Competing quantum Hall phases in the second
Landau level of 2D electron systems
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).
Giant plasticity in the quantum solid HCP 4He
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).
Quantum spin ice: pyrochlore quantum magnet
Tb2Ti2O7 at ultra-low temperatures
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).
Novel fractional quantum Hall States in
two-dimensional electron systems
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).
Bose glass and Mott glass of quasiparticles
in a doped quantum magnet
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).
Interplay of frustration and magnetic field in the two-dimensional quantum antiferromagnet Cu(tn)Cl2
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/kB = 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, Bsat= 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 Bsat 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).
Direct measurement of the Bose-Einstein condensation universality class in NiCl2-4SC(NH2)2 at ultralow temperatures
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 Hc1 approximately 2 T and a upper critical field Hc2 approximately 12 T. The results show a power-law temperature dependence of the phase transition line Hc1(T)- Hc1(0)=aT α; with α=1.47+/-0.10 and Hc1(0)=2.053 T, consistent with the 3D BEC universality class. Near Hc2, 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).
A1 and A2 transitions in superfluid
3He in 98% porosity aerogel
Superfluid 3He in high porosity aerogel is the system in which the effects of static impurities on a p-wave superfluid can be investigated in a systematic manner. Shear acoustic impedance measurements were carried out on this system (98% porosity aerogel) in the presence of magnetic fields up to 15 T at the sample pressures of 28.4 and 33.5 bars. We observed the splitting of the superfluid transition into two transitions in high fields in both bulk and liquid in aerogel. The field dependence of the splitting in aerogel resembles that of the bulk superfluid 3He caused by the presence and growth of the A1 phase. The results provide the first evidence of the A1 phase in superfluid 3He in aerogel.
Reference: H. C. Choi, A. J. Gray, C. L. Vicente, J. S. Xia, G. Gervais, W. P. Halperin, N. Mulders, and Y. Lee, Physical Review Letters 93, 145302 (2004).