Critical Field Curve Research Articles

Multiband Superconductivity in High-Pressure Sulfur Hydrides

The temperature dependence of the two superconducting gaps in pressurised H3S at 155 GPa with a critical temperature of 203 K has been determined using a data analysis of the experimental curve of the upper critical magnetic field as a function of temperature in the framework of the two-band s-wave Eliashberg theory. Two different phonon-mediated intra-band Cooper pairing channels in a regime of moderate strong couplings have the key role of the pair-exchange interaction between the two gaps, giving the two non-diagonal terms of the coupling tensor, which are missing in the single-band s-wave Eliashberg theory. The results provide a prediction of the different temperature dependence of the small and large gaps as a function of temperature, which provides evidence of multigap superconductivity in H3S.

In-field neutron diffraction investigation of metamagnetism in Nd7Rh3

The binary intermetallic compound, Nd7Rh3 shows exotic magnetic properties which are rare among the R7X3 type compounds. Previous magnetic measurements on Nd7Rh3 revealed two antiferromagnetic transitions in Nd7Rh3 (TN1 = 32 K and TN2 = 16 K). This compound exhibits metamagnetic transition at 2 K, as in isostructural counterparts Nd7Ni3 or Tb7Rh3. But there are ample differences in the metamagnetic transition features. Further investigations on Nd7Rh3 gave evidence for phase-coexistence, which is rarely observed in stoichiometric compounds. Electronic phase separation is observed due to the first order transition brought out by a change in temperature or magnetic field (H). This compound was also shown to exhibit features attributable to devitrification of the arrested metastable state, after travelling across metamagnetic transition. In the present work, neutron diffraction studies on Nd7Rh3 were carried out in external magnetic fields to have an in-depth knowledge of kinetic arrest due to field-induced first order phase transition, phase coexistence and devitrification of the arrested metastable state, using the CHUF (cooling and heating in unequal fields) protocol. The sample was cooled in a field of 3 T (T) and was warmed in different fields (0–0.8 T). On the basis of these experiments, a field (H)-Temperature (T) phase diagram could be inferred which shows the critical field (HK) and temperature (TK) curve across which the devitrification of the arrested metastable state begins. Above the (HK, TK) curve, the system is in the arrested (high field) state, whereas below the curve the system has returned partially to its magnetic de-arrested state.

Magnetic field in holographic superconductor with dark matter sector

Based on the analytical technique the effect of the static magnetic field on the s-wave holographic superconductor with dark matter sector of U(1)-gauge field type coupled to the Maxwell field has been examined. In the probe limit, we obtained the mean value of the condensation operator. The nature of the condensate in an external magnetic field as well as the behaviour of the critical field close to the transition temperature has been revealed. The obtained upturn of the critical field curves as a function of temperature, both in four and five spacetime dimensions, is a fingerprint of the strong coupling approach.

Magnetic phase transitions and magnetic entropy in the XY antiferromagnetic pyrochlores (Er1-x Y x )2Ti2O7.

We present the results of experimental determination of the heat capacity of the pyrochlore Er2Ti2O7 as a function of temperature (0.35-300 K) and magnetic field (up to 9 T), and for magnetically diluted solid solutions of the general formula (Er1-x Y x )2Ti2O7 (x≤0.471). On either doping or increase of magnetic field, or both, the Néel temperature first shifts to lower temperature until a critical point above which there is no well-defined transition but a Schottky-like anomaly associated with the splitting of the ground state Kramers doublet. By taking into account details of the lattice contribution to the heat capacity, we accurately isolate the magnetic contribution to the heat capacity and hence to the entropy. For pure Er2Ti2O7 and for (Er1-x Y x )2Ti2O7, the magnetic entropy as a function of temperature evolves with two plateaus: the first at [Formula: see text], and the other at [Formula: see text]. When a very high magnetic field is applied, the first plateau is washed out. The influence of dilution at low values is similar to the increase of magnetic field, as we show by examination of the critical temperature versus critical field curve in reduced terms.

Dimensional Crossover of the Fulde–Ferrell–Larkin–Ovchinnikov State in Strongly Pauli-Limited Quasi-One-Dimensional Superconductors

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is examined in quasi-one-dimensional s-wave and d-wave superconductors with particular attention paid to the effect of the Fermi-surface anisotropy. The upper critical field $H_{c2}(T)$ is found to exhibit a qualitatively different behavior depending on the ratio of the hopping energies $t_b/t_a$ and the direction of the FFLO modulation vector q, where $t_a$ and $t_b$ are the intra- and interchain hopping energies, respectively. In particular, when $t_b/t_a < 0.1$ and $\mathbf{q} \parallel \mathbf{a}$, we find a novel dimensional crossover of $H_{c2}(T)$ from one dimension to two dimensions, where a is the lattice vector of the most conductive chain. Just below the tricritical temperature $T^*$, the upper critical field $H_{c2}(T)$ increases steeply as in one-dimensional systems, but when the temperature decreases, the rate of increase in $H_{c2}(T)$ diminishes and a shoulder appears. Near $T = 0$, $H_{c2}(T)$ shows a behavior typical of the FFLO state in two-dimensional systems, i.e., an upturn with a finite field at $T = 0$. When the angle between $\mathbf{q}$ and $\mathbf{a}$ is large, the upper critical field curve is convex upward at low temperatures, as in three-dimensional systems, but the magnitude is much larger than that of a three-dimensional isotropic system. For $t_b/t_a > 0.15$, the upper critical fields exhibit a two-dimensional behavior, except for a slight shoulder in the range of $0.2 > t_b/t_a > 0.15$. The upper critical field is maximum for $\mathbf{q} \parallel \mathbf{a}$ both for s-wave and d-wave pairings, while it is only slightly larger than the Pauli paramagnetic limit for $\mathbf{q} \perp \mathbf{a}$. The relevance of the present results to the organic superconductor ${\rm (TMTSF)_2ClO_4}$ is discussed.

Reply to “Comment on ‘Diamagnetism and Cooper pairing aboveTcin cuprates' ”

We analyze recent torque measurements of the magnetization $M_d$ vs. magnetic field $H$ in optimally doped YBa$_2$Cu$_3$O$_{7-y}$ (OPT YBCO) to argue against a recent proposal by Rey et al that the magnetization results above $T_c$ are consistent with Gaussian fluctuations. We find that, despite its strong interlayer coupling, OPT YBCO displays an anomalous non-monotonic dependence of $M_d$ on $H$ which represents direct evidence for the locking of the pair wavefunction phase $\theta_n$ at $T_c$ and the subsequent unlocking by a relatively weak $H$. These unusual features characterize the unusual nature of the transition to the Meissner state in cuprate superconductors. They are absent in low-$T_c$ superconductors to our knowledge. We also stress the importance of the vortex liquid state, as well as the profiles of the melting field $H_m(T)$ and the upper critical field curve $H_{c2}(T)$ in the $T$-$H$ plane. Contrary to the claims of Rey et al, we show that the curves of the magnetization and the Nernst signal illustrate the inaccessibility of the $H_{c2}$ line near $T_c$. The prediction of the $H_{c2}$ line by Rey et al is shown to be invalid in OPT YBCO.

Concentration dependence in kinetic arrest of the first-order magnetic transition in Ta doped HfFe2

Magnetic behavior of the pseudo-binary alloy Hf1−xTaxFe2 has been studied, for which the zero-field ferromagnetic (FM) to antiferromagnetic (AFM) transition temperature is tuned near to T = 0 K. Our studies show that such composition lies around x = 0.230. Detailed magnetization studies on x = 0.225, 0.230 and 0.235 show thermomagnetic irreversibility at low temperature due to kinetic arrest of the first-order AFM–FM transition. All three compositions studied show a reentrant transition in the zero-field-cooled warming curve and non-monotonic variation of the upper critical field. The region in H–T space where these features of kinetic arrest manifest themselves increases with increasing Ta concentration.

The effect of Si addition in MgB2 thin films by hybrid physical–chemical vapor deposition using silane as the dopingsource

We report the synthesis and characterization of silicon dopedMgB2 films fabricated by hybrid physical–chemical vapor deposition using silaneas the doping source. Various amounts of Si up to 9% were added intoMgB2 thin films, which were epitaxially grown in situ on SiC. Compared with that ofthe undoped film, the superconducting transition temperature of the doped filmdoes not show significant change; however the magnetic critical current density (Jc) has beenincreased. Jc for the film with 5.0 at.% doping goes up to2.7 × 105 A cm − 2 for a 3 T perpendicular field. And the slope of the upper critical fieldcurve for the doped film is slightly higher than that for the undopedMgB2 film.

Bose-Einstein condensation and the magnetically ordered state ofTlCuCl3

The dimerized S=1/2 spins of the Cu ions in TlCuCl3 are ordered antiferromagnetically in the presence of a field larger than about 54 kOe in the zero-temperature limit. Within the mean-field approximation all thermal effects are frozen out below 6 K. Nevertheless, experiments show significant changes of the critical field and the magnetization below this temperature, which reflect the presence of low-energetic dimer-spin excitations. We calculate the dimer-spin correlation functions within a self-consistent random-phase approximation, using as input the effective exchange coupling parameters obtained from the measured excitation spectra. The calculated critical field and magnetization curves exhibit the main features of those measured experimentally, but differ in important respects from the predictions of simplified boson models.

Pressure-induced antiferromagnetic bulk superconductor EuFe2As2

We present the magnetic and superconducting phase diagram of EuFe2As2 for B‖c and B‖ab. The antiferromagnetic phase of the Eu2+ moments is completely enclosed in the superconducting phase. The upper critical field vs. temperature curves exhibit strong concave curvatures, which can be explained by the Jaccarino–Peter compensation effect due to the antiferromagnetic exchange interaction between the Eu2+ moments and conduction electrons.

Effect of pressure on the structural phase transition and superconductivity in(Ba1−xKx)Fe2As2(x=0and 0.45) andSrFe2As2single crystals

The effects of pressure up to $\ensuremath{\sim}20\text{ }\text{kbar}$ on the structural phase transition of ${\text{SrFe}}_{2}{\text{As}}_{2}$ and lightly Sn-doped ${\text{BaFe}}_{2}{\text{As}}_{2}$ as well as on the superconducting transition temperature and upper critical field of $({\text{Ba}}_{0.55}{\text{K}}_{0.45}){\text{Fe}}_{2}{\text{As}}_{2}$ single crystals have been studied. All the transition temperatures decrease with pressure in an almost linear fashion. Under pressure, the upper critical-field curve ${H}_{c2}(T)$ for $({\text{Ba}}_{0.55}{\text{K}}_{0.45}){\text{Fe}}_{2}{\text{As}}_{2}$ shifts down in temperature to follow the zero-field ${T}_{c}$ with very little change in slope. Composite $P\text{\ensuremath{-}}T$ phase diagrams for three parent compounds $A{\text{Fe}}_{2}{\text{As}}_{2}$ ($A=\text{Ba}$,Sr,Ca) are constructed and appear to be remarkably similar: (i) having a structural (antiferromagnetic) phase-transition line with a negative slope and (ii) showing signs of the emerging superconducting state at intermediate pressures.

Multiple superconducting transitions in theSr3Ru2O7region ofSr3Ru2O7-Sr2RuO4eutectic crystals

We report the superconducting properties of ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}{\text{-Sr}}_{2}{\text{RuO}}_{4}$ eutectic crystals, which consist of the spin-triplet superconductor ${\text{Sr}}_{2}{\text{RuO}}_{4}$ with a monolayer stacking of ${\text{RuO}}_{2}$ planes and the metamagnetic normal metal ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ with a bilayer stacking. Although ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ so far has not been reported to exhibit superconductivity, our ac susceptibility measurements revealed multiple superconducting transitions that occur in the ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ region of the eutectic crystals. The diamagnetic shielding essentially reached the full fraction at low ac fields parallel to the $c$ axis. However, both the shielding fraction and the onset temperature are easily suppressed by ac fields larger than 0.1 mT rms and no anomaly was observed in the specific heat. Moreover, the critical field curves of these transitions have a positive curvature near zero fields, which is different from the upper critical field curve of the bulk ${\text{Sr}}_{2}{\text{RuO}}_{4}$. These facts suggest that the superconductivity observed in the ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ region is not a bulk property. To explain these experimental results, we propose the scenario that stacking ${\text{RuO}}_{2}$ planes, which are the building blocks of superconducting ${\text{Sr}}_{2}{\text{RuO}}_{4}$, are contained in the ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ region as stacking faults.

Evolution of Superconducting and Hidden Order Phases in URu2Si2 Under Applied Pressure

AbstractElectrical resistivity measurements performed under applied hydrostatic pressure and in magnetic fields have been used to probe the hidden order (HO) and superconducting (SC) states of URu2Si2, which have ambient-pressure transition temperatures TO = 17.5 K and Tc = 1.5 K, respectively. TO increases with applied pressure and a distinct kink in its pressure dependence is observed at 15 kbar; this feature is associated with the onset of antiferromagnetism. The pressure dependence of the SC upper critical field has been measured with the external field aligned parallel to both crystalline axes. The SC phase is smoothly suppressed to a critical pressure of about 15 kbar and no qualitative change in the critical field curves is observed. The co-evolution of the HO and SC phases is discussed within the context of a model in which the two phases compete for Fermi surface fraction.

Antiferromagnetic superconductors with effective-mass anisotropy in magnetic fields

We derive critical field H_c2 equations for antiferromagnetic \textit{s}-wave, d_{x^2-y^2}-wave, and d_{xy}-wave superconductors with effective mass anisotropy in three dimensions, where we take into account (i) the Jaccarino-Peter mechanism of magnetic-field-induced superconductivity (FISC) at high fields, (ii) an extended Jaccarino-Peter mechanism that reduces the Pauli paramagnetic pair-breaking effect at low fields where superconductivity and an antiferromagnetic long-range order with a canted spin structure coexist, and (iii) the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) state. As an example, experimental phase diagrams observed in organic superconductor kappa-(BETS)_2FeBr_4 are theoretically reproduced. In particular, the upper critical field of low-field superconductivity is well reproduced without any additional fitting parameter other than those determined from the critical field curves of the FISC at high fields. Therefore, the extended Jaccarino-Peter mechanism seems to occur actually in the present compound. It is predicted that the FFLO state does not occur in the FISC at high fields in contrast to the compound lambda-(BETS)_2FeCl_4, but it may occur in low-field superconductivity for s-wave and d_{x^2-y^2}-wave pairings. We also briefly discuss a possibility of compounds that exhibit unconventional behaviors of upper critical fields.

Improved Superconducting Properties of In-Situ PIT-Processed&amp;lt;tex&amp;gt;$rm MgB_2$&amp;lt;/tex&amp;gt;Tapes

We fabricated pure and SiC-added MgB/sub 2//Fe composite tapes using a MgH/sub 2/ starting powder and applying heat treatments at 600-800/spl deg/C. At 4.2 K, upper critical field (B/sub c2/) of 27 T and 22.5 T were obtained for the SiC-added tapes. This B/sub c2/ at 4.2 K is comparable to that of the bronze-processed Nb/sub 3/Sn. For both the pure and SiC-doped tapes, the critical temperature (T/sub c/) values increased with increasing heat treatment temperature. The slope of the upper critical field vs. temperature (B/sub c2/-T) curves, dB/sub c2//dT, of the pure MgB/sub 2/ tape decreased with increasing temperature, a change leading to the lower B/sub c2/ at 4.2 K. Thus, the critical current density (J/sub c/) of pure MgB/sub 2/ tapes in high magnetic fields at 4.2 K decreased with increasing heat treatment temperature. The dB/sub c2//dT of the SiC-added tape, on the other hand, is almost independent of heat treatment temperature. Thus, the J/sub c/ at 4.2 K of SiC-added tape is also independent of heat treatment temperature. In the high temperature region, due to the higher T/sub c/ values, the J/sub c/ of the tapes heat-treated at 800/spl deg/C exhibit higher values than the tapes heat-treated at 600/spl deg/C. At 20 K, B/sub c2/ values of 9-10 T were obtained for the pure MgB/sub 2/ tape heat-treated at 600/spl deg/C and the SiC doped tape heat-treated at 800/spl deg/C. These B/sub c2/ values at 20 K are comparable to that of commercial Nb-Ti at 4.2 K. The 5 at.% SiC-added tape heat-treated at 800/spl deg/C shows J/sub c/(MgB/sub 2/ core) values higher than 10/sup 4/ A/cm/sup 2/ at 20 K in 5 T and at 30 K in 1 T. These J/sub c/ values are among the highest obtained for MgB/sub 2/ tapes.

Critical-field curves for switching toggle mode magnetoresistance random access memory devices (invited)

The magnetization behavior of generalized synthetic antiferromagnet (SAF) is investigated by analytic/numeric method, applying Stoner–Wohlfarth model to each layer. Critical conditions for toggle-mode switching are given using the critical-field curves obtained from the field trajectories giving a constant value to the average angle of the magnetizations of the two layers or to the angle between the two, leaving the other variable. The critical fields for the toggle operation are found to be expressed by analytical formulas including only controllable element parameters, facilitating the parameter optimization for the memory element. The results obtained for the generalized SAF are useful in determining the tolerance for the parameter imbalance. Half-select disturb effect is still one of the important issues to be taken into account in designing the memory element.

Optimization of magnetic parameters for toggle magnetoresistance random access memory

The magnetic parameters of the synthetic antiferromagnetic (SAF) elements for toggle-mode magnetoresistance random access memories (Toggle-MRAMs) have been optimized using the critical field curves obtained by analytical method with the aid of numerical calculations, to maximize the operating field margin taking into account the required memory density, storage lifetime, half-select disturb robustness, and the available strength of operating field. The control of especially low-exchange coupling strength in the SAF in addition to the increase of the operating field has been found to be essential for the development of toggle-MRAM in near future.

Non-Fermi-liquid behaviour in CeRhSn

Measurements of the temperature T dependence of the electrical resistivity ρ, specific heat C and magnetic susceptibility χ have been made on the compound CeRhSn from about 10 K to temperatures as low as 0.1 K. Over certain temperature intervals, these properties are observed to have non-Fermi-liquid (NFL) temperature T dependences: with and . While the NFL behaviour can be described by the Griffiths–McCoy model, it may also be related to a weak magnetic transition at 6.2 K that depends on atomic disorder. The resistively determined upper critical field versus temperature curve of the isostructural reference compound without f electrons, LaRhSn, is also reported.

Multipolar excitations in the antiferroquadrupolar phase of CeB 6

The hexaboride compound CeB 6 exhibits quadrupolar O xy type and magnetic order due to the four-fold degenerate Ce 3+–Γ 8 CEF-states. The B- T phase diagram shows anomalous large positive slope of the critical field curve. This is due to a new type of induced octupolar T xyz order parameter. Inelastic neutron scattering experiments by Bouvet have provided direct evidence for the existence of collective excitations. Their nature is shown to be of mixed multipolar character and their dispersion and intensities are calculated using a RPA-type approach and a Holstein–Primakoff approach which show excellent agreement. Temperature and field dependence of the multipolar excitations are calculated and the latter is compared to experimental results.

Vortex Lattice Structure of High Temperature Superconductors

We study vortex lattice structure of high temperature superconductors by using the Ginzburg–Landau model. The structure of the vortex lattice is oblique at the temperatures well below the transition temperature Tc, where the mixed s–d state is expected to have the lowest energy. Whereas, very close to Tc, the dx2-y2wave is slightly lower in energy, and a triangular vortex lattice recovers. The coexistence and the coupling between the s- and d-waves account for the upward curvature of the upper critical field curve HC2(T).