Ehrenfest relations and magnetoelastic effects in field-induced ordered phases

Magnetoelastic properties in field-induced magnetic ordered phases are studied theoretically based on a Ginzburg-Landau theory. A critical field for the field-induced ordered phase is obtained as a function of temperature and pressure, which determine the phase diagram. It is found that magnetic field dependence of elastic constant decreases discontinuously at the critical field, Hc, and that it decreases linearly with field in the ordered phase (H>Hc). We found an Ehrenfest relation between the field dependence of the elastic constant and the pressure dependence of critical field. Our theory provides the theoretical form for magnetoelastic properties in field- and pressure-induced ordered phases.Comment: 7 pages, 3 figure

Neutron scattering study of the field-dependent ground state and the spin dynamics in S=1/2 NH4CuCl3

Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH4CuCl3, which shows plateaus in the magnetization curve at m=1/4 and m=3/4 of the saturation value. Two structural phase transitions at T1≈156  K and at T2=70  K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below TN=1.3  K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model

Spinon localization in the heat transport of the spin-1/2 ladder compound (C5_5H12_{12}N)2_2CuBr4_4

We present experiments on the magnetic field-dependent thermal transport in the spin-1/2 ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$. The thermal conductivity $\kappa(B)$ is only weakly affected by the field-induced transitions between the gapless Luttinger-liquid state realized for $B_{c1}< B < B_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to the heat transport. We observe, however, that the thermal conductivity is strongly suppressed by the magnetic field deeply within the Luttinger-liquid state. These surprising observations are discussed in terms of localization of spinons within finite ladder segments and spinon-phonon umklapp scattering of the predominantly phononic heat transport.Comment: 4 pages, 3 figure

Quantum and classical criticality in a dimerized quantum antiferromagnet

A quantum critical point (QCP) is a singularity in the phase diagram arising due to quantum mechanical fluctuations. The exotic properties of some of the most enigmatic physical systems, including unconventional metals and superconductors, quantum magnets, and ultracold atomic condensates, have been related to the importance of the critical quantum and thermal fluctuations near such a point. However, direct and continuous control of these fluctuations has been difficult to realize, and complete thermodynamic and spectroscopic information is required to disentangle the effects of quantum and classical physics around a QCP. Here we achieve this control in a high-pressure, high-resolution neutron scattering experiment on the quantum dimer material TlCuCl3. By measuring the magnetic excitation spectrum across the entire quantum critical phase diagram, we illustrate the similarities between quantum and thermal melting of magnetic order. We prove the critical nature of the unconventional longitudinal ("Higgs") mode of the ordered phase by damping it thermally. We demonstrate the development of two types of criticality, quantum and classical, and use their static and dynamic scaling properties to conclude that quantum and thermal fluctuations can behave largely independently near a QCP.Comment: 6 pages, 4 figures. Original version, published version available from Nature Physics websit

High resolution millimeter wave SAR interferometry

High resolution millimeter wave synthetic aperture radar (SAR) interferometry is presented using the MEMPHIS multi-baseline InSAR system. A complete processing chain is used to generate digital elevation models starting from the radar raw data. A deeper focus is laid on the phase unwrapping step, which is achieved using the multi-baseline properties of the system. In November 2006, an experiment was realized including two test sites in Switzerland; the actual results are presented and discussed

Microscopic model for the magnetization plateaus in NH4CuCl3

A simple model consisting of three distinct dimer sublattices is proposed to describe the magnetism of NH4CuCl3. It explains the occurrence of magnetization plateaus only at 1/4 and 3/4 of the saturation magnetization. The field dependence of the excitation modes observed by ESR measurements is also explained by the model. The model predicts that the magnetization plateaus should disappear under high pressure.Comment: 4 pages, 5 figures, REVTeX

Uniaxial pressure dependencies of the phase boundary of TlCuCl_3

We present a thermal expansion and magnetostriction study of TlCuCl_3, which shows a magnetic-field induced transition from a spin gap phase to a Neel ordered phase. Using Ehrenfest relations we derive huge and strongly anisotropic uniaxial pressure dependencies of the respective phase boundary, e.g. the transition field changes by about $\pm 185$ GPa depending on the direction of uniaxial pressure.Comment: 2 pages, e figures; presented at SCES200

Thermodynamics of the Spin Luttinger-Liquid in a Model Ladder Material

The phase diagram in temperature and magnetic field of the metal-organic, two-leg, spin-ladder compound (C5H12N)2CuBr4 is studied by measurements of the specific heat and the magnetocaloric effect. We demonstrate the presence of an extended spin Luttinger-liquid phase between two field-induced quantum critical points and over a broad range of temperature. Based on an ideal spin-ladder Hamiltonian, comprehensive numerical modelling of the ladder specific heat yields excellent quantitative agreement with the experimental data across the complete phase diagram.Comment: 4 pages, 4 figures, updated refs and minor changes to the text, version accepted for publication in Phys. Rev. Let

A portable, modular, self-contained recirculating chamber to measure benthic processes under controlled water velocity

Citation: Ruegg, J., Brant, J. D., Larson, D. M., Trentman, M. T., & Dodds, W. K. (2015). A portable, modular, self-contained recirculating chamber to measure benthic processes under controlled water velocity. Freshwater Science, 34(3), 831-844. doi:10.1086/682328We report the design, construction, and functional characteristics of a sealable, portable chamber for measuring benthic metabolic process rates, particularly those under unidirectional flow as found in streams. The design optimizes inherent tradeoffs, such as size, stability, and cost, associated with chambers built for field-based measurements. The chamber is small enough to be portable and minimizes the water-volume: benthic surface-area ratio. In addition, the chamber is clear to allow measurement of photosynthetic rates. The design minimizes power draw to sustain water velocities found at stream field sites and is modular to allow easy disassembly and cleaning. The design is relatively simple, thereby increasing sturdiness, minimizing construction costs, and decreasing the expertise required to build the unit. We demonstrated the performance characteristics, specifically amperage needed to achieve desired water velocity, flow heterogeneity and turbulence in the working area, the degree of isolation from atmosphere, mixing rate of solute injectate, and heating rate of the chamber. We provide proof of concept with data for in situ benthic rates (gross community production, community respiration, and NH4+ uptake). Publications on metabolic chambers built for in situ use do not typically report performance characteristics, so it is difficult to compare our design to existing literature. We include chamber characteristics to clarify the advantages and limitations of benthic rates measured in such chambers