Superconducting Phase Diagram Research Articles

Superconductivity without diamagnetism in systems with broken time-reversal symmetry

We analyze the phase diagram of superconductors with a broken time-reversal symmetry in an external magnetic field. Ferromagnetism (broken time-reversal symmetry) originates either from the electron spin or the intrinsic angular momentum of Cooper pairs (chiral $p$-wave superconductors such as ${\text{Sr}}_{2}{\text{RuO}}_{4}$). In addition to the Meissner and the mixed states, the phase diagram includes also the cryptoferromagnetic state with intrinsic domain structure independent of sample shape and size. The cryptoferromagnetic state is not diamagnetic and can be detected by observation of magnetization curves predicted in the present analysis.

Quantum oscillations in an overdoped high-Tc superconductor

As well as their elevated superconducting transition temperatures (Tcs), the copper-oxide superconductors display a variety of novel physical properties that have inspired many theories to explain their electronic ground state. One topic of note is the nature of the metallic phase in these compounds, in particular, how coherent electronic states or quasiparticles emerge from the antiferromagnetic insulator with doping — the addition of trace amounts of impurities. Vignolle et al. report the observation of quantum oscillations, that is, oscillations in the electrical and magnetic response as a function of magnetic field, in the overdoped superconductor Tl2Ba2CuO6+δ. These oscillations point to the existence of a large Fermi surface of well-defined quasiparticles covering two-thirds of the Brillouin zone. The measurements confirm the applicability of a generalized Fermi-liquid picture to the overdoped side of the superconducting dome and mean that the important charge carriers involved in superconductivity are essentially similar to those found in normal metals. This paper reports the observation of quantum oscillations in the overdoped superconductor Tl2Ba2CuO6+δ that show the existence of a large Fermi surface of well-defined quasiparticles covering two-thirds of the Brillouin zone. These measurements firmly establish the applicability of a generalized Fermi-liquid picture on the overdoped side of the superconducting dome. The nature of the metallic phase in the high-transition-temperature (high-Tc) copper oxide superconductors, and its evolution with carrier concentration, has been a long-standing mystery1. A central question is how coherent electronic states, or quasiparticles, emerge from the antiferromagnetic insulator with doping. Recent quantum oscillation experiments on lightly doped copper oxides have shown evidence for small pockets of Fermi surface2,3,4,5, the formation of which has been associated with the opening of the pseudogap—an anisotropic gap in the normal state excitation spectrum of unknown origin1. As the doping is increased, experiments suggest that the full Fermi surface is restored6,7, although the doping level at which the pseudogap closes and the nature of the electronic ground state beyond this point have yet to be determined. Here we report the observation of quantum oscillations in the overdoped superconductor Tl2Ba2CuO6+δ that show the existence of a large Fermi surface of well-defined quasiparticles covering two-thirds of the Brillouin zone. These measurements confirm that, in overdoped superconducting copper oxides, coherence is established at all Fermi wavevectors, even near the zone boundary where the pseudogap is maximal and electronic interactions are strongest; they also firmly establish the applicability of a generalized Fermi-liquid picture on the overdoped side of the superconducting phase diagram.

Field Re-entrant Superconductivity Induced by the Enhancement of Effective Mass in URhGe

High quality single crystals of a ferromagnetic superconductor URhGe were successfully grown. The electrical resistivity was measured for the field along b -axis in the orthorhombic crystal structure in order to study precisely its field re-entrant superconductivity which occurs in the vicinity of the field H R , where the easy magnetization switches from c - to b -axis. The field re-entrant superconductivity of URhGe is analyzed with special focus on its dependence with the value of residual resistivity ρ 0 and the coefficient A of T 2 term of the resistivity. The experimental results are well explained by a crude model related with the field dependence of the effective mass m * , where the corresponding critical temperature T sc ( m * ) and the upper critical field H c2 are strongly enhanced. Discussion is made on the interplay between magnetic and superconducting phase diagram as well as the link between T sc and A in other heavy fermion superconductors.

Effect of Structural Parameters on Superconductivity in Fluorine-Free LnFeAsO1-y (Ln = La, Nd)

The crystal structure of LnFeAsO 1- y (Ln = La, Nd) has been studied by the powder neutron diffraction technique. The superconducting phase diagram of NdFeAsO 1- y is established as a function of oxygen content which is determined by Rietveld refinement. The small As–Fe bond length suggests that As and Fe atoms are connected covalently. FeAs 4 -tetrahedrons transform toward a regular shape with increasing oxygen deficiency. Superconducting transition temperatures seem to attain maximum values for regular FeAs 4 -tetrahedrons.

Impact of oxygen annealing on the heat capacity and magnetic resonance of superconductingPr0.88LaCe0.12CuO4−δ

We use thermodynamic and neutron-scattering measurements to study the effect of oxygen annealing on the superconductivity and magnetism in Pr0.88LaCe0.12CuO4�. Although the transition temperature Tc measured by susceptibility and superconducting coherence length increases smoothly with gradual oxygen removal from the annealing process, bulk superconductivity, marked by a specific-heat anomaly at Tc and the presence of a neutron magnetic resonance, only appears abruptly when Tc is close to the largest value. These results suggest that the effect of oxygen annealing must first be determined in order to establish a Ce doping dependence of antiferromagnetism and superconductivity phase diagram for electron-doped copper oxides.

Experimental evidence of two distinct charge carriers in underdoped cuprate superconductors

We present the results on heavily underdoped ${\text{Y}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{Ba}}_{2}{\text{Cu}}_{3}{\text{O}}_{6+y}$, which provide the evidence that the doping mechanism (cation substitution or oxygen loading) directly determines whether the corresponding injected mobile holes contribute to superconductivity or only to high-temperature transport. We argue that this hole tagging is a signature of the complexities of single-hole doping in Mott insulators, and it calls for a subtler description of the correlated bands than the usual one. We also map in great detail the underdoped superconducting phase diagram ${T}_{c}$ vs hole doping, which shows that the total number of mobile holes is not the driving parameter for superconductivity.

Double superconducting transition in the filled skutteruditePrOs4Sb12and sample characterizations

Through the characterization of many samples of the filled skutterudite compound ${\text{PrOs}}_{4}{\text{Sb}}_{12}$, we found that even though the double superconducting transition in the specific heat (${T}_{c1}\ensuremath{\sim}1.89\text{ }\text{K}$ and ${T}_{c2}\ensuremath{\sim}1.72\text{ }\text{K}$) appears in samples of good quality, there are various pieces of evidence that cast doubt on its intrinsic nature. First, three samples exhibit a single sharp transition of $\ensuremath{\sim}15\text{ }\text{mK}$ width at ${T}_{c}\ensuremath{\sim}1.7\text{ }\text{K}$. The quality of the samples exhibiting a single transition is clearly better; the width of the transition is smallest and the normalized specific heat jump ${\phantom{(}(C\ensuremath{-}{C}_{\text{normal}})/{C}_{\text{normal}})}_{{T}_{c}}$ is larger than the sum of the two specific heat jumps when a double transition exists. Second, the ratio of the two specific heat jumps $\frac{\ensuremath{\Delta}C({T}_{c1})}{\ensuremath{\Delta}C({T}_{c2})}$ exhibits a wide variation with samples not only among different batches but also even within a batch. Finally, this ratio was strongly reduced by polishing a sample down to $\ensuremath{\sim}100\text{ }\ensuremath{\mu}\text{m}$, which points to bulk inhomogeneities as an origin of the spurious transition. Our x-ray diffraction analysis points to Pr-vacancy problem in the samples, but it does not show that they are the origin of the double transition. We provide the superconducting phase diagram under magnetic field of a sample exhibiting a single transition and fit the curve ${H}_{c2}(T)$ with a two-band model taking into account the appropriate values for the gap as deduced from thermal conductivity measurements.

Superconductivity phase diagrams for the electron-doped cupratesR2−xCexCuO4(R=La, Pr, Nd, Sm, and Eu)

The superconductivity phase diagrams of electron-doped cuprates of the form ${R}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ (with $R=\mathrm{La}$, Pr, Nd, Sm, and Eu) have been determined for cerium compositions $0<x<0.36$ in a consistent series of epitaxial thin films grown by reactive molecular beam epitaxy. The use of epitaxial thin films allows the growth of materials away from thermodynamical equilibrium expanding the accessible phase space beyond the availability of bulk material. The superconducting phase space systematically increases with the rare-earth ionic size. The doping concentration where the maximal transition temperature occurs in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ is considerably shifted to lower doping $(x\ensuremath{\sim}0.09)$ compared to ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ $(x\ensuremath{\sim}0.15)$. At the same time, the width of the superconducting region is broadened.

Effect of Pressure and Magnetic Field on the Superconducting State of a Heavy Fermion Superconductor NpPd5Al2

We have carried out the electrical resistivity measurements under high pressures and high magnetic fields in NpPd 5 Al 2 in order to investigate a quantum critical behavior and the stability of the superconducting phase. A T -linear dependence of the electrical resistivity is changed into the Fermi liquid relation of ρ∼ A T 2 under the magnetic fields, and A value indicates a divergent feature around the upper critical field H c2 , indicating the existence of a field-tuned quantum critical point (QCP) around H c2 . With increasing pressure, the superconducting transition temperature T sc decreases and becomes zero above 5.7 GPa, together with a decrease of the upper critical field, indicating that the electronic state of NpPd 5 Al 2 is located in the vicinity of QCP at ambient pressure and deviates from QCP with increasing pressure. Furthermore, we have constructed the superconducting phase diagram at 3.71 GPa.

Superconductivity in Sc metal and Li(Mg) alloy under extreme pressure

Li and Sc are two of the 23 elements which only superconduct under high pressure. In previous studies T c for Li reaches 14 K at 30 GPa, but for Sc only 0.35 K at 21 GPa. We determined T c ( P ) for Sc to be 74.2 GPa and found that T c increases monotonically with pressure to 8.2 K. Changes in the superconducting phase diagram of monovalent Li are studied upon alloying with 10% divalent Mg.

Unconventional superconductivity and antiferromagnetic quantum phase transition in [formula omitted]: 115In-NQR study under pressure

We report on the pressure ( P)-induced phase diagram of antiferromagnetism (AFM) and superconductivity (SC) which emerges at their border at zero magnetic field. The nuclear-quadrupole-resonance (NQR) measurements have revealed that AFM can take place in the superconducting state with T c ∼ 2.1 K at P = 2.05 GPa and the AFM quantum phase transition (QPT) occurs near P = 2.1 GPa . These results reveal the existence of the tetracritical point in the P– T phase diagram of AFM and SC in CeRnIn 5 . We also found the gapless nature in the uniformly coexisting phase of AFM and SC, indicating an intimate relationship between AFM and SC.

Effects of a magnetic field on superconductivity and quantum criticality in quasi-two-dimensional systems with Dirac electrons

We study the effects of an external magnetic field on the superconducting phase diagram of a quasi-two-dimensional system of Dirac electrons at an arbitrary temperature. At zero temperature, there is a quantum phase transition connecting a normal and a superconducting phase, occurring at a critical line that corresponds to a magnetic field dependent critical coupling parameter, which should be observed in planar materials containing Dirac electrons, such as Cu x TiSe 2 . Moreover, the superconducting gap is obtained as a function of temperature, magnetic field and coupling parameter ( λ R ) . From this, we extract the critical magnetic field B c as a function of the temperature. For small values of B c , we obtain a linear decay of the critical field, which is similar to the behavior observed experimentally in the copper doped dichalcogenide Cu x TiSe 2 and also in intercalated graphite.

High-field superconducting phase diagrams including Fulde-Ferrell-Larkin-Ovchinnikov vortex states

Motivated by a striking observation of a Fulde-Ferell-Larkin-Ovchinnikov (FFLO) vortex state in the heavy fermion material $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$ in fields perpendicular to the superconducting planes $(\mathbf{H}\ensuremath{\Vert}c)$, superconducting phase diagrams including an FFLO state of uniaxially anisotropic superconductors are systematically studied. In the ballistic limit with no quasiparticle (QP) relaxation, the high-field superconducting state in $\mathbf{H}\ensuremath{\Vert}c$ and in the low temperature limit should be not the FFLO state modulating along $\mathbf{H}$, appearing in $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$, but a different vortex state with a modulation perpendicular to the field. It is argued that an enhancement near ${H}_{c2}(0)$ of the QP relaxation rate, presumably originating from a nonsuperconducting quantum critical fluctuation, in this material is the origin of the absence of the latter modulated state and of the strange $\mathbf{H}\ensuremath{\Vert}c$ phase diagram in which the FFLO state is apparently different from that in $\mathbf{H}\ensuremath{\perp}c$.

Tuning Co Valence State in Cobalt Oxyhydrate Superconductor by Postreduction

We report a successful tuning of Co valence state in cobalt oxyhydrate superconductor via a facile postreduction using NaOH as reducing agent. The change in Co valence was precisely determined by measuring the volume of the released oxygen. The possible hydronium incorporation was greatly suppressed in concentrated NaOH solution, making the absolute Co valence determinable. As a result, an updated superconducting phase diagram was obtained, which shows that the superconducting transition temperature increases monotonically with increasing Co valence in a narrow range from +3.58 to +3.65.

Pressure-induced superconductivity in Sc to74GPa

Using a diamond anvil cell with nearly hydrostatic helium pressure medium we have significantly extended the superconducting phase diagram Tc(P) of Sc, the lightest of all transition metals. We find that superconductivity is induced in Sc under pressure, Tc increasing monotonically to 8.2 K at 74.2 GPa. The Tc(P) dependences of the trivalent d-electron metals Sc, Y, La, and Lu are compared and discussed within a simple s-d charge transfer framework.

Quantum phase diagram of antiferromagnetism and superconductivity with a tetracritical point inCeRhIn5in zero magnetic field

We report on a pressure-temperature phase diagram of antiferromagnetism (AFM) and superconductivity (SC) in $\mathrm{Ce}\mathrm{Rh}{\mathrm{In}}_{5}$ studied by the nuclear-quadrupole-resonance measurements under pressure. It is demonstrated that an AFM phase transition takes place inside the superconducting state. This result reveals that a tetracritical point exists in the phase diagram of AFM and SC at zero magnetic field. The finding of unconventional SC characteristics in the uniformly coexisting phase of AFM and SC suggests that both phases may be mediated by the same magnetic interaction.

NMR Initiatives on Understanding High‐temperature Superconductivity

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Doping-dependent reduction of the Cu nuclear magnetic resonance intensity in the electron-doped superconductorPr2−xCexCuO4

$^{63}\mathrm{Cu}$ nuclear magnetic resonance (NMR) measurements on the electron-doped high-temperature superconducting cuprates ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ reveal a large temperature-dependent reduction in the Cu NMR intensity for $x\ensuremath{\leqslant}0.125$ and below room temperature. Our results show that the previous report of a reduction in the $^{63}\mathrm{Cu}$ NMR intensity in ${\mathrm{Sr}}_{0.9}{\mathrm{La}}_{0.1}\mathrm{Cu}{\mathrm{O}}_{2}$ with 0.1 doped electrons per Cu is not specific to this superconductor. Rather, the doping-dependent reduction appears to be intrinsic to the electron-doped superconductors. Our results provide further evidence that there is a symmetry in the electron-doped and hole-doped superconducting phase diagrams where a significant temperature-dependent reduction of the Cu NMR intensity occurs for less than $\ensuremath{\sim}0.13$ doped holes or doped electrons per Cu.

Lattice parameter and superconductivity in Nax(H3O)zCoO2 · yH2O

We synthesized Nax(H3O)zCoO2·yH2O samples with various chemical compositions, and measured their magnetic properties to draw the superconducting phase diagram. Although the superconductivity is very sensitive to both the Na content and the Co valence, the c-axis length seems to be a good parameter governing the phase diagram. The relation between the c-axis length and superconductivity is discussed.

Magnetization measurements on HoNi2B2C single crystals before and after neutron irradiation

A single crystal of HoNi2B2C was fully characterised by magnetization measurements. The magnetic and the superconducting phase diagram were determined in different crystal directions and the superconductive properties evaluated as a function of temperature and applied field. The critical current density was calculated from magnetization loops using the Bean model. The results on the critical current density reveal bulk pinning in those regions of the phase diagram, where superconductivity is not suppressed by metamagnetically ordered structures of the Ho 4f-moments. These measurements were repeated after neutron irradiation of the sample.