Pressure Dependence Of Tc Research Articles

Pressure-induced suppression of charge density wave and emergence of superconductivity in 1T−VSe2

We report pressure evolution of charge density wave (CDW) order and emergence of superconductivity (SC) in 1T-VSe2 single crystal by studying resistance and magnetoresistance behavior under high pressure. With increasing quasi-hydrostatic pressure the CDW order enhances with increase ofthe ordering temperature up to 240K at 12 GPa. Upon further increase of pressure, the resistance anomaly due to CDW order gets suppressed drastically and superconductivity emerges at ~15 GPa, with the onset critical temperature (Tc) ~ 4K. The pressure dependence of Tc is found negligible, different from the significant increase or a dome-shape seen in iso-structural layered diselenide superconductors. The high pressure magnetoresistance and Hall measurements suggest successive electronic structural changes with Fermi surface modifications at 6 GPa and 12GPa. From the observed negative magnetoresistance in this pressure range and absence of coexisting CDW and SC phases, we propose that intra-layer spin-fluctuation can play a role in the emergence of superconductivity in the high pressure phase.

A Review of Magneto-Elastic Interaction and Its Theoretical Descriptions in Type-II Superconductors

It is well known that the interaction between defect-induced strain field and vortex lattice (VL) gives rise to vortex pinning behaviors. Likewise, magneto-elastic (ME) interaction is caused by the difference between specific volumes of normal phase and superconducting (SC) phase. Acting as inhomogeneous strain sources, the normal-state vortex cores produce local deformation in the surrounding SC matter and thus affect VL energy. ME interaction belongs to the fundamental physics of interdependences between vortex matter and crystal lattice. The review starts with the effects of ME interaction on magnetization, vortex morphology and intervortex interaction. In Sect. 2, theoretical descriptions of ME interaction are classified in terms of different simplifications. The core ideas and outstanding results of these theories are given. It is followed by Sect. 3, where the correlation and difference between ME and pinning interaction are outlined. Although pinning differs from ME interaction in their formation mechanisms, the pinning interaction is established upon the vortex-induced strain field from Ginzburg-Landau (GL) viewpoint. Section 4 goes further with the GL treatment for solving ME and pinning problem; the general thoughts and steps can be found in this section. The last section gives some outlooks which involve pressure dependence of Tc and anisotropy.

Enhancement of superconducting properties and flux pinning mechanism on Cr0.0005NbSe2 single crystal under Hydrostatic pressure

Superconducting properties of Cr0.0005NbSe2 (Tc~6.64 K) single crystals have been investigated through the temperature dependent resistivity (~8 GPa) and DC magnetization (~1 GPa) measurements. Further, the critical current density (Jc) as a function of applied magnetic field has been studied from magnetic isotherms. The vortex pinning mechanisms have also been systematically analyzed using weak collective pinning theory as a function of pressure. The Jc corresponds to the flux flow enhanced by the application of pressure due to increase of Tc and vortex changes. We found that the pressure is responsible for the spatial variations in the charge carrier mean free path (δl pinning). We find that core point pinning is more dominant than surface pinning which is caused by the application of pressure. In addition, Jc(H = 0) increases from 3.9 × 105 (0 GPa) to 1.3 × 106 (1.02 GPa) A/cm2 at 2 K as the pressure is increased from normal pressure to 1.02 GPa. The pressure dependence of Tc (dTc/dP) becomes 0.91 K/GPa and 0.75 K/GPa from magnetization and resistivity measurements respectively. We found that the pressure promotes the anisotropy nature, and decrease of coherence length and resulting in pathetic interface of the vortex core with pinning centers.

High- Tc superconductivity up to 55 K under high pressure in a heavily electron doped Li0.36(NH3)yFe2Se2 single crystal

We report a high-pressure study on the heavily electron doped Lix(NH3)yFe2Se2 single crystal by using the cubic anvil cell apparatus. The superconducting transition temperature Tc = 44 K at ambient pressure is first suppressed to below 20 K upon increasing pressure to Pc = 2 GPa, above which the pressure dependence of Tc(P) reverses and Tc increases steadily to ca. 55 K at 11 GPa. These results thus evidenced a pressure-induced second high-Tc superconducting (SC-II) phase in Lix(NH3)yFe2Se2 with the highest Tcmax = 55K among the FeSe-based bulk materials. Hall data confirm that in the emergent SC-II phase the dominant electron-type carrier density undergoes a fourfold enhancement and tracks the same trend as Tc(P). Interesting, we find a nearly parallel scaling behavior between Tc and the inverse Hall coefficient for the SC-II phases of both Lix(NH3)yFe2Se2 and (Li,Fe)OHFeSe. The present work demonstrates that high pressure offers a distinctive means to further raising the maximum Tc of heavily electron doped FeSe-based materials by increasing the effective charge carrier concentration via a plausible Fermi surface reconstruction at Pc.

Pressure-structure relationships in the 10 K layered carbide halide superconductor Y2C2I2

The electronic structures of the 10 K layered yttrium carbide halide superconductor Y2C2I2 is characterized by bands of low dispersion and narrow peak-valley features in the electronic density of states at the Fermi level. In order to investigate to what extent the superconducting properties can be modified by external pressure we have studied the pressure dependence of the superconducting critical temperature and the crystal structure of Y2C2I2 to pressures of 7.4 GPa. Up to ~2.5 GPa we observe an increase of Tc from 10 K to about 12 K. A structural phase transition from a 1s to a 3s stacking variant occurs at about 2.5 GPa above which Tc rapidly decreases to a value of ~7.5 K at 7.5 GPa. Density functional calculations corroborate the structural phase transition to occur at a critical cell volume of ~270 Å3 corresponding to a pressure of ~2.4 GPa, in good agreement with the experimental findings. The pressure dependence of Tc and inter-atomic distances and angles are discussed with respect to the results of density functional calculations of the electronic and crystal structure.

Novel structures and superconductivities of calcium–lithium alloys at high pressures: A first-principles study

Exposing a material to high pressures can fundamentally influence its crystal and electronic structure, leading to the formation of new materials with unique physical and chemical properties. Here, we have conducted a systematic search for Ca–Li alloys by using a global minima search based on particle-swarm optimization algorithm in combination with density functional theory calculations. We predict that Calcium and Lithium with a high Ca composition CaLi, Ca2Li and Ca3Li exist, and a strikingly decomposition-combination-decomposition oscillating behavior with pressure is revealed. All predicted Ca–Li compounds are metallic and good electron–phonon superconductors with transition temperatures (Tc) of around 8–19 K. The superconductivity mainly originates from the low-energy Ca vibrations and the pressure dependence of Tc is dominated by the phonon softening/hardening.

Fabrication of new superconducting materials, CaxK1−xCy (0 < x < 1)

Metal intercalation to graphite produces various types of superconductors. The highest superconducting transition temperature Tc (onset temperature, Tconset, of 11.5 K) was found in Ca intercalated graphite, denoted CaC6. Tconset increased up to 15.1 K at 7.5 GPa, implying a positive pressure dependence. However, no new metal-intercalated graphite superconductors with Tconset higher than 11.5 K at ambient pressure have so far been reported. To search for new graphite superconductors, we successfully synthesized binary-element-intercalated graphite, CaxK1−xCy. Their structure resembles that of KC8. Tc increased continuously with increasing x. Furthermore, the pressure dependence of Tc in Ca0.6K0.4C8 was investigated over a wide pressure range from 0–43 GPa. Tc (= 9.6 K at 0 GPa) increased to 11.6 K at 3.3 GPa, and decreased to 2.0 K at 41 GPa. This behavior is similar to that of CaC6, albeit with a lower maximum Tc. X-ray diffraction patterns were measured under high pressures of 0–24 GPa, and suggest a structural transition at 15 GPa. Evidence is given for superconducting graphite involving binary metal intercalation.

Superconductivity in the presence of disorder in skutterudite-related La3Co4Sn13 and La3Ru4Sn13 compounds: Electrical transport and magnetic studies

La3Co4Sn13 and La3Ru4Sn13 were categorized as BCS superconductors. In a plot of the critical field Hc2 vs T, La3Ru4Sn13 displays a second superconducting phase at the higher critical temperature Tc⋆, characteristic of inhomogeneous superconductors, while La3Co4Sn13 shows bulk superconductivity below Tc. We observe a decrease in critical temperatures with external pressure and magnetic field for both compounds. Additionally, for La3Ru4Sn13 we find that dTc⋆/dP>dTc/dP. The pressure dependences of Tc are interpreted according to the McMillan theory and understood to be a consequence of lattice stiffening. The investigation of the superconducting state of La3CoxRu4−xSn13 shows a Tc⋆ that is larger then Tc for x < 4. This unique and unexpected observation is discussed as a result of the local disorder and/or the effect of chemical pressure when Ru atoms are partially replaced by smaller Co atoms.

Universal V-shaped temperature-pressure phase diagram in the iron-based superconductorsKFe2As2,RbFe2As2, andCsFe2As2

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor RbFe2As2, at a critical pressure Pc = 11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal V-shaped phase diagram for Tc vs P in these fully hole-doped 122 materials, when measured relative to the critical point (Pc, Tc). From measurements of the upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe the same two-fold jump in (1/Tc)(-dHc2/dT) across Pc, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of Pc. We discuss a possible link between scattering and pairing, and a scenario where a d-wave state favored by high-Q scattering at low pressure changes to a state with s+- symmetry favored by low-Q scattering at high pressure.

Superconductivity in K- and Na-doped BaFe2As2: What can we learn from heat capacity and pressure dependence of Tc

A brief overview of changes in the superconducting transition temperature under pressure and evolution of specific heat capacity jump at Tc for two related families of iron-based superconductors, Ba 1-x K x Fe 2 As 2 (0.2 ≤ x ≤ 1.0) and Ba 1-x Na x Fe 2 As 2 (0.2 ≤ x ≤ 0.9) will be given. For Ba 1-x K x Fe 2 As 2 the specific heat capacity jump at Tc measured over the whole extent of the superconducting dome shows clear deviation from the empirical, [Formula: see text], scaling (known as the BNC scaling) for x &gt; 0.7. At the same concentrations range apparent equivalence of effects of pressure and K -substitution on Tc fails. These observations suggests a significant change of the superconducting state for x &gt; 0.7. In contrast, the data for the large portion of Ba 1-x Na x Fe 2 As 2 (0.2 ≤ x ≤ 0.9) series follow the BNC scaling. However, the pressure dependence of Tc (measured up to ~12 kbar) have clear nonlinearities for Na concentration in 0.2–0.25 region, that may be consistent with Tc crossing the phase boundaries of the emergent, narrow, antiferromagnetic/tetragonal phase. Results will be discussed in context other studies of these two related families of iron-based superconductors.

Pressure dependence of Tc in LnFeAsO1-y (Ln = La, Ce, Nd, Tb)

We measured the temperature dependence of electrical resistivity of non-fluorine- substituted, oxygen-deficient LnFeAsO1-y (Ln = La, Ce, Nd, Tb) under hydrostatic high pressure up to '8 GPa in order to observe pressure dependence of superconducting transition temperature Tc. In LaFeAsO0.65, onset Tc initially enhances below 4 GPa, then decreases by applying further pressure. However, the zero-resistivity temperature does not show enhancement by applying pressure. In the case of NdFeAsO1-y and TbFeAsO1-y, Tc shows monotonic decrease as increasing pressure. Tc is much more likely to go down faster if Ln with small ionic size was taken. Therefore, the bulk superconductivity is suppressed finally at ~7 GPa in TbFeAsO0.7.

Pressure Dependence of the Superconducting Transition Temperature of Filled Skutterudites LaT4P12 (T = Fe, Ru)

The filled skutterudite intermetallic compounds with the general formula LnT4X12 (Ln = Lanthanide, T = transition metal, X = pnictogen) have attracted much attention for exhibiting novel physical phenomena, such as superconductivity,1) heavy fermion behavior,2) metal–insulator transition,3) magnetic ordering,4) and their prospect in thermoelectric applications.5) Among the various physical properties, at least 19 filled skutterudites have been reported to be superconductors with the superconducting transition temperature Tc up to 17K. Most of these have been classified as Bardeen– Cooper–Schrieffer (BCS) ones. In BCS superconductivity Tc generally decreases under pressure predominantly due to a stiffness of the lattice with increasing pressure.6) This general rule is followed for most filled skutterudite superconductors in the series of LnT4X12, except for T = Fe.7) In a previous high-pressure study7) in the LaM4P12 (M = Fe, Ru, Os) up to 1.8GPa, it has been found that the Tc of LaFe4P12 increases at a rate of 0.72K/GPa, and those of LaRu4P12 and LaOs4P12 decrease at rates 10.16 and 10.095K/GPa, respectively. The distinct pressure dependence of Tc has been interpreted in terms of two competing contributions: a depression of Tc due to a compression of the filled skutterudite sublattice, and an enhancement of Tc due to a compression of La and its immediate environment. However, their interpretation is based only on the features of the filled skutterudite structure. Although the model gives a feasible explanation for the unique positive pressure coefficient dTc=dP of LaFe4P12, La atoms are inside (FeP6)4 cages and well separated, which makes the model less convincing. Actually, electronic structure calculations8) on LaFe4P12 have implied that its superconductivity is primarily associated with the electrons of its phosphorus sublattice, which form a strong covalentbonded P4 ring and dominate the density of states at the Fermi energy NðEFÞ. Very recently, high pressure studies in the RT4P12 (R = La, Y, T = Fe, Ru) up to 8GPa have been performed,9,10) and found that the dTc=dP’s for LaFe4P12 and YFe4P12 show positive and those for LaRu4P12 and YRu4P12 exhibit negative. Cheng et al.9) have revealed from a closer inspection of the characteristics of crystal and electronic structures in the series RT4P12 that the pressure effect on Tc of the filled skutterudite should have little to do with that of the corresponding R metals. The observed dTc=dP > 0 of YFe4P12 was further confirmed by the band-structure calculations that show an enhancement of both NðEFÞ and the Hopfield parameter11) (©) under pressure at least up to 8GPa. On the other hand, a dTc=dP < 0 for YRu4P12 was also explained by similar calculations. In this note, we present theoretical results of the pressure dependence of Tc for LaT4P12 (T = Fe, Ru) by performing band-structure calculations. Electronic structure calculations are carried out using a full-potential linearized augmented plane wave method (FLAPW) with the local density approximation (LDA) for the exchange correlation potential. For the LDA, the formula proposed by Gunnarsson and Lundqvist is used.12) Calculations are performed at ambient pressure (denoted as 0GPa) and 8GPa. Experimental crystal structure including lattice constants and internal parameters is assumed throughout the calculation.10,13,14) The parameters of the atomic positions at 0GPa are used for 8GPa. The sampling points are uniformly distributed in the irreducible Brillouin Zone, 225 k-points for potential convergence, and 3025 k-points for the final band structures. The used LAPW basis functions are 5425 at the ¥ point. The present calculations for LaT4P12 (T = Fe, Ru) are in good agreement with previous reports15,16) at 0GPa. We evaluate the pressure dependence of Tc using McMillan’s strong-coupling equation17) as well as the case of YT4P12 (T = Fe, Ru).9) By taking the derivative with respect to volume of the equation, we can obtain the relation:

Sudden reversal in the pressure dependence ofTcin the iron-based superconductor CsFe2As2: A possible link between inelastic scattering and pairing symmetry

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at the zero temperature limit, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and investigate what parameters control this transition. Comparing samples of different residual resistivity, we find that a 6-fold increase in impurity scattering does not shift Pc. From a study of X-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of these lattice parameters suggests that Pc should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2. Resistivity measurements under pressure reveal a change of regime across Pc, suggesting a possible link between inelastic scattering and pairing symmetry.

Superconductivity of BaLi4 under pressure

We studied the pressure-induced superconductivity of BaLi4 up to 53 GPa by means of electrical resistivity in a diamond anvil cell. Superconductivity in BaLi4 is first observed at a pressure of 5.4 GPa with a superconducting critical temperature (Tc) of 4.5 K. Below 2 GPa, superconductivity is not observed above the minimum temperature achievable in the current study, 2 K. Between 5.4 and 12 GPa, the Tc increases steeply to its maximum value of 7 K. Above 12 GPa, the pressure dependence of Tc is complex and the sign of dTc/dP changes several times in going up to the maximum pressure studied, of 53 GPa.

Superconducting properties of barium in three phases under high pressure from first principles

Electron—phonon coupling (EPC) in the three high-pressure phases of Ba is investigated using a pseudopotential plane-wave method based on density functional perturbation theory. The calculated values of superconducting critical temperature Tc of Ba—I and Ba—II under pressure are consistent well with the trends observed experimentally. Moreover, Ba—V is found to be superconducting with a maximum Tc exceeding 7.8 K at 45 GPa. With the increase of pressure, the values of Tc increase in Ba I and Ba—II but the value of Tc decreases in Ba—V. For Ba—I at pressures below 2 GPa, the increases of logarithmic average frequency ωlog and electron—phonon coupling parameters λ both contribute to the enhancement of Tc. For all the three phases at pressures above 2 GPa, Tc is found to be primarily determined by λ. Further investigation reveals that for all the three phases, the change in λ with pressure can be explained mainly by change in the phonon frequency Thus for Ba—II and Ba—V, although they exhibit completely different superconducting behaviors, their superconductivities have the same origin; the pressure dependence of Tc is determined finally by the pressure dependence of phonon frequency.

Sudden reversal in the pressure dependence of Tc in the iron-based superconductor KFe2As2

The pairing mechanism in iron-based superconductors is the subject of ongoing debate. Proximity to an antiferromagnetic phase suggests that pairing is mediated by spin fluctuations, but orbital fluctuations have also been invoked. The former typically favour a pairing state of extended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces (s+-), while the latter yield a standard s-wave state without sign change (s++). Here we show that applying pressure to KFe2As2 induces a change of pairing state. The critical temperature Tc decreases with pressure initially, and then suddenly increases, above a critical pressure Pc. The constancy of the Hall coefficient through Pc rules out a change in the Fermi surface. There is compelling evidence that the pairing state below Pc is d-wave, from bulk measurements at ambient pressure. Above Pc, the high sensitivity to disorder argues for a particular kind of s+- state. The change from d-wave to s-wave is likely to proceed via an unusual s + id state that breaks time-reversal symmetry. The proximity of two distinct pairing states found here experimentally is natural given the near degeneracy of d-wave and s+- states found theoretically. These findings make a compelling case for spin-fluctuation-mediated superconductivity in this key iron-arsenide material.

Transport properties of Y1-xNdxCo2 compounds

Electrical resistivity ρ and thermopower S of light rare earth-based pseudo-binary Y1–xNdxCo2 alloys have been measured at temperatures from 2 K to 300 K and under pressures up to 3.5 GPa. The Curie temperature of the alloys, TC, determined from characteristic features in the temperature dependences of the transport properties, decreases with decreasing Nd concentration x and vanishes around xc = 0.3. The residual resistivity has a pronounced maximum at x = xc. The temperature coefficient of thermopower dS/dT at low temperature limit shows a complex dependence on alloy composition: it changes its sign from negative to positive at x ≍ 0.2, having a maximum at x = xc, and is nearly composition independent at x > 0.5. The pressure dependences of TC and ρ0 of Yo.6Ndo.4Co2 reveal the behavior similar to that observed in the Y1-xRxHCo2 (RH = heavy rare earth) alloy systems, which implies that the magnetic state of the Co-3d electron subsystem is responsible for the transport properties in the Y1-xNdxCo2 alloys.

Effect of Mn Substitution on the Thermal Expansion of EuBa2Cu3O7–δ

Abstract In the present communication, effect of Mn substitution on thermal expansion of EuBa2Cu3–xMnxO7–δ has been investigated. Thermal expansion measurements have been carried out using a high resolution dilatometer based on three terminal capacitance method. The present set–up is sensitive enough to detect small changes in linear thermal expansion coefficient, α of less than 10 –8 per Kelvin. Results show that the pristine sample exhibits a jump in thermal expansion coefficient around its transition temperature which is a characteristic of a second order phase transition.–8 –1For the pure sample a jump in α, of the order of 9.5¥10Khas been observed. It is seen that Mn doping reduces the jump in thermal expansion. Using the well–known Ehrenfest relations, both the pressure dependence of Tc, dTc/dP and the discontinuity in compressibility, ΔK, are determined and are seen to decrease with increase in Mn concentration.

Sample dependence of superconductivity for V3Si under high pressure

V3Si is one of the superconducting A15 compounds and has relatively high Tc = 17 K at ambient pressure. The superconductivity of V3Si and the pressure effect of that were strongly dependent on the sample. The pressure dependence of Tc was considered to be influenced by the composition and the crystal disorder.

Theoretical study on superconductivity inCeMIn5 (M = Co, Rh, Ir): pairing symmetry and pressure dependence

We study theoretically heavy fermion superconductorsCeMIn5 (M = Co,Rh, Ir). CeCoIn5 and CeIrIn5 that become superconducting at ambient pressure withTc = 2.3 K and0.4 K, respectively. Onthe other hand, CeRhIn5 is an antiferromagnet at ambient pressure and becomes superconducting under pressuresgreater than 1.6 GPa. With regards to the superconductivity, the existence of line nodes isindicated by nuclear-quadrupole-resonance (NQR), thermal conductivity, specific heatand electrical resistivity measurements. However, the pairing symmetry betweendx2−y2 and dxy is controversial. Therefore, we investigate the gap structure ofCeMIn5 by a detailed calculation. We introduce a three-dimensional periodicAnderson model (3D-PAM) in order to reproduce the band characteristics ofCeMIn5. Thus, we identifythe gap structure of CeMIn5 as the dx2−y2 symmetry by solving the Èliashberg equations. In addition, we discuss the pressure dependence ofTc and show that twofactors determine Tc. One factor is the momentum dependence of quasi-particle interaction and the other factor isthe wavefunction renormalization factor. Thus, we have explained the superconductivity inCeMIn5 using the Fermi liquid theory.