Phase Diagram Of UPt3 Research Articles

Nearly isotropic d-wave pairing in UPt 3

The available phase diagram of UPt 3 in H-T-P coordinates imposes strong restrictions on all theories which are going to explain it. We present a self-consistent solution of this problem. In our picture UPt 3 is a d-wave nearly isotropic superconductor with small splitting of critical temperature according to the irreducible representations of a D 6h group. Two of these representations A 1g and E 1g are responsible for the phase transitions in UPt 3. The order of high- and low-temperature representations reverses under pressure. This scenario explains all the peculiarities of the UPt 3 phase diagram such as the existence of a tetracritical point in the H-T plane, a kink in H c2( T) for different orientations of magnetic field, the behavior of superconducting phases under pressure and others.

Muon spin relaxation in heavy fermion systems

Heavy fermion systems have received a great deal of study by a wide variety of techniques, includingμSR. In a number of systems, coexisting superconducting and magnetic states have been reported, leading to speculation of an intimate connection between magnetism and superconductivity in these compounds. We observe a spontaneous magnetic field in the superconducting phase of UPt3. In addition, the broadening of the transverse field muon precession signal only onsets approximately 60 mK below the superconductingTc. Our results provide evidence that the “lower superconducting phase” in theH-T phase diagram of UPt3 is characterised by broken time-reversal symmetry. Measurements of URu2Si2 and CeCu2.2Si2 indicate that the magnetically ordered volume fraction is temperature dependent in both systems.

Ultrasound study of the superconducting phases of UPt3 under uniaxial stress

Abstract We have studied for the first time experimentally the phase-diagram of UPt 3 under a simultaneous application of both uniaxial stress and magnetic field parallel to the c -axis. We find that while the splitting between the two transition temperatures observed in zero field decreases with increasing pressure the splitting in a field observed at low temperatures increases with applied stress. This new result appears to contradict the predictions of several widely discussed theoretical models proposed to explain the phase diagram of UPt 3 .

Two independent order parameters for superconducting UPt3

The H - T phase diagram of UPt 3 , with the vector nature of H kept in mind, severely constraints Ginzburg-Landau (GL) theories. For H | c , it cannot be explained by the popular two-component vector order parameter model. Among two independent like-parity order parameters, only the case where one behaves as an A and the other as a B representation of D 6h (the point group of UPt 3 ) is allowed. This model is studied with special emphasis on the possible two-order-parameter flux lattices and possible transitions between them, and shown to compare fairly with experiment.

Ultrasonic studies of superconducting phase diagram of UPt3

We present a superconducting phase diagram of UPt3 for a field at 45° to the c-axis constructed from high-resolution ultrasonic velocity measurements. It is unambiguously determined that the HFL(T) line intersects the Hc2(T) line where a kink is observed. The available data favors the interpretation that the H(Tc∗) line terminates on the HFL(T) line at a different point.

Constraints on the order parameter of superconducting UPt 3 from the phase diagram

The tetracritical point seen in the H T phase diagram of UPt 3 for several orientations of H strongly constrains Ginzburg-Landau (GL) theories, as the GL “Hamiltonian” must have a conserved quantity Q. A two-component vector order parameter model has a Q for H∥c but no tetracritical point. Among accidentally nearly degenerate like-parity order parameters, only the case where one behaves as an A and the other as a B representation of D 6h (the point group of UPt 3) is allowed. This model is studied and compared to experiment.

Phase transitions in the mixed state of superconductors with anisotropic pairing

The symmetry approach is used to classify the superconducting lattices near H c2 and to show that for the unconventional superconductors the phase transition from one-quantum hexagonal lattice to the lattice with double or triplet magnetic flux per unit sell or with reduced number of rotational elements is quite characteristic. In the context of present theory the H T phase diagram of UPt 3 with two close superconducting phase transitions is analyzed.

Phase diagram in a heavy Fermion superconductor UPt 3

We construct a schematic phase diagram with the characteristics of the H vs. T phase diagram of UPt 3, which consists of at least three phases. We investigate possible vortex structures based on our 1D scenario where the triplet pairing function is described by a one-dimensional representation with odd-parity when the spin-orbit coupling is weak. The G-L free energy functional is numerically minimized to yield a vortex solution. For feasible values of β = 1 + 2 β 2 / β 1 = 1.2 ∼ 2.0 we have obtained a phase diagram topologically similar to that observed experimentally.

Two-order-parameter theory of the phase diagram of superconducting UPt3.

The [ital H]-[ital T] phase diagram of UPt[sub 3] has previously been shown to be incompatible with a two-component-vector order parameter. It is studied in this paper within the [ital AB]'' model, consisting of two nearly degenerate order parameters, one transforming as an [ital A] and the other as a [ital B] representation of [ital D][sub 6[ital h]]. The nature of the different one- and two-order-parameter vortex-lattice phases and the possible transitions between them are examined in detail. A quantitative comparison is made with the experimental diagram, and the parameters in the Ginzburg-Landau free energy are deduced. It is found that the flux lattice evolves continuously in the low-[ital H], low-[ital T] phase with no additional transitions between different types of two-order-parameter lattices, and that its orientation is in accord with neutron-diffraction studies near one of the inner transition lines.

Thermodynamic stability of the superconducting phase diagram of UPt3

The superconducting phase diagram ofUPt 3, as determined by accurate dilatometry as function of temperature and magnetic field (B‖c andB⊥c), exhibits three superconducting phases which meet in a tetracritical point. We show that the measured phase diagrams are thermodynamically stable and that all four phase lines are of second order. Via Ehrenfest relations the pressure dependence of the various phases has been determined. The so-calledC phase is the most stable phase under uniaxial pressure along the c-axis (p c>2.5 kbar). The resulting phase diagram in thep c -T plane yields stringent constraints on the fourth order coupling constants in the Ginzburg-Landau scenario of a vector superconducting order parameter coupled to a symmetry breaking field.

Superstructures and Superconductivity

Heavy fermion materials - so named because their conduction electrons behave as though they had extra mass - are like the cuprates in that they exhibit unusual superconducting properties. By the time the cuprates had been discovered, a good understanding of these materials was in hand. Unlike theories of high-[Tc] superconductivity, however, ideas about heavy fermions have not been the subject of great controversy. Thus, most of the effort in this backwater of condensed matter physics has focused on certain details of the behavior of one particularly well-studied compounds, UPt[sub 3]. The cause for sustained interest was that the process of developing ever more elaborate explanations for ever more elaborate experiments did not seem to converage. A recent paper by Midgley et al. reporting modulations in the crystal lattice of UPt[sub 3] suggests that theory and experiment might finally converge in a way that, while it does not threaten the broad understanding of heavy fermion systems, involves a degree of freedom ignored until now even in the face of past experience with elemental metallic uranium. Their transmission electron micrograph evidence for the existence of an incommensurate lattice modulation in UPt[sub 3] implicates this modulation as a probable source of themore » double superconducting transitions. Remarkably, the superconducting and magnetic coherence lengths, and the now discovered modulation period, are all of the same magnitude. For some time people have felt that stacking faults might be relevant to the properties of UPt[sub 3], but these new results are distinct from this. What Midgley et al. suggest is that the complicated superconducting phase diagram of UPt[sub 3] derives from the internal strain field caused by the modulation, and that this strain field lifts the degeneracy associated with unconventional pairing.« less

Superconducting phase diagram of UPt3 studied by thermal expansion and specific heat

The specific heat and thermal expansion of UPt3 were measured in the vicinity of the superconducting transition for magnetic fields up to 10 kOe, applied along the hexagonalc axis. Both thermodynamic techniques show clear evidence of two transitions in zero field, which merge at a critical field of approximately 8 kOe. The upper critical field curve determined by the resistivity for the upper transition exhibits a small but abrupt change of slope at a comparable value of the field. The phase diagram that emerges forH∥c is therefore qualitatively similar to that previously found forH⊥c, this time with a critical point (or region) aroundH*=8 kOe,T*=0.36 K. The implications of a nearly isotropic phase diagram are discussed in connection with recent theories.