“HO”

1. “HO” Using Pressure to Change the Stability of Hidden Order PI: Gregory S. Boebinger, Director National High Magnetic Field Laboratory Supported by NSF (No. DMR-0084173), and State of Florida The nature of the “hidden order” (“HO”) phase in URu2Si2 at low temperatures remains one of the more puzzling problems in condensed matter physics. It has been previously reported that intense (36 tesla) magnetic fields suppress the HO phase (black line at right). High magnetic fields (>25T) also reveal magnetoresistance oscillations whose oscillation frequencies, F, provide a direct measurement of the Fermi surface. An abrupt reconstruction of the Fermi surface within the HO phase occurs at ~31T (blue line at right), as evidenced by the shift of frequencies in the SdH spectra (red and blue arrows at lower right). Taken together with reported Hall effect results, this implies an increase in the effective carrier density and suggests that the magnetic-field suppression of the HO-state is ultimately related to destabilizing a gap in the spectrum of itinerant quasiparticles. These phase separation between HO and antiferromagnetism These observations indicate an adiabatic continuity between antiferromagnetism and the hidden order in URu2Si2, with magnetic field and pressure changing their relative weight. Y.J. Jo; L. Balicas; C. Capan; K. Behnia; P. Lejay; J. Flouquet; J.A. Mydosh; and P. Schlottmann, Physical Review Letters 98, 166404 (2007). 2007

2. P = 1 bar “HO” P = 8 kbar “HO” P = 11 kbar “HO” Using Pressure to Change the Stability of Hidden Order PI: Gregory S. Boebinger, Director National High Magnetic Field Laboratory Supported by NSF (No. DMR-0084173), and State of Florida Pressure favors Antiferromagnetism to the detriment of the “Hidden Order” State The nature of the phase at temperatures below ~17.5 K in URu2Si2 remains one of the more puzzling questions in condensed matter physics. There is strong evidence of some sort of electric or magnetic ordering in this phase, but conventional types of order, such as antiferromagnetism and charge density waves, have all been ruled out experimentally. This unexplained phase has thus been called the “hidden-order” (“HO”) state. It is located in the H-T (Temperature-Magnetic Field) phase diagram below the black line (right). Measurements of resistance at high magnetic fields find a new transition within the HO phase, shown as a blue line in the figure. This new transition is characterized by an abrupt change in the Fermi surface. While hydrostatic pressure favors antiferromagnet order in “detriment” to the hidden order, it has a modest effect on the complex H-T phase diagram. These observations indicate a continuity between the two orderings, with field and pressure changing their relative weight. Figure: Phase diagram of URu2Si2 at different pressures. Resistance measurements find a new transition (blue line) within the “Hidden Order” phase (“HO”, oulined in black). 2007