Evidence for broken time-reversal symmetry in the superconducting phase of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$

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Evidence for broken time-reversal symmetry in the superconducting phase of ${URu}_{2} {Si}_{2}$

E. R. Schemm, R. E. Baumbach, P. H. Tobash, F. Ronning, E. D. Bauer, and A. Kapitulnik

Phys. Rev. B 91, 140506(R) – Published 30 April 2015

Abstract

Recent experimental and theoretical interest in the superconducting phase of the heavy-fermion material ${URu}_{2} {Si}_{2}$ has led to a number of proposals in which the superconducting order parameter breaks time-reversal symmetry (TRS). In this study we measure the polar Kerr effect (PKE) as a function of temperature for several high-quality single crystals of ${URu}_{2} {Si}_{2}$ . We find an onset of PKE below the superconducting transition that is consistent with a TRS-breaking order parameter. This effect appears to be independent of an additional, possibly extrinsic, PKE generated above the hidden order transition at $T_{HO} = 17.5$ K, and contains a structure below $T_{c}$ suggestive of additional physics within the superconducting state.

Received 8 October 2014
Revised 25 March 2015

DOI:https://doi.org/10.1103/PhysRevB.91.140506

Authors & Affiliations

E. R. Schemm^1,2, R. E. Baumbach^3,*, P. H. Tobash³, F. Ronning³, E. D. Bauer³, and A. Kapitulnik^1,2,4,†

¹Department of Physics, Stanford University, Stanford, California 94305, USA
²Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
³Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
⁴Department of Applied Physics, Stanford University, Stanford, California 94305, USA

^*Present address: National High Magnetic Field Laboratory, Tallahassee, FL 32310, USA.
^†Author to whom correspondence should be addressed: aharonk@stanford.edu

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Vol. 91, Iss. 14 — 1 April 2015

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Figure 1
Resistivity data for the $RRR = 267 {URu}_{2} {Si}_{2}$ single crystal studied in this work. Analogous characterization of the two other samples yielded RRR values of 318 and 453. (a) The high-temperature data shows the broad Kondo peak associated with the formation of heavy charge carriers. The transitions at (b) $T_{c} (midpoint) = 1.49$ K and at (c) $T_{HO} = 17.5$ K are clearly resolved.
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Figure 2
Kerr effect data taken on a ${URu}_{2} {Si}_{2}$ single crystal with $RRR = 267$ upon warmup in a true zero-field ( $μ − metal$ shielded) environment. Right axis: Resistivity data (open blue points) from Fig. 1 showing the superconducting transition at $T_{c} (midpoint) = 1.49$ K. Left axis: Change in the Kerr angle (solid red circles) relative to the background at 5 K, showing an onset of the Kerr signal at $T_{c}$ and a sharp anomaly at $\sim 1$ K. Error bars in the Kerr effect data are statistical.
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Figure 3
Field training of PKE in the superconducting phase of ${URu}_{2} {Si}_{2}$ . Open red circles: Zero-field warmup data following +100 G cooldown in the same crystal. Solid blue circles: Zero-field warmup data following $- 100$ G cooldown, showing a complete reversal of the Kerr signal.
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Figure 4
Kerr effect in the hidden order phase. Main panel: Kerr effect in the HO regime following high field training for the $RRR = 318$ crystal. The remanent Kerr rotation depends on the magnitude of the training field, up to about 1 T, in contrast to the field-training response expected for TRSB developing through a second-order phase transition. The collection of $θ_{K} (T)$ curves does not converge until well above $T_{HO}$ . Inset: A small, field-trainable Kerr rotation begins in the vicinity of $T_{HO} = 17.5$ K in all samples. This signal saturates by $T = 5$ K, permitting a mostly independent examination of PKE in the superconducting state relative to this background. $PFC = positive$ (+100 G) field-cooled, $NFC = negative$ ( $- 100$ G) field-cooled, $ZFC = zero$ -field-cooled.
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Figure 5
Independent field training of the hidden order and superconducting PKE signals in ${URu}_{2} {Si}_{2}$ for the sample with $RRR = 267$ . Red and blue (open and solid symbols) indicate whether a positive or negative training field was applied during cooldown through each temperature range. The solid line is a guide to the eye of the form $[1 - {(T / T_{onset})}^{2}]$ . Inset: Magnification of the lower left portion of the main panel data, showing more clearly the reversal of Kerr signal, including the $\sim 1$ K feature, in the superconducting state, with no change in background.
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Physical Review B

covering condensed matter and materials physics

Evidence for broken time-reversal symmetry in the superconducting phase of ${URu}_{2} {Si}_{2}$

E. R. Schemm, R. E. Baumbach, P. H. Tobash, F. Ronning, E. D. Bauer, and A. Kapitulnik

Phys. Rev. B 91, 140506(R) – Published 30 April 2015

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Physical Review B

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Evidence for broken time-reversal symmetry in the superconducting phase of URu2Si2

E. R. Schemm, R. E. Baumbach, P. H. Tobash, F. Ronning, E. D. Bauer, and A. Kapitulnik

Phys. Rev. B 91, 140506(R) – Published 30 April 2015

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Evidence for broken time-reversal symmetry in the superconducting phase of ${URu}_{2} {Si}_{2}$