Non-superconducting State Research Articles

Fragile charge order in the nonsuperconducting ground state of the underdoped high-temperature superconductors

The normal state in the hole underdoped copper oxide superconductors has proven to be a source of mystery for decades. The measurement of a small Fermi surface by quantum oscillations on suppression of superconductivity by high applied magnetic fields, together with complementary spectroscopic measurements in the hole underdoped copper oxide superconductors, point to a nodal electron pocket from charge order in YBa2Cu3(6+δ). Here, we report quantum oscillation measurements in the closely related stoichiometric material YBa2Cu4O8, which reveals similar Fermi surface properties to YBa2Cu3(6+δ), despite the nonobservation of charge order signatures in the same spectroscopic techniques, such as X-ray diffraction, that revealed signatures of charge order in YBa2Cu3(6+δ). Fermi surface reconstruction in YBa2Cu4O8 is suggested to occur from magnetic field enhancement of charge order that is rendered fragile in zero magnetic fields because of its potential unconventional nature and/or its occurrence as a subsidiary to more robust underlying electronic correlations.

Two Sources of Paramagnetic Curie-Type Contribution to the Normal State Magnetic Susceptibility of HTSC YBa<sub>2</sub>Cu<sub>3</sub>O<sub>6+δ</sub>

The experimental evidence for the existence of chain paramagnetic Curie-type contribution to the temperature dependence of the normal (non-superconducting) state magnetic susceptibility χ (T) was obtained for pure high-Tc superconductors (HTSC) YBa2Cu3O6+δ series with different oxygen contents (0.6 < δ ≤ 1). It was shown that this contribution became clearly evident on curves χ (T) only at the temperature T < 150 K. It increases with the amount of oxygen vacancies increase in Cu1–O4 chains, and also depends on the way of the oxygen vacancies ordering.

Wavelength-dependent optical enhancement of superconducting interlayer coupling inLa1.885Ba0.115CuO4

We analyze the pump wavelength dependence for the photo-induced enhancement of interlayer coupling in La1.885Ba0.115CuO4, which is promoted by optical melting of the stripe order. In the equilibrium superconducting state (T < Tc = 13 K), in which stripes and superconductivity coexist, time-domain THz spectroscopy reveals a photo-induced blue-shift of the Josephson Plasma Resonance after excitation with optical pulses polarized perpendicular to the CuO2 planes. In the striped, non-superconducting state (Tc < T < T_SO = 40 K) a transient plasma resonance similar to that seen below Tc appears from a featureless equilibrium reflectivity. Most strikingly, both these effects become stronger upon tuning of the pump wavelength from the mid-infrared to the visible, underscoring an unconventional competition between stripe order and superconductivity, which occurs on energy scales far above the ordering temperature.

Massive electrons signify correlations

Superconductivity Thirty years on, and the mechanism of superconductivity in copper-oxide superconductors remains a mystery. Knowledge of their normal nonsuperconducting state is also incomplete; however, we do know that the more robust the superconductivity, the higher the magnetic fields required to suppress it. Ramshaw et al. studied samples of three different compositions of the copper-oxide YBa2Cu3O6+δ in magnetic fields exceeding 90 T. They found that as the oxygen content increased toward the point of the maximum transition temperature, the conducting electrons became heavier and heavier. This mass enhancement reflected an increase in electronic correlations, which in turn may be a signature of a quantum critical point. Science , this issue p. [317][1] [1]: /lookup/doi/10.1126/science.aaa4990

Sign reversal of the Hall response in a crystalline superconductor

We consider the Hall conductivity due to the motion of a vortex in a lattice-model of a clean superconductor, using a combination of general arguments, unrestricted Hartree-Fock calculations, and exact diagonalization. In the weak coupling limit, $k_F \xi \gg 1$, the sign of the Hall response of the superconducting state is the same as that of the normal (non-superconducting) state. For intermediate and strong coupling, however, ($k_F \xi \sim 1$) we find that the sign of the Hall response in the superconducting state can be opposite to that of the normal state. In addition, we find that the sign reversal of the Hall response is correlated with a discontinuous change in the density profile at the vortex core. Implications for experiments in the cuprate superconductors are discussed.

Controlling Superconductivity in La2−xSr x CuO4+δ by Ozone and Vacuum Annealing

In this study we performed a series of ozone and vacuum annealing experiments on epitaxial La2−xSr x CuO4+δ thin films. The transition temperature after each annealing step has been measured by the mutual inductance technique. The relationship between the effective doping and the vacuum annealing time has been studied. Short-time ozone annealing at 470 °C oxidizes an underdoped film all the way to the overdoped regime. The subsequent vacuum annealing at 350 to 380 °C slowly brings the sample across the optimal doping point back to the undoped, non-superconducting state. Several ozone and vacuum annealing cycles have been done on the same sample and the effects were found to be repeatable and reversible Vacuum annealing of ozone-loaded La2−xSr x CuO4+δ (LSCO) films is a very controllable process, allowing one to tune the doping level of LSCO in small steps across the superconducting dome, which can be used for fundamental physics studies.

A copper oxide's electronic structure

Physics![Figure][1] Superconducting copper-oxide electronic structurePHOTO: NICOLLE R. FULLER Physicists still do not understand why some copper-oxide compounds become superconducting at relatively high temperatures. Even more basic issues have remained controversial as well, such as the electronic structure of the normal (i.e., nonsuperconducting) state from which superconductivity emerges. Sebastian et al. chose YBa2Cu3O6.56 as a good representative of a subclass of these compounds and used high magnetic fields to suppress its superconductivity and reach its normal state. They analyzed the wiggles of the electrical resistivity to map out the shape of the so-called Fermi surface, which separates the quantum states filled with electrons from the empty ones. They found an undulating shape that suggested that the electronic structure was organized differently from what one expects based on the crystal lattice structure alone. Nature 10.1038/nature13326 (2014). [1]: pending:yes

Possible way to turn MgFeGe into an iron-based superconductor

We propose that the contrasting low-temperature behaviors observed experimentally among isostructural and isoelectronic materials, like non-superconducting and nonmagnetic MgFeGe, magnetically ordered NaFeAs, and superconducting LiFeAs, can be well understood from itinerant weak coupling limit. We find that stronger $(\pi,\pi)$ instability appearing in the d$_{x^2-y^2}$ orbital of NaFeAs is responsible for the occurrence of weak magnetism while weaker but still prominent $(\pi,\pi)$ instability in LiFeAs leads to a superconducting state. In contrast, multiple competing instabilities coexisting in orbital-resolved momentum-dependent susceptibilities, serving as magnetic frustrations from itinerant electrons, may account for the nonmagnetic state in MgFeGe, while poorer Fermi surface nesting leads to a non-superconducting state. Based on above findings, we predict a possible way to make MgFeGe a new Fe-based superconductor.

Quench Distribution During an Overcurrent Fault Condition in Highly Homogeneous REBCO Coated Conductor

One of the most important factor for resistive-type fault current limiters is uniform voltage distribution under fault condition. Homogeneous distribution of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and resistance of nonsuperconducting state in the longitudinal direction are essential for uniform quenching. Recently, we have succeeded in manufacturing the highly homogeneous superconducting RE BCO coated conductor. In this paper, we report the voltage distribution in our coated conductor under fault condition, at 50 Hz for a half cycle. The experimental results show that applied voltage, which is necessary to quench over the entire length of sample, depends on I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and normal state resistance on T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> .

Spectroscopic evidence for Fermi liquid-like energy and temperature dependence of the relaxation rate in the pseudogap phase of the cuprates

Cuprate high-Tc superconductors exhibit enigmatic behavior in the nonsuperconducting state. For carrier concentrations near "optimal doping" (with respect to the highest Tcs) the transport and spectroscopic properties are unlike those of a Landau-Fermi liquid. On the Mott-insulating side of the optimal carrier concentration, which corresponds to underdoping, a pseudogap removes quasi-particle spectral weight from parts of the Fermi surface and causes a breakup of the Fermi surface into disconnected nodal and antinodal sectors. Here, we show that the near-nodal excitations of underdoped cuprates obey Fermi liquid behavior. The lifetime τ(ω, T) of a quasi-particle depends on its energy ω as well as on the temperature T. For a Fermi liquid, 1/τ(ω, T) is expected to collapse on a universal function proportional to (ℏω)(2) + (pπk(B)T)(2). Magneto-transport experiments, which probe the properties in the limit ω = 0, have provided indications for the presence of a T(2) dependence of the dc (ω = 0) resistivity of different cuprate materials. However, Fermi liquid behavior is very much about the energy dependence of the lifetime, and this can only be addressed by spectroscopic techniques. Our optical experiments confirm the aforementioned universal ω- and T dependence of 1/τ(ω, T), with p ∼ 1.5. Our data thus provide a piece of evidence in favor of a Fermi liquid-like scenario of the pseudogap phase of the cuprates.

Energetics of superconductivity in the two-dimensional Hubbard model

The energetics of the interplay between superconductivity and the pseudogap in high-temperature superconductivity is examined using the eight-site dynamical cluster approximation to the two-dimensional Hubbard model. Two regimes of superconductivity are found: a weak-coupling/large-doping regime in which the onset of superconductivity causes a reduction in potential energy and an increase in kinetic energy, and a strong-coupling regime in which superconductivity is associated with an increase in potential energy and a decrease in kinetic energy. The crossover between the two regimes is found to coincide with the boundary of the normal-state pseudogap, providing further evidence of the unconventional nature of superconductivity in the pseudogap regime. However, the absence, in the strongly correlated but nonsuperconducting state, of discernibly nonlinear response to an applied pairing field suggests that resonating valence bond physics is not the origin of the kinetic-energy driven superconductivity.

Magnetoresistance studies of homogenously disordered 3-dimensional NbN thin films

We present magnetoresistance studies on a series of homogenously disordered 3-dimensional NbN thin films with disorder ranging from the moderately clean limit (kFl~10.12) to the extremely dirty limit where the superconducting ground state is completely destroyed (kFl~0.42). We find that for samples with kFl >1, the magnetoresistance is positive up to 12T and as disorder increases it decays more slowly with temperature. On the other hand, for samples with kFl<1, we observe a peak in the magnetoresistance which vanishes at a temperature close to the pseudogap temperature of disordered superconducting samples. These observations are consistent with the idea of the disorder-driven non-superconducting state being comprised of pre-formed Cooper pairs.

Coherent transport in extremely underdoped Nd1.2Ba1.8Cu3Oz nanostructures

Proximity-effect and resistance magneto-fluctuation measurements in submicron Nd1.2Ba1.8Cu3Oz (NBCO) nano-loops are reported to investigate coherent charge transport in the non-superconducting state. We find an unexpected inhibition of Cooper pair transport, and a destruction of the induced superconductivity, by lowering the temperature from 6 K to 250 mK. This effect is accompanied by a significant change in the conductance-voltage characteristics and the zero bias conductance response to the magnetic field, pointing to the activation of a strong pair-breaking mechanism at lower temperature. The data are discussed in the framework of mesoscopic effects specific to superconducting nanostructures, proximity effect and high temperature superconductivity.

Stripes and electronic quasiparticles in the pseudogap state of cuprate superconductors

This article is devoted to a discussion of stripe and electron-nematic order and their connection to electronic properties in the pseudogap regime of copper-oxide superconductors. We review basic properties of these symmetry-breaking ordering phenomena as well as proposals which connect them to quantum-oscillation measurements. Experimental data indicate that these orders are unlikely to be the cause of the pseudogap phenomenon, implying that they occur on top of the pseudogap state which itself is of different origin. Specifically, we discuss the idea that the non-superconducting pseudogap ground state hosts electron-like quasiparticles which coexist with a spin liquid, realizing a variant of a fractionalized Fermi liquid. We speculate on how stripe order in such a pseudogap state might offer a consistent description of ARPES, NMR, quantum-oscillation, and transport data.

Mixed state of aπ-striped superconductor

A model of an anti-phase modulated d-wave superconductor has been proposed to describe the decoupling between Cu-O planes in 1/8 doped La$_{2-x}$Ba$_{x}$CuO$_{4}$. Unlike a uniform d-wave superconductor, this model exhibits an extended Fermi surface. Within Bogoliubov-de Gennes theory, we study the mixed state of this model and compare it to the case of a uniform d-wave superconductor. We find a periodic structure of the low-energy density of states, with a period that is proportional to $B$, corresponding to Landau levels that are a coherent mixture of particles and holes. These results are also discussed in the context of experiments which observe quantum oscillations in the cuprates, and are compared to those for models in which the Fermi surface is reconstructed due to translational symmetry breaking in the non-superconducting state and to a model of a Fermi-arc metal.

Temperature and Pressure Evolution of the Crystal Structure of Ax(Fe1–ySe)2 (A = Cs, Rb, K) Studied by Synchrotron Powder Diffraction

Temperature-dependent synchrotron powder diffraction on Cs0.83(Fe0.86Se)2 revealed first order I4/m to I4/mmm structural transformation around 216{\deg}C associated with the disorder of the Fe vacancies. Irreversibility observed during the transition is likely associated with a mobility of intercalated Alkali atoms. Pressure-dependent synchrotron powder diffraction on Cs0.83(Fe1-ySe)2, Rb0.85(Fe1-ySe)2 and K0.8(Fe1-ySe)2 (y ~ 0.14) indicated that the I4/m superstructure reflections are present up to pressures of 120 kbar. This may indicate that the ordering of the Fe vacancies is present in both superconducting and non-superconductive states.

A Comparison of Methods for Computing the Residual Resistivity Ratio of High-Purity Niobium.

We compare methods for estimating the residual resistivity ratio (RRR) of high-purity niobium and investigate the effects of using different functional models. RRR is typically defined as the ratio of the electrical resistances measured at 273 K (the ice point) and 4.2 K (the boiling point of helium at standard atmospheric pressure). However, pure niobium is superconducting below about 9.3 K, so the low-temperature resistance is defined as the normal-state (i.e., non-superconducting state) resistance extrapolated to 4.2 K and zero magnetic field. Thus, the estimated value of RRR depends significantly on the model used for extrapolation. We examine three models for extrapolation based on temperature versus resistance, two models for extrapolation based on magnetic field versus resistance, and a new model based on the Kohler relationship that can be applied to combined temperature and field data. We also investigate the possibility of re-defining RRR so that the quantity is not dependent on extrapolation.

Mind the pseudogap

The discovery of predicted collective electronic behaviour in copper-oxide superconductors in the non-superconducting state provides clues to unlocking the 24-year-old mystery of high-temperature superconductivity. See Letter p.283 The pseudogap phenomenon, a discontinuity in the energy level of a material's electronic spectrum, is a universal characteristic of the high transition temperature (Tc) copper oxides. The nature of the pseudogap has been a central question in condensed matter physics for more than a decade, but many of its properties remain unexplained. Recent studies have pointed to the universal existence of an unusual magnetic order below T*, the temperature below which the anomalous properties associated with the pseudogap become apparent. If confirmed, this would have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. The results of inelastic neutron scattering experiments on the superconductor HgBa2CuO4+δ (Hg1201) now reveal a fundamental collective magnetic mode associated with the unusual order, providing further support for this picture.

Interlayer magnetotransport in the overdoped cuprateTl2Ba2CuO6+x: Quantum critical point and its downslide in an applied magnetic field

Fundamental explanations of high-temperature (high-${T}_{c}$) superconductivity must account for the profound differences in the properties of the ``normal'' (nonsuperconducting) state at the two extremes of charge doping: heavy and light. On the light doping side, its properties clearly violate the standard Fermi-liquid theory of metals. The key to the nature of superconducting pairing lies in understanding the transition to a conventional behavior on the overdoped side. We report a convergence of the pseudogap energy scale and the boundary that separates unconventional from a conventional metal in the zero-temperature limit, both boundaries framing a V-shaped area of ``strange metal'' state in the temperature-doping phase space. By accessing the low-temperature regions of the phase diagram via a high-field interlayer magnetotransport in heavily doped ${\text{Tl}}_{2}{\text{Ba}}_{2}{\text{CuO}}_{6+x}$, we show that the pseudogap boundary has the hallmarks of a quantum phase transition with a zero entropy jump. The critical doping (linkage) point consistently downshifts with magnetic field in unison with the suppression of ${T}_{c}$, suggesting that quantum critical fluctuations that destabilize the pseudogap are connected to the superconductivity with high-${T}_{c}$.

Superconductivity induced in iron telluride films by low-temperature oxygen incorporation

We report superconductivity induced in films of the nonsuperconducting, antiferromagnetic parent material FeTe by low-temperature oxygen incorporation in a reversible manner. X-ray absorption shows that oxygen incorporation changes the nominal Fe valence state from $2+$ in the nonsuperconducting state to mainly $3+$ in the superconducting state. Thus superconductivity in O-doped FeTe occurs in a quite different charge and strain state than the more common ${\text{FeTe}}_{1\ensuremath{-}x}{\text{Se}}_{x}$. This work also suggests a convenient path for conducting doping experiments in situ with many measurement techniques.