G. Reményi

Heat capacity of CuGeO3 : Sensitivity to crystalline quality

Abstract Specific heat measurements were carried out on high quality monocrystalline samples of copper germanate CuGeO3. Ageing and fragility of the high quality samples is evidenced through comparison of the various available literature data. These together with our results are analysed within the mean field theory for the spin-Peierls transition.

Interplay between phase defects and spin polarization in the specific heat of the spin-density-wave compound (TMTTF)2Br in a magnetic field.

Equilibrium heat relaxation experiments provide evidence that the ground state of the commensurate spin-density-wave compound (TMTTF)2Br after the application of a sufficient magnetic field is different from the conventional ground state. The experiments are interpreted on the basis of the local model of strong pinning as the deconfinement of soliton-antisoliton pairs triggered by the Zeeman coupling to spin degrees of freedom, resulting in a magnetic-field-induced density-wave glass for the spin carrying the phase configuration.

Specific heat at the spin-peierls transition in CuGeO3 under magnetic fields up to 22 tesla

We report on heat capacity measurements of the spin-Peierls compound CuGeO3 under magnetic fields between 0 and 22 teslas. We have determined the magnetic phase diagram between the three different phases (U=uniform,I=incommensurate,D=dimerised) of this system, unknown up to now forH>14 T, and which can be well interpreted within the Hartree-Fock approximation. We have studied the 2nd order phase transition forH=const. at the crossing ofD-U andI-U phases. The spin-Peierls transition gives rise to a well pronounced specific heat jump. The phase diagram and the specific heat results are discussed in the framework of a B.C.S.-like theoretical model.

Equilibrium low temperature heat capacity of the spin density wave compound (TMTTF)2 Br : effect of a magnetic field

Abstract.We have investigated the effect of the magnetic field (B) on the very nlow-temperature equilibrium heat capacity ceq of the quasi-1 D organic ncompound (TMTTF)2Br, characterized by a commensurate Spin Density nWave (SDW) ground state. Below 1xa0K, ceq is dominated by a Schottky-like nAST-2 contribution, very sensitive to the experimental time scale, na property that we have previously measured in numerous DW compounds. Under napplied field (in the range 0.2–7xa0T), the equilibrium dynamics, and hence nceq extracted from the time constant, increases enormously. For B n≥ 2–3xa0T, ceq varies like B2, in agreement with a nmagnetic Zeeman coupling. Another specific property, common to other nCharge/Spin density wave (DW) compounds, is the occurrence of metastable nbranches in ceq, induced at very low temperature by the field exceeding na critical value. These effects are discussed within a generalization to nSDWs in a magnetic field of the available Larkin-Ovchinnikov local model of nstrong pinning. A limitation of the model when compared to experiments is npointed out.nn

Singlet Orbital Ordering in Bilayer Sr3Cr2O7

We perform an extensive study of Sr_{3}Cr_{2}O_{7}, the n=2 member of the Ruddlesden-Popper Sr_{n+1}Cr_{n}O_{3n+1} system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, Rln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations. It is accompanied by anomalous asymmetric distortions of the CrO_{6} octahedra. Strong coupling and Lanczos calculations on a derived Kugel-Khomskii Hamiltonian yield a simultaneous orbital and moment ordering. Our results favor an exotic ordered phase of orbital singlets not originated by frustration.

Determination of the phase diagram of 2% Zn-doped by means of ultrasound measurements in high magnetic fields

Measurements of the elastic constant of the 2% Zn-doped spin - Peierls system under high magnetic fields up to 20 T are reported. The successive magnetic ordering and the spin - Peierls transition are confirmed. The magnetic phase diagram has been determined. A transition line between the magnetic ordered phase and the uniform phase is observed at very high magnetic fields above 15 T.

Specific heat fluctuations in the vicinity of the spin-Peierls transition of CuGeO3

We present an analysis of experimental specific heat data measured in the vicinity of the spin-Peierls transition of a CuGeO3 single crystal. The aim of the investigation was to analyse the type of the transition, including the character of the fluctuations, and to give a proper determination of the critical temperature. We used and successively compared the Gaussian and critical fluctuations models to fit the experimental points detected in magnetic fields up to 21 T. Our results present new information about the influence of a magnetic field on fluctuations in the CuGeO3 compound. Within the framework of the Gaussian fluctuations model, there is no significant magnetic field dependence of the fluctuations. Using this model, the critical region is estimated to be as small as 10 mK in zero field, which excludes the possibility of observation of critical fluctuations in the present experiments. In spite of this, a critical fluctuations type of analysis can also be carried out, with a critical exponent value lying between the three-dimensional XY and the Heisenberg universality classes. We also note the absence of broadening of the fluctuations regime under magnetic field. Taking into account the scatter of the data, the two models are equally suitable in the fitting process.