Neutron diffraction from the vortex lattice in the heavy-fermion superconductorUPt3

Abstract

We have used neutron diffraction to observe the vortex lattice of UPt3. This is the first such measurement in a heavy-fermion system, a superconductor below 1 K, or in a system with such a long magnetic penetration depth (6000 ± 75 A). It also provides the first value for the pair coherence length, 111 ± 2 A, to be obtained directly within the pairing state of UPta. The lattice is oblique hexagonal with conventional quantization. Its anisotropy can be explained by considering a combination of gap and Fermisurface anisotropics. The lattice does not appear to change near the transitions between the superconducting phases identified by other techniques.PACS numbers: 74.70.Tx, 61.12.Bt, 74.60.GeThere is considerable indirect evidence that the heavyfermion material UPts is not a conventional 5-wave superconductor [l]. This evidence consists primarily of the observation of power-law, rather than exponential, behavior in its thermal and transport properties. Transverse ultrasound [2] and muon spin relaxation (ju'*'SR) measurements [3] show diiTerent power laws for diff"erent crystalline directions in a manner consistent with the E\g symmetry [4]. Further indications of higher-/ pairing lie in torsional oscillator [5], ultrasound [6], and specific-heat measurements [7] which show evidence for possible phase transitions within the superconducting state. At r=0 ultrasound anomalies occur [8] when //^ 0.65//^ 2 (Hllc) and H^0.25Hc2(H±c). Since the transition occurs as a function of magnetic field, i.e., vortex density, and the probes observing it are sensitive to vortex motion, it is natural to suggest [9] that this transition corresponds to changes in the vortex lattice at this field. Because of this and the possibility that the unconventional superconducting state manifests itself directly in the vortex lattice, we have used neutron diffraction to study the vortex lattice in UPt3.Our sample is a right circular cylinder ^ 5 mm in diameter and 30 mm long oriented with c along the cylinder axis. The boule was Czochralski grown and float zone purified, and then annealed in vacuum for 150 h at 1300°C and cooled slowly to room temperature. The extrapolated r=0 resistivity along the a axis was 1.3 /iHcm, equivalent to a mean free path /-1700 A, and the superconducting transition in zero field was clearly split (from specific-heat measurement [10]), with Tc~ = 0.451 and r/=0.507 K.Neutron scattering from the vortex lattice was first suggested by de Gennes and Matricon [11], first measured in niobium by Cribier et al. [12], and subsequently observed in a number of cubic materials [13][14][15], in technetium [15], and in YBa2Cu307 [16]. Our experiment was performed using the small-angle neutron scattering (SANS) apparatus in the cold-neutron guide hall of the Ris</i DR3 reactor. A 6-m-pinhole collimator gave incident beam divergences of --0.14° FWHM (horizontal) and ~0.30° FWHM (vertical). The dimensions of the beam impinging on the sample were 4.5 mm (horizontal) x25 mm (vertical). The incident neutron wavelength Xn was varied bet...

Phase diagram of the unconventional superconductorUPt3in the weak-crystal-field model

“…Therefore, it is the one-quantum flux lattices which should be observed in the neutron diffraction measurements in UPt 3 . 22 The period multiplication in the vortex lattice can be found only by phase sensitive techniques.…”

“…As is known from neutron scattering experiments, 18 the tiny magnetic moments on U atoms are ordered antiferromagnetically with a very short correlation length aϷ150 Å, which is comparable to the superconducting coherence length at zero temperature, 0 Ϸ110 Å. 22 In order to have a nearly isotropic H c2 in the basal plane 23 one should assume that the moments rotate freely, maintaining their perpendicular orientation with respect to the magnetic field. Even if this theoretical assumption is correct, a serious problem still exists for zero-field behaviors.…”

Absence of zero field muon spin relaxation induced by superconductivity in the B phase of UPt3

“…and Broholm et al [20] are consistent; iii) the sample of these latter authors which is described in some detail in Refs. [20] and [21] was not annealed, i.e. it was not of the best possible quality.…”

“…We have inferred this conclusion using the following reasoning : i) additional transverse field µSR measurements on our samples show that the transverse relaxation rates are clearly larger that in the samples of Luke et al [19]; ii) the transverse relaxation rates measured by Luke et al and Broholm et al [20] are consistent; iii) the sample of these latter authors which is described in some detail in Refs. [20] and [21] was not annealed, i.e. it was not of the best possible quality.…”

Nonlinear magneto-optical response ofs- andd-wave superconductors

“…Nevertheless, we believe that the nonlinear optic can yield information complementary to the tunneling experiments and might be also of importance in view of the application on heavy fermion superconductors, where it was also manifested that the superconducting state is anomalous. [35][36][37] …”