Thermodynamic Properties of the Spin-1/2 Antiferromagnetic ladder Cu2(C2H12N2)2Cl4 under Magnetic Field

Specific heat ($C_V$) measurements in the spin-1/2 Cu$_2$(C$_2$H$_{12}$N$_2$)$_2$Cl$_4$ system under a magnetic field up to $H=8.25 T$ are reported and compared to the results of numerical calculations based on the 2-leg antiferromagnetic Heisenberg ladder. While the temperature dependences of both the susceptibility and the low field specific heat are accurately reproduced by this model, deviations are observed below the critical field $H_{C1}$ at which the spin gap closes. In this Quantum High Field phase, the contribution of the low-energy quantum fluctuations are stronger than in the Heisenberg ladder model. We argue that this enhancement can be attributed to dynamical lattice fluctuations. Finally, we show that such a Heisenberg ladder, for $H>H_{C1}$, is unstable, when coupled to the 3D lattice, against a lattice distortion. These results provide an alternative explanation for the observed low temperature ($T_C\sim 0.5K$ -- $0.8K$) phase (previously interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped state.Comment: Minor changes, list of authors complete

Field-Induced Magnetic Order in Quantum Spin Liquids

We study magnetic field-induced three-dimensional ordering transitions in low-dimensional quantum spin liquids, such as weakly coupled, antiferromagnetic spin-1/2 Heisenberg dimers and ladders. Using stochastic series expansion quantum Monte Carlo simulations, thermodynamic response functions are obtained down to ultra-low temperatures. We extract the critical scaling exponents which dictate the power-law dependence of the transition temperature on the applied magnetic field. These are compared with recent experiments on candidate materials and with predictions for the Bose-Einstein condensation of magnons obtained in mean-field theory.Comment: RevTex, 4 pages with 5 figure

Aging in a topological spin glass

We have examined the nonconventional spin glass phase of the 2-dimensional kagome antiferromagnet (H_3 O) Fe_3 (SO_4)_2 (OH)_6 by means of ac and dc magnetic measurements. The frequency dependence of the ac susceptibility peak is characteristic of a critical slowing down at Tg ~ 18K. At fixed temperature below Tg, aging effects are found which obey the same scaling law as in spin glasses or polymers. However, in clear contrast with conventional spin glasses, aging is remarkably insensitive to temperature changes. This particular type of dynamics is discussed in relation with theoretical predictions for highly frustrated non-disordered systems.Comment: 4 pages, 4 figure

Low-temperature specific heat and thermal conductivity of glycerol

We have measured the thermal conductivity of glassy glycerol between 1.5 K and 100 K, as well as the specific heat of both glassy and crystalline phases of glycerol between 0.5 K and 25 K. We discuss both low-temperature properties of this typical molecular glass in terms of the soft-potential model. Our finding of an excellent agreement between its predictions and experimental data for these two independent measurements constitutes a robust proof of the capabilities of the soft-potential model to account for the low-temperature properties of glasses in a wide temperature range.Comment: 4 pages, 3 figures. To be published in Phys. Rev. B (2002

The Magnetic Spin Ladder (C_{5}H_{12}N)_{2}CuBr_{4}: High Field Magnetization and Scaling Near Quantum Criticality

The magnetization, $M(H \leq 30$ T, 0.7 K $\leq T \leq 300$ K), from single crystals and powder samples of (C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$ has been used to identify this system as an $S=1/2$ Heisenberg two-leg ladder in the strong coupling limit, $J_{\perp} = 13.3$ K and $J_{\parallel} = 3.8$ K, with $H_{c1} = 6.6$ T and $H_{c2} = 14.6$ T. An inflection point in $M(H, T = 0.7$ K) at half-saturation, $M_{s}/2$, is described by an effective \emph{XXZ} chain. The data exhibit universal scaling behavior in the vicinity of $H_{c1}$ and $H_{c2}$, indicating the system is near a quantum critical point.Comment: 4 pages, 4 figure

Investigation of thermal and magnetic properties of defects in a spin-gap compound NaV2O5

The specific heat, magnetic susceptibility and ESR signals of a Na-deficient vanadate Na_xV_2O_5 (x=1.00 - 0.90) were studied in the temperature range 0.07 - 10 K, well below the transition point to a spin-gap state. The contribution of defects provided by sodium vacancies to the specific heat was observed. It has a low temperature part which does not tend to zero till at least 0.3 K and a high temperature power-like tail appears above 2 K. Such dependence may correspond to the existence of local modes and correlations between defects in V-O layers. The magnetic measurements and ESR data reveal S=1/2 degrees of freedom for the defects, with their effective number increasing in temperature and under magnetic field. The latter results in the nonsaturating magnetization at low temperature. No long-range magnetic ordering in the system of defects was found. A model for the defects based on electron jumps near vacancies is proposed to explain the observed effects. The concept of a frustrated two-dimensional correlated magnet induced by the defects is considered to be responsible for the absence of magnetic ordering.Comment: 6 pages, 8 figure

Magnetic properties of a new molecular-based spin-ladder system: (5IAP)2CuBr4*2H2O

We have synthesized and characterized a new spin-1/2 Heisenberg antiferromagnetic ladder: bis 5-iodo-2-aminopyridinium tetrabromocuprate(II) dihydrate. X-ray diffraction studies show the structure of the compound to consist of well isolated stacked ladders and the interaction between the Cu(2+) atoms to be due to direct Br...Br contacts. Magnetic susceptibility and magnetization studies show the compound to be in the strong-coupling limit, with the interaction along the rungs (J' ~ 13 K) much greater than the interaction along the rails (J ~ 1 K). Magnetic critical fields are observed near 8.3 T and 10.4 T, respectively, establishing the existence of the energy gap.Comment: 10 pages, 4 figures, submitted to Phys. Rev. B Figure 4 did not print. *.eps files replaced with figures.ps fil

Specific heat of an S=1/2 Heisenberg ladder compound Cu2_2(C5_5H12_{12}N2_2)2_2Cl4_4 under magnetic fields

Specific heat measurements down to 0.5 K have been performed on a single crystal sample of a spin-ladder like compound Cu$_{2}$(C$_{5}$H$_{12}$N$_{2}$)$_{2}$Cl$_{4}$ under magnetic fields up to 12 T. The temperature dependence of the observed data in a magnetic field below 6 T is well reproduced by numerical results calculated for the S=1/2 two-leg ladder with $J_{\rm{rung}}$/$J_{\rm{leg}}$=5. In the gapless region above 7 T ($H_{\rm{c1}}$), the agreement between experiment and calculation is good above about 2 K and a sharp and a round peak were observed below 2 K in a magnetic field around 10 T, but the numerical data show only a round peak, the magnitude of which is smaller than that of the observed one. The origin of the sharp peak and the difference between the experimental and numerical round peak are discussed.Comment: 14 pages, 11 figures, Submitted to PR