Research Highlights

New physics has had to be invoked to explain the existence of exotic quantum Hall states such as the n =5/2 and 7/2 states. Recent progress in fabrication of high-quality low-density samples allows one to probe these states in a new regime where the electron-electron interactions are strong. The results reveal the existence of anisotropic transport for n = 7/2 in a high-quality very dilute 2D electron system. The new behavior is attributed to a large Landau level mixing effect that perturbs the pairing stability of composite fermions in the dilute limit.

Reference: W. Pan, A. Serafin, J. S. Xia, L. Yin, N. S. Sullivan, K. W. Baldwin, K. W. West, L. N. Pfeiffer, and D. C. Tsui, Physical Review B 89, 24130R (2014).

High precision NMR studies of dilute impurities in solid ⁴He have demonstrated the existence of an unexpected lattice relaxation at low temperatures (T < 0.2 K). This new effect is attributed to the quantum plasticity reported in studies of the elastic constants in the same temperature regime by Rojas, Balibar, Beamish and colleagues.

Reference: D. Candela, C. Huan, S. S. Kim, L. Yin, J. S. Xia, and N. S. Sullivan, Journal of Physics Conference Series 568 (1), 012017 (2014).

New research at the lab’s High B/T facility supports the proposal that the disordered ground state of terbium titanate is a quantum spin ice.

Reference: L. Yin, J. S. Xia, Y. Takano, N. S. Sullivan, Q. J. Li, and X. F. Sun, Physical Review Letters 110, 137201 (2013).

This experiment probes the nature of the 12/5 fractional quantum Hall state by using a hydraulic-driven rotator to tilt the two-dimensional system in a magnetic field.

Reference: C. Zhang, C. Huan, J. S. Xia, N. S. Sullivan, W. Pan, K. W. Baldwin, K. W. West, L. N. Pfeiffer, and D. C. Tsui, Physical Review B 85, 241302 (2012).

A Bose glass state of field-induced magnetic quasiparticles has been observed in a doped quantum magnet (bromine-doped dichloro-tetrakis-thiourea-nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand canonical ensemble; bromine doping introduces disorder into the hopping and interaction strength of the bosons, leading to their localization into a Bose glass down to zero field, where it becomes an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the Bose–Einstein condensate (corresponding to a magnetically ordered phase) is marked by a universal exponent that governs the scaling of the critical temperature with the applied field.

Reference: R. Yu, L. Yin, N. S. Sullivan, J. S. Xia, C. Huan, A. Paduan-Filho, N. F. Oliveira Jr, S. Haas, A. Steppke, C. F. Miclea, F. Weickert, R. Movshovich, E.-D. Mun, B. L. Scott, V. S. Zapf, and T. Roscilde, Natur 489, 379-384 (2012).

Specific heat and ac magnetic susceptibility measurements at low temperatures (T ≥ 40 mK) and high-magnetic fields (B ≤14 T), have been performed on a two-dimensional (2D) antiferromagnet Cu(tn)Cl₂ (tn = 1,3-diaminopropane = C₃H₁₀N₂). The compound represents a S=1/2 spatially anisotropic triangular antiferromagnet realized by a square lattice with nearest-neighbor (J/k_B = 3 K), frustrating next-nearest-neighbor (0 < J' /J <0.6), and interlayer (|J"/J| ≈ 10⁻³) interactions.The absence of long-range magnetic order down to T = 60 mK in B = 0 and the T² behavior of the specific heat for T ≤ 0.4 K and B ≥ 0 are considered evidence of a high degree of 2D magnetic order. In fields lower than the saturation field, B_sat= 6.6 T , a specific heat anomaly, appearing near 0.8 K, is ascribed to bound vortex-antivortex pairs stabilized by the applied magnetic field. The resulting magnetic phase diagram is remarkably consistent with the one predicted for a square lattice without a frustrating interaction, expect that B_sat is shifted to values lower than expected.

Reference: A. Orendáčová, E. Čižmár, L. Sedláková, J. Hanko, M. Kajňaková, M. Orendáč, A. Feher, J. S. Xia, L. Yin, D. M. Pajerowski, M. W. Meisel, V. Zeleňák, S. Zvyagin, and J. Wosnitza, Physical Review B 80, 144418 (2010).

A field-induced Bose-Einstein condensation (BEC) in the organic compound NiCl₂-4SC(NH₂)₂ using ac susceptibility measurements down to 1 mK. The Ni S=1 spins exhibit 3D XY antiferromagnetism between a lower critical field H_c1 approximately 2 T and a upper critical field H_c2 approximately 12 T. The results show a power-law temperature dependence of the phase transition line H_c1(T)- H_c1(0)=aT ^α; with α=1.47+/-0.10 and H_c1(0)=2.053 T, consistent with the 3D BEC universality class. Near H_c2, a kink was found in the phase boundary at approximately 150 mK.

Reference: L. Yin, J. S. Xia, V. S. Zapf, N. S. Sullivan, and A. Paduan-Filho, Physical Review Letters 101, 187205 (2008).