The pressure-temperature phase diagram of URu 2 Si 2 under hydrostatic conditions
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1264-Z10-03
The pressure-temperature phase diagram of URu2Si2 under hydrostatic conditions Nicholas P. Butch,1 Jason R. Jeffries,2 William J. Evans,2 Songxue Chi,3 Juscelino B. Leão,3 Jeffrey W. Lynn,3 Stanislav V. Sinogeikin,4 James J. Hamlin,5 Diego A. Zocco,5 and M. Brian Maple5 1
Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, MD 20742, USA 2 Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA 3 NIST Center for Neutron Research, Gaithersburg, MD 20899, USA 4 HPCAT, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA 5 Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA ABSTRACT The pressure dependence of the hidden order phase of the heavy fermion superconductor URu2Si2 has been a subject of intense research since shortly after the discovery of the compound decades ago. Applied pressure increases the critical temperature of the paramagnetic / hidden order transition and brings about a transition to long-range antiferromagnetism. The reported pressures and temperatures of these phase boundaries vary between studies: 4 – 7 kbar at low temperature and 12 – 15 kbar at high temperature. We review experimental evidence that the measured values of pressure and temperature are very sensitive to the chosen pressure transmitting medium. Recent x-ray diffraction measurements suggest that the relative position of the silicon atom in the unit cell is changing as a function of pressure. Recent neutron diffraction measurements show that the zero-temperature limit of the hidden order / antiferromagnetic transition occurs at pressures greater than 7.5 kbar. INTRODUCTION The moderately heavy fermion superconductor URu2Si2 is notorious for its hidden order (HO) phase, whose order parameter remains unidentified. The transition from a heavy paramagnetic state into the HO phase at approximately 17 K [1-3] is characterized by a large BCS-like specific heat anomaly [1], anomalies in ultrasound [4] and thermal expansion [5], a substantial increase in thermal conductivity [6], a hump in electrical resistivity [1], a kink in magnetic susceptibility [1], the opening of a large gap in the incommensurate spin excitation spectrum [7], the crossing of the chemical potential by a heavy electron band [8], and a small seemingly extrinsic strain-induced magnetic moment [9,10]. These phenomena point to a partial gapping of the Fermi surface at the transition into the HO state. While the existence of a small dipole moment, too small to account for the entropy released at the transition, has long been an associated mystery, it is becoming accepted that this small moment appears due to residual strain in samples. In the absence of a large dipole moment, it has been theoretically necessary to invoke exotic ordering of either local or itinerant electrons as the underlying cause. Many theories describing the HO parameter have been suggested over the years; recent proposals include nest
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