Slope Of Upper Critical Field Research Articles

Pressure-induced linear enhancement of the superconducting transition in Nd0.8Sr0.2NiO2 thin films

We report the pressure (P) effect on the superconducting transition temperature T c and the upper critical field μ 0 H c2 of infinite-layer Nd0.8Sr0.2NiO2 thin films by measuring the electrical transport properties under various hydrostatic pressures to 4.6 GPa. At ambient pressure, it shows the clear superconducting transition with T c ∼ 10 K. Based on the evolution of resistance R(T), we found that the T c is monotonically enhanced to ∼14 K upon increasing pressure to 2.9 GPa. The constructed temperature–pressure phase diagram indicates that the calculated slope dT c/dP is about 1.14 K GPa−1 and the superconducting T c shows no signatures of saturation with pressure. It thus gives the possibility to further enhance T c by employing higher pressures or heterostructure engineering. In addition, the normalized slope of upper critical field μ 0 H c2(0) implies that the electron correlations are gradually decreasing with pressure, which exhibits an opposite evolution with superconducting T c. Our work further confirms the positive pressure effects in nickelate superconductors and gives more insight to further enhance its superconducting transition temperature.

Two Qualitatively Different Superconducting Phases under High Pressure in Single-Crystalline CeNiGe3

We have measured the temperature dependence of resistivity in single-crystalline CeNiGe$_{3}$ under hydrostatic pressure in order to establish the characteristic pressure-temperature phase diagram. The transition temperature to AFM-I phase $T_{\rm N1}$ = 5.5 K at ambient pressure initially increases with increasing pressure and has a maximum at $\sim$ 3.0 GPa. Above 2.3 GPa, a clear zero-resistivity is observed (SC-I phase) and this superconducting (SC) state coexists with AFM-I phase. The SC-I phase suddenly disappears at 3.7 GPa simultaneously with the appearance of an additional kink anomaly corresponding to the phase transition to AFM-II phase. The AFM-II phase is continuously suppressed with further increasing pressure and disappears at $\sim$ 6.5 GPa. In the narrow range near the critical pressure, an SC phase reappears (SC-II phase). A large initial slope of upper critical field $\mu_0H_{\rm c2}$ and non-Fermi liquid behavior indicate that the SC-II phase is mediated by antiferromagnetic fluctuations. On the other hand, the robust coexistence of the SC-I phase and AFM-I phase is unusual on the contrary to superconductivity near a quantum critical point on most of heavy-fermion compounds.

Effect of hydrostatic pressure on the superconducting properties of quasi-1D superconductor K2Cr3As3

K2Cr3As3 is a newly discovered quasi-1D superconductor with a Tc = 6.1 K and an upper critical field µ0Hc2(0) ≈ 40 T three times larger than the Pauli paramagnetic limit µ0Hp that is suggestive of a spin-triplet Cooper pairing. In this paper, we have investigated the effects of hydrostatic pressure on its Tc and µ0Hc2 by measuring the ac magnetic susceptibility χ'(T) under magnetic fields at various hydrostatic pressures up to 7.5 GPa. The major findings include: (1) Tc is suppressed gradually to below 2 K at 7.5 GPa; (2) the estimated µ0Hc2(0) decreases dramatically to below µ0Hp above ~2 GPa and becomes slight lower than the orbital limiting field estimated from the initial slope of upper critical field via = −0.73TcdHc2/ in the clean limit; (3) the estimated Maki parameter α = √2/Hp drops from 4 at ambient pressure to well below 1 at P > 2 GPa, suggesting the crossover from Pauli paramagnetic limiting to orbital limiting in the pair breaking process upon increasing pressure. These observations suggested that the application of hydrostatic pressure could drive K2Cr3As3 away from the ferromagnetic instability and lead to a breakdown of the spin-triplet pairing channel. We have also made a side-by-side comparison and discussed the distinct effects of chemical and physical pressures on the superconducting properties of K2Cr3As3.

Rutgers relation for the analysis of superfluid density in superconductors

It is shown that the thermodynamic Rutgers relation for the second order phase transitions can be used for the analysis of the superfluid density data irrespective of complexities of the Fermi surface, structure of the superconducting gap, pairing strength, or scattering. The only limitation is that critical fluctuations should be weak, so that the mean field theory of the second order phase transitions is applicable. By using the Rutgers relation, the zero temperature value of the London penetration depth, $\lambda(0)$, is related to the specific heat jump $\Delta C$ and the slope of upper critical field $dH_{c2}/dT$ at the transition temperature $T_c$, provided the data on $\Delta\lambda=\lambda(T)-\lambda(0)$ are available in a broad temperature domain. We then provide a new way of determination of $\lambda(0)$, the quantity difficult to determine within many techniques.

Ambient/low pressure synthesis and fast densification to achieve 55 KTc superconductivityin NdFeAsO0.75F0.25

55 K superconductivity in NdFeAsO0.75F0.25 is demonstrated using two stage, ambient pressure synthesis (APS) with two zone temperaturecontrol for maintaining the As vapour pressure. The density of the APS material was subsequentlyincreased by spark plasma sintering (SPS) for short times (60 s) at relatively low pressures (<80 MPa), giving density values above 95% of the theoretical value whilemaintaining very high slopes of upper critical field of up to 8.5 T K−1. TheAPS+SPS method developed here is ideal for fabricating tapes in a rapid and safe way.

Magnetic and Superconducting Properties of CeTX3 (T: Transition Metal and X: Si and Ge) with Non-centrosymmetric Crystal Structure

We grew single crystals of non-centrosymmetric compounds CeTSi 3 and CeTGe 3 (T: transition metal), and studied the magnetic properties by measuring the electrical resistivity, magnetic susceptibility and magnetization. We also studied the effect of pressure on the electronic states in antiferromagnets CeTGe 3 (T: Co, Rh, Ir) by measuring the resistivity under pressure. No noticeable change of the Neel temperature was observed up to 8 GPa in CeRhGe 3 and CeIrGe 3 , which are far from the magnetic quantum critical point. On the other hand, the Neel temperature in CeCoGe 3 was strongly decreased as a function of pressure, and pressure-induced superconductivity was observed in the pressure region from 5.4 GPa to about 7.5 GPa. The slope of upper critical field H c2 at 6.5 GPa is found to be extremely large, with an upturn curvature of H c2 with decreasing temperature: -d H c2 /d T = 200 kOe/K at the superconducting transition temperature T sc = 0.69 K, revealing unconventional superconductivity.

Bulk nanocrystalline superconducting YBa2Cu3O7-x

A method to fabricate high density nanocrystalline superconducting YBa2Cu3O7-x(YBCO) is presented in which commercially available YBCO powder is ball milled and then hot isostatically pressed (HIP'ed) at 0.2 GPa and 400 C. Ball milling decreased the average particle size from 2 μm to ~ 20 nm within 2 hours and to ~ 4 nm after milling for 30 hours with no noticeable strain. Ball milling increases the oxygen sublattice disorder and the Y/Ba anti-site disorder and drives the orthorhombic YBCO phase towards a disordered metastable cubic phase Y1/3Ba2/3CuO3-x consistent with the work by Simoneau et al. [14, 19]. Powders milled for 30 hours were HIPed at temperatures from 400 C to 550 C. At 450 C and above, the powder decomposed into the parent oxides. At 400 C, the metastable cubic phase reordered to form bulk YBCO with an average grain size of 5 - 18 nm and a relative mass density of ~ 96%. The bulk YBCO material has an onset melting temperature and associated enthalpy of 970 °C and 157 J.g-1 respectively which can be compared to 965 °C and 156 J.g-1 for the starting commercial powder. These results are consistent with XRD data and they suggest a predominantly single-phase nanocrystalline YBCO has been fabricated. The superconducting transition temperature (TC) of the nanocrystalline YBCO determined from ac. susceptibility measurements is 88.8 K, with a slope of upper critical field near TC of ~ 0.12 T.K-1.

Reversible magnetization and critical fluctuations in systematically dopedYBa2Cu3O7−δsingle crystals

The temperature and field dependence of reversible magnetization have been measured on a YBa$_2$Cu$_3$O$_{7-\delta}$ single crystal at six different doping concentrations. It is found that the data above 2 T can be described by the scaling law based on the GL-LLL (lowest Landau level approach based on Ginzburg-Landau theory) critical fluctuation theory yielding the values of the slope of upper critical field $-\mathrm{d}H_{\mathrm{c2}}(T)/\mathrm{d}T$ near $T_\mathrm{c}$. This set of values is self-consistent with that obtained in doing the universal scaling for the six samples. Based on a simple Ginzburg-Landau approach, we determined the doping dependence of the coherence length $\xi$ which behaves in a similar way as that determined from $\xi= \hbar v_\mathrm{F}/E_\mathrm{sc}$ with $E_\mathrm{sc}$ the superconducting energy scale. Our results may suggest a growing coherence length towards more underdoping.

Comparative study of the characteristic length scales and fields of Hg-based high-Tc superconductors.

The equilibrium magnetization was studied in magnetically aligned Hg-based cuprate superconductors with a crystal structure containing one, two, and three adjacent Cu-O layers. The mixed state magnetization was measured with the magnetic field applied perpendicular to the layers, along with low field Meissner effect and normal state susceptibility studies for volume correction factors. The internally consistent analysis is used to identify systematics as the number {open_quote}{open_quote}{ital n}{close_quote}{close_quote} of Cu-O layers increases. The study reveals a linear increase in the slope of upper critical field {minus}{ital dH}{sub {ital c}2}/{ital dT} with {open_quote}{open_quote}{ital n}{close_quote}{close_quote}. Most importantly, we find a nearly constant carrier doping per Cu-O layer. This result is compared with other studies, e.g., Rietveld analysis of neutron diffractometry, and its implications are discussed. {copyright} {ital 1996 The American Physical Society.}

Upper Critical Field of High Tc Superconducting Oxides

The upper critical field of the superconducting oxide La2-xSrxCuO4-y was studied as a function of the Sr concentration by electrical resistance measurements under magnetic fields up to 26 T. The slope of upper critical field near Tc and the electrical resistivity show anomalous concentration dependences. The results are related to the orthorhombic-to-tetragonal phase transition.