62 research outputs found
Low-temperature transport in ultra-thin tungsten films grown by focused-ion-beam deposition
We have fabricated tungsten-containing films by focused-ion-beam (FIB)-induced chemical vapour deposition. By using ion-beam doses below 50 pC/μm² on a substrate of amorphous silicon, we have grown continuous films with thickness below 20 nm. The low-temperature electron transport properties were investigated by measuring current-voltage characteristics for temperatures down to 400 mK and in magnetic fields up to 8 T. FIB-deposited tungsten films are known to have an enhanced transition temperature compared to bulk tungsten [1], and films with thickness down to 50 nm have been investigated [2]. The films in this work are closer to the limit of two-dimensional system and are superconducting below 1 K, with a finite resistance at zero bias current. This work is supported by EPSRC. [1] E S Sadki, S Ooi, and K Hirata, Appl. Phys. Lett. 85, 6206 (2004); I J Luxmoore, I M Ross, A G Gullis, P W Fry, J Orr, P D Buckle, and J H Jefferson, Thin Solid Films 515, 6791 (2007). [2] W Li, J C Fenton, Y Wang, D W McComb, and P A Warburton, J. Appl. Phys. 104, 093913 (2008)
Thermo-electric properties of one-dimensional constrictions
This thesis describes low-temperature transport measurements in low-dimensional systems fabricated in high-mobility GaAs/AlGaAs heterostructures. These low-dimensional systems are formed by electrostatically constricting the electrons in the two-dimensional electron gas at the interface of the heterostructure, by applying a voltage to a pair of metallic gates known as a split-gate. At low temperatures the electrical conduction occurs without scattering. The aim is to measure the thermal conductance of these one-dimensional ballistic conductors. The thermal conductance of a split-gate device was measured as a function of gate voltage, over a wide range of temperatures and in the absence of a magnetic field. The electrons on one side of the constriction were heated with an electric current, and the temperature drop across the split-gate was measured using the thermopower of another split-gate. The measurements show that the thermal conductance displays plateaux corresponding to the one-dimensional subbands, confirming previous results. The design of the samples allows a quantitative test of the Wiedemann-Franz law in one-dimensional constrictions. The results strongly suggest that the Wiedemann-Franz law is satisfied, and new information was obtained regarding the anomaly in the conductance known as 0.7 structure, which can provide a new insight into the nature of this anomaly. It is found that the thermal conductance corresponding to the anomaly is suppressed with respect to the value expected from a single-particle picture
Magnetoacoustics of the Low-Dimensional Quantum Antiferromagnet Cs2CuCl4 with Spin Frustration
We report on results of sound-velocity and sound-attenuation measurements in the triangular-lattice spin-1/2 antiferromagnet Cs₂CuCl₄ (T<sub>N</sub> = 0.6 K), in external magnetic fields up to 14 T, applied along the <em>b</em> axis, and at temperatures down to 300 mK. The results are analyzed with a quasi-two-dimensional hard-core boson theory based on exchange-striction coupling. There is a good qualitative agreement between theoretical and experimental results.Citation: Sytcheva, A. et al. (2010). 'Magnetoacoustics of the low-dimensional quantum antiferromagnet Cs₂CuCl₄', Journal of Low Temperature Physics, 159(1-2), 109-113. [Available at http://www.springer.com/materials/journal/10909]. © The Authors 2010. This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any non-commercial use, distribution, and reproduction in any medium, provided the original authors and source are credited.
Spin-strain coupling in NiCl2-4SC(NH2)2
We report results of ultrasonic investigations of the quantum S = 1 spin-chain magnet NiCl2-4SC(NH2)2, also known as DTN, in magnetic fields up to 18 T and temperatures down to 0.3 K. A field H along the [001] direction induces a transition into an antiferromagnetic phase with T(N)max ≈ 1.2 K. Accordingly, at T = 0 there are two quantum critical points at ~2.1 T and at ~12.6 T. The acoustic c33 mode, propagating along the spin chains, shows a pronounced softening close to the phase transition, accompanied by energy dissipation of the sound wave. The H-T phase diagram obtained from our measurements is compared with results from other experimental investigations and the low-temperature acoustic anomalies are traced up to T > T(N). We also report frequency-dependent effects, which open the possibility to investigate the spin fluctuations in the critical regions. Our observations show an important role of the spin-phonon coupling in DTN
Magnetoelectric effects in an organo-metallic quantum magnet
We observe a bilinear magnetic field-induced electric polarization of 50 in single crystals of NiCl-4SC(NH) (DTN). DTN forms a
tetragonal structure that breaks inversion symmetry, with the highly polar
thiourea molecules all tilted in the same direction along the c-axis.
Application of a magnetic field between 2 and 12 T induces canted
antiferromagnetism of the Ni spins and the resulting magnetization closely
tracks the electric polarization. We speculate that the Ni magnetic forces
acting on the soft organic lattice can create significant distortions and
modify the angles of the thiourea molecules, thereby creating a magnetoelectric
effect. This is an example of how magnetoelectric effects can be constructed in
organo-metallic single crystals by combining magnetic ions with electrically
polar organic elements.Comment: 3 pages, 3 figure
Interplay of Spin and Lattice Degrees of Freedom in the Frustrated Antiferromagnet CdCr_2O_4: High-field and Temperature Induced Anomalies of the Elastic Constants
Temperature and magnetic field studies of the elastic constants of the
chromium spinel CdCr_2O_4 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
theory based on an exchange-striction mechanism leads to an estimate of the
strength of the magneto-elastic 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 discussed and we compare the
spin-phonon coupling in CdCr_2O_4 in both the ordered and disordered states.Comment: 12 pages, 8 figures; Appendix added,To appear in Phys Rev.
Character of magnetic excitations in a quasi-one-dimensional antiferromagnet near the quantum critical points: Impact on magneto-acoustic properties
We report results of magneto-acoustic studies in the quantum spin-chain
magnet NiCl-4SC(NH) (DTN) having a field-induced ordered
antiferromagnetic (AF) phase. In the vicinity of the quantum critical points
(QCPs) the acoustic mode manifests a pronounced softening accompanied
by energy dissipation of the sound wave. The acoustic anomalies are traced up
to , 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.Comment: 5 pages, 4 figures. Accepted for publication by Phys. Rev
Quantum fluctuations and strong mass renormalization in NiCl2-4SC(NH2)2
In a number of quantum paramagnets, magnetic field can induce a quantum phase
transition to an antiferromagnetic state which exists for a range of fields Hc1
< H < Hc2. Generally, these compounds exhibit a significant asymmetry in their
properties at low- and high-field transitions. Here we present detailed
specific heat and thermal conductivity measurements in NiCl2-4SC(NH2)2 together
with analytical and numerical results. We show that the asymmetry is caused by
a strong renormalization of the effective mass of spin excitations due to
quantum fluctuations for H<Hc1 that are absent for H<Hc2.Comment: 4 pages, 3 figures. Acepted for publication in Phy. Rev. Let
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