We use an electron thermometer to measure the temperature rise of approximately 2 x 10(5) electrons in a two-dimensional box, due to heat flow into the box through a ballistic one-dimensional (1D) constriction. Using a simple model we deduce the thermal conductance kappa(Vg) of the 1D constriction, which we compare to its electrical conductance characteristics; for the first four 1D subbands the heat carried by the electrons passing through the wire is proportional to its electrical conductance G(Vg). In the vicinity of the 0.7 structure this proportionality breaks down, and a plateau at the quantum of thermal conductance pi(2)k(2/B)T/3h is observed.

Low-field magnetotransport measurements of topological insulators such as Bi2Se3 are important for revealing the nature of topological surface states by quantum corrections to the conductivity, such as weak-antilocalization. Recently, a rich variety of high-field magnetotransport properties in the regime of high electron densities (∼1019 cm−3) were reported, which can be related to additional two-dimensional layered conductivity, hampering the identification of the topological surface states. Here, we report that quantum corrections to the electronic conduction are dominated by the surface states for a semiconducting case, which can be analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the case of strong spin-orbit interaction. However, in the metallic-like case this analysis fails and additional two-dimensional contributions need to be accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance prove as strong indications for the two-dimensional layered metallic behavior. Temperature-dependent magnetotransport properties of high-quality Bi2Se3 single crystalline exfoliated macro and micro flakes are combined with high resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy, confirming the structure and stoichiometry. Angle-resolved photoemission spectroscopy proves a single-Dirac-cone surface state and a well-defined bulk band gap in topological insulating state. Spatially resolved core-level photoelectron microscopy demonstrates the surface stability.

We report results of magneto-acoustic studies in the quantum spin-chain magnet NiCl2-4SC(NH2)2 (DTN) having a field-induced ordered antiferromagnetic (AF) phase. In the vicinity of the quantum critical points (QCPs) the acoustic c33 mode manifests a pronounced softening accompanied by energy dissipation of the sound wave. The acoustic anomalies are traced up to T > TN , where the thermodynamic properties are determined by fermionic magnetic excitations, the "hallmark" of one-dimensional (1D) spin chains. On the other hand, as established in earlier studies, the AF phase in DTN is governed by bosonic magnetic excitations. Our results suggest the presence of a crossover from a 1D fermionic to a 3D bosonic character of the magnetic excitations in DTN in the vicinity of the QCPs. The interest in quasi-1D quantum spin systems has grown considerably during the last decade. This is fostered by the progress in preparing materials with welldefined 1D spin subsystems and the possibility of analyzing the experimental data with the help of nonperturbative theories for 1D models.1 In addition, such systems often manifest quantum phase transitions at T =0 which are governed by parameters other than the temperature. True 1D models do not exhibit any longrange order at finite temperatures.1 Real quasi-1D antiferromagnetic (AF) materials, containing weakly coupled spin chains with gapless spectra of their low-lying excitations, are usually magnetically ordered at low temperatures. At temperatures higher than the Neél temperature, T N , but of the order of the exchange constant, these systems behave as quantum spin chains, where any longrange magnetic order is destroyed by enhanced quantum fluctuations.1 One should note that quasi-1D magnets, in which the low-energy eigenstates of their 1D subsystems have spin gaps, usually do not manifest long-range magnetic ordering.2 However, an external magnetic field can close the spin gap, ∆, and for H > H c ∼ ∆ a quantum phase transition to a phase with gapless spin excitations takes place. A further increase of the field yields a second quantum phase transition to a spin-polarized phase at H > H s . In the spin-polarized phase the low-energy excitations are also gapped. Hence, the magnetically ordered phase can be observed in the field domain where spin excitations are gapless, and the Néel temperature in such systems is field dependent. The magnetic susceptibilities of a quasi-1D spin system in mean-field approximation can be written aswhere the superscript (1) denotes the susceptibility of one chain, α = x, y, z, J ⊥ is the weak interchain exchange constant, Z is the coordination number, and q is the wave vector. The quasi-1D spin system becomes ordered when the denominator becomes zero (which defines T N (H)). The low-T thermodynamics of a state with long-range magnetic order is determined by bosonic excitations, magnons. Recently, several groups have observed phenomena in some AF systems that have been interpreted as Bose-Einstein condensation (BEC) of magnons, viz., as a thermodynami...

Temperature and magnetic field studies of the elastic constants of the chromium spinel CdCr2O4 show pronounced anomalies related to strong spin-phonon coupling in this frustrated antiferromagnet. A detailed comparison of the longitudinal acoustic mode propagating along the [111] direction with a theory based on an exchange-striction mechanism leads to an estimate of the strength of the magnetoelastic interaction. The derived spin-phonon coupling constant is in good agreement with previous determinations based on infrared absorption. Further insight is gained from intermediate and high magnetic field experiments in the field regime of the magnetization plateau. The role of the antisymmetric Dzyaloshinskii-Moriya interaction is discussed.

Helically spin-polarized Dirac fermions (HSDF) in protected topological surface states (TSS) are of high interest as a new state of quantum matter. In three-dimensional (3D) materials with TSS, electronic bulk states often mask the transport properties of HSDF. Recently, the high-field Hall resistance and low-field magnetoresistance indicate that the TSS may coexist with a layered two-dimensional electronic system (2DES). Here, we demonstrate quantum oscillations of the Hall resistance at temperatures up to 50 K in nominally undoped bulk Bi2Se3 with a high electron density n of about 2·1019 cm−3. From the angular and temperature dependence of the Hall resistance and the Shubnikov-de Haas oscillations we identify 3D and 2D contributions to transport. Angular resolved photoemission spectroscopy proves the existence of TSS. We present a model for Bi2Se3 and suggest that the coexistence of TSS and 2D layered transport stabilizes the quantum oscillations of the Hall resistance.