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abstract: 'We investigated low field vortex dynamics in a single crystal of U$_{.9725}$Th$_{.0275}$Be$_{13}$.We found a sharp transition in the vortex creep rate at the lower transition temperature $T_{c2}$, coincident with the second jump in the specific heat. In the high-temperature phase, rather strong creep rates are observed. In the low temperature phase, the rates drop to undetectabely low levels. This behaviour indicates that a very strong pinning mechanism is present in the low temperature phase of U$_{.9725}$Th$_{.0275}$Be$_{13}$, which could be explained by the existence of domain walls, separating discreetly degenerate states of a superconductor, that can sustain fractional vortices and thus act as very strong pinning centers.'
address:
- 'Laboratorium für Festkörperpkysik, ETH Hönggerberg, 8093 Zürich, Switzerland'
- 'Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA'
author:
- Elisabeth Dumont
- Ana Celia Mota
- 'James L. Smith'
title: 'Unconventional strong pinning in the low temperature phase of U$_{.9725}$Th$_{.0275}$Be$_{13}$'
---

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Heavy Fermions; Superconductivity; Vortex creep

If a small amount of uranium is substituted for thorium in the heavy fermion superconductor UBe$_{13}$, the superconducting transition temperature shows a non-monotonic dependence on thorium concentration $x$. Moreover, in a small critical range $0.019<x<0.045$ a second transition occurs below the superconducting jennifernicholson@example.com. The nature of this phase transition is still a matter of controversy. In zero field $\mu$SR measurements, a continuous increase of $\mu$-spin-relaxation rate, which sets in at $T_{c2}$ has been observed@muSR_UTB. This has been interpreted as originating from a very weak spontaneous magnetic field. Superconducting origin of the transition at $T_{c2}$ has been deduced from the observation of a distinct increase in the slope of the lower critical field $H_{c1}$@Hc1_UTB at $T_{c2}$.

Recently, the low-field magnetic properties of the heavy fermion superconductor UPt$_{3}$ have been investigated @andreas. In the low–field, low–temperature B–phase, bulk vortices remain so strongly pinned that the creep rates drop from rather high finite rates observed in the A–phase to undetectabely low levels at $T_{c}^{-}$. The observation of zero initial creep in the B–phase of UPt$_{3}$ was then interpreted as originating from an intrinsic pinning mechanism in multicomponent superconducting phases. In such phases fractional vortices get trapped in domain walls separating discreetly degenerate states of the jennifernicholson@example.com. These experimental results show that vortex creep measurements can be used as a powerful probe to give information about the character of a superconducting phase.

We present here relaxation measurements of the remanent magnetization on a single crystal of <span>U$_{.9725}$Th$_{.0275}$Be$_{13}$</span>(size: $1.00\times 0.89 \times 2.25$mm$^3$) prepared in Los Alamos National jennifernicholson@example.com. The experimental arrangement has been described earlier @andreas. The single crystal has a transition temperature $T_{c1}=523$mK. Specific heat measurements @art show a second jump at $T_{c2}=350$mK. Typical relaxation measurements of the remanent magnetization are shown in fig. \[decays\]. All the values of $M_{rem}$ are taken with the sample in the fully critical state.

![Isothermal relaxation measurements of remanent $M_{rem}$ in <span>U$_{.9725}$Th$_{.0275}$Be$_{13}$</span>. Temperatures are given in mK on the right hand side of the graph. <span data-label="decays"></span>](UTB_decays.eps)

We observe two different regimes of vortex creep, separated by a dramatic change in creep rates at $T=T_{c2}$. For temperatures above $T_{c2}$, we observe strong vortex creep and a logarithmic time dependence of the decays. For temperatures below $T_{c2}$, the initial vortex creep drops to zero. At longer times the decays start deviating from logarithmic behaviour. This acceleration of vortex creep at long times has also been observed in jennifernicholson@example.com. Here we only discuss the initial slope of the decays. The normalized creep rates ($\partial \ln M/ \partial \ln t$) are plotted in Fig.\[rates\] as a function of temperature. At $T=T_{c2}$, we observe a sharp transition in creep rates. They drop by 3 orders of magnitude to zero within our sensitivity ($\partial \ln M/ \partial 
\ln t \approx 10^{-5}$). Also shown in Fig.\[rates\] are measurements of the lower critical field $H_{c1}$ on the same crystal. $H_{c1}$ was obtained as the field where the first deviation from the initial slope of isothermal magnetization–curves occurs.

![Creep rates (closed circles, right axis) and lower critical field $H_{c1}$ (open diamonds, left axis) of <span>U$_{.9725}$Th$_{.0275}$Be$_{13}$</span>plotted vs $T^2$. The lines are guides to the eye.<span data-label="rates"></span>](UTB_Hc1.eps)

In conclusion, we have observed a sharp transition in vortex pinning in <span>U$_{.9725}$Th$_{.0275}$Be$_{13}$</span>at $T_{c2}$, similar to the one observed in UPt$_{3}$ at $jennifernicholson@example.com. This might hint towards a new type of strong pinning structures and/or a new type of vortices being present in the low–T phases of these two multiphase superconductors.

<span>9</span> H. R. Ott et al., Phys. Rev. B <span>**31**</span> (1985) 1651. R. H. Heffner et al., Phys. Rev. B <span>**40**</span> (1989)806. U. Rauchschwalbe et al., Europhys. Lett. <span>**3**</span> (1987)757. A. Amann et al., Phys. Rev. B <span>**57**</span> (1998) 3640. M. Sigrist, K. Ueda, Rev. Mod. Phys. <span>**63**</span> (1992) 239. A. Ramirez, private communication. J.L. Smith, Phil. Mag. B, <span>**65**</span> (1992) 1367.

{'source': 'arXiv_out/cond-mat0001182_extract_LT22new'}