry that leads to relativistic dispersion of the spectrum of quasiparticles.

Statistics in a dissipative medium.

We consider a gas of bosons or fermions where the particles are coupled dissipatively to an external medium. The effect of dissipation is to reduce the weight in the partition function of paths which permute particle positions, an effect which can ultimately destroy the distinctions between fermions and bosons. The crossover temperature kT, is given by the solutions of 4\ensuremath{\gamma}[\ensuremath{\psi}(1+\ensuremath{\gamma}/kT) -\ensuremath{\psi}(1)+ln2]=${\mathit{kT}}_{0}$-kT. $gamma = hbar eta /2 pi M---, \ensuremath{\eta} is the fraction coefficient, M is the particle mass, ${\mathit{T}}_{0}$ is the degeneracy temperature in the absence of dissipation, and k is Boltzmann's constant. Thus for overdamped particles \ensuremath{\gamma}\ensuremath{\gg}${\mathit{kT}}_{0}$ Bose and Fermi statistics are indistinguishable. We suggest that the results may provide a model for charge carriers in the normal state of high-temperature superconductors.

Evidence for the heavy-fermion normal state of high-temperature superconductors from the theory of spectroscopy.

We show that recent angle-resolved photoemission and inverse photoemission spectra of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ agree well with a numerical theory with no adjustable parameters based on the heavy-fermion normal state. The band structure deduced from the angle-resolved photoemission is in essential accord with the calculation. The three most obvious features of the inverse photoemission spectrum, the Fermi edge, the large width of the empty portion of the band, and the satellite at 2--3 eV are in agreement with the calculation. These agreements support the heavy-fermion state as the normal state of this material.