N. Harrison

Bose-Einstein condensation of S = 1 nickel spin degrees of freedom in NiCl2-4SC(NH2)2.

It has recently been suggested that the organic compound NiCl2-4SC(NH2)2 (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni S = 1 spins exhibit three-dimensional XY antiferromagnetism above a critical field H(c1) approximately 2 T. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law H - H(c1) proportional variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.

Magnetic-Field-Induced Quantum Critical Point and Competing Order Parameters in URu2Si2

A comprehensive transport study, as a function of both temperature and magnetic field in continuous magnetic fields up to 45 T reveals that URu2Si2 possesses all the essential hallmarks of quantum criticality at temperatures above 5.5 K and fields around 38 T, but then collapses into multiple low temperature phases in a hierarchically-organized phase diagram as the temperature is reduced. Although certain generic features of the phase diagram are very similar to those in the cuprates and heavy fermion superconductors, the existence of multiple ordered hysteretic phases near the field-tuned quantum critical point is presently unique to URu2Si2. This finding suggests the existence of many competing order parameters separated by small energy difference in URu2Si2.

Competition between Pauli and orbital effects in a charge-density-wave system

We present angular-dependent magnetotransport and magnetization measurements on

Observation of a multiferroic critical end point

\ensuremath{\alpha}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{MHg}(\mathrm{SCN}{)}_{4}

Critical state in a low-dimensional metal induced by strong magnetic fields

compounds at high magnetic fields and low temperatures. We find that the low-temperature ground state undergoes two subsequent field-induced density-wave-type phase transitions above a critical angle of the magnetic field with respect to the crystallographic axes. This new phase diagram may be qualitatively described assuming a charge-density-wave ground state which undergoes field-induced transitions due to the interplay of Pauli and orbital effects.

Spin-density wave fermi surface reconstruction in underdoped YBa2Cu3O6+x.

The study of abrupt increases in magnetization with magnetic field known as metamagnetic transitions has opened a rich vein of new physics in itinerant electron systems, including the discovery of quantum critical end points with a marked propensity to develop new kinds of order. However, the electric analogue of the metamagnetic critical end point, a “metaelectric” critical end point, has been rarely studied. Multiferroic materials wherein magnetism and ferroelectricity are cross-coupled are ideal candidates for the exploration of this novel possibility using magnetic-field (H) as a tuning parameter. Herein, we report the discovery of a magnetic-field-induced metaelectric transition in multiferroic BiMn2O5, in which the electric polarization (P) switches polarity along with a concomitant Mn spin–flop transition at a critical magnetic field Hc. The simultaneous metaelectric and spin–flop transitions become sharper upon cooling but remain a continuous cross-over even down to 0.5 K. Near the P = 0 line realized at μ0Hc ≈ 18 T below 20 K, the dielectric constant (ɛ) increases significantly over wide field and temperature (T) ranges. Furthermore, a characteristic power-law behavior is found in the P(H) and ɛ(H) curves at T = 0.66 K. These findings indicate that a magnetic-field-induced metaelectric critical end point is realized in BiMn2O5 near zero temperature.

The use of experimental data in constraining the tight-binding band parameters of quasi-two-dimensional organic molecular metals: application to alpha-(BEDT-TTF)2KHg(SCN)4

We present the results of magnetotransport and magnetic torque measurements on the alpha-(BEDT-TTF)2KHg(SCN)4 charge-transfer salt within the high magnetic field phase, in magnetic fields extending to 33 T and temperatures as low as 27 mK. While the high magnetic field phase (at fields greater than ~ 23 T) is expected, on theoretical grounds, to be either a modulated charge-density wave phase or a charge/spin-density wave hybrid, the resistivity undergoes a dramatic drop below ~ 3 K within the high magnetic field phase, falling in an approximately exponential fashion at low temperatures, while the magnetic torque exhibits pronounced hysteresis effects. This hysteresis, which occurs over a broad range of fields, is both strongly temperature-dependent and has several of the behavioural characteristics predicted by critical-state models used to describe the pinning of vortices in type II superconductors in strong magnetic fields. Thus, rather than exhibiting the usual behaviour expected for a density wave ground state, both the transport and the magnetic properties of alpha-(BEDT-TTF)2KHg(SCN)4, at high magnetic fields, closely resembles those of a type II superconductor.

Catastrophic Fermi Surface Reconstruction in the Shape-Memory Alloy AuZn

We consider the reconstruction expected for the Fermi surface of underdoped YBa(2)Cu(3)O(6+x) in the case of a collinear spin-density wave with a characteristic vector Q = (pi[1 +/- 2delta],pi), assuming an incommensurability delta approximately 0.06 similar to that found in recent neutron scattering experiments. A Fermi surface possibly consistent with the multiple observed quantum oscillation frequencies is obtained. From the low band masses expected using this model as compared with experiment, a uniform enhancement of the quasiparticle effective mass over the Fermi surface by a factor of approximately 7 is indicated. Further predictions of the Fermi surface topology are made, which may potentially be tested by experiment to indicate the relevance of this model to underdoped YBa(2)Cu(3)O(6+x).

Magnetocrystalline anisotropy in a single crystal of CeNiGe 2

Whilst tight-binding bandstructure calculations are very successful in describing the Fermi-surface configuration in many quasi-two-dimensional organic molecular metals, the detailed topology of the predicted Fermi surface often differs from that measured in experiments. This is very significant when, for example, the formation of a density-wave state depends critically on details of the nesting of Fermi-surface sheets. These differences between theory and experiment probably result from the limited accuracy to which the -orbitals of the component molecules (which give rise to the transfer integrals of the tight-binding bandstructure) are known. In order to surmount this problem, we have derived a method whereby the transfer integrals within a tight-binding bandstructure model are adjusted until the detailed Fermi-surface topology is in good agreement with a wide variety of experimental data. The method is applied to the charge-transfer salt -(BEDT-TTF)2KHg(SCN)4, the Fermi surface of which has been the source of much speculation in recent years. The Fermi surface obtained differs in detail from previous bandstructure calculation findings. In particular, the quasi-one-dimensional component of the Fermi surface is more strongly warped. This implies that upon nesting of these sheets, significant parts of the quasi-one-dimensional sheets remain, leading to a complicated Fermi-surface topology within the low-temperature, low-magnetic-field phase. In contrast to previous models of this phase, the model for the reconstructed Fermi surface in this work can explain virtually all of the current experimental observations in a consistent manner.

Localized f electrons in Ce x La 1 − x RhIn 5 : de Haas–van Alphen measurements

AuZn undergoes a shape-memory transition at 67 K. The de Haas-van Alphen effect persists to 100 K enabling the observation of a change in the quantum oscillation spectrum indicative of a catastrophic Fermi surface reconstruction at the transition. The coexistence of both Fermi surfaces at low temperatures suggests an intrinsic phase separation in the bulk of the material. In addition, Dingle analysis reveals a sharp change in the scattering mechanism at a threshold cyclotron radius, attributable to the underlying microstructure driving the shape-memory effect.