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تسجيل مستخدم جديدDiverging thermal expansion of the spin-ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$
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We present high-resolution measurements of the $c^star$-axis thermal expansion and magnetostriction of piperidinium copper bromide hp. The experimental data at low temperatures is well accounted for by a two-leg spin-ladder Hamiltonian. The thermal expansion shows a complex behaviour with various sign changes and approaches a $1/sqrt{T}$ divergence at the critical fields. All low-temperature features are semi-quantitatively explained within a free fermion model; full quantitative agreement is obtained with Quantum Monte Carlo simulations.
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Magnetic excitations in the spin-ladder material (C$_5$H$_{12}$N)$_2$CuBr$_4$ [BPCB] are probed by high-resolution multi-frequency electron spin resonance (ESR) spectroscopy. Our experiments provide a direct evidence for a biaxial anisotropy ($sim 5%
$ of the dominant exchange interaction), that is in contrast to a fully isotropic spin-ladder model employed for this system previously. It is argued that this anisotropy in BPCB is caused by spin-orbit coupling, which appears to be important for describing magnetic properties of this compound. The zero-field zone-center gap in the excitation spectrum of BPCB, $Delta_0/k_{B}=16.5$ K, is detected directly. Furthermore, an ESR signature of the inter-ladder exchange interactions is obtained. The detailed characterization of the anisotropy in BPCB completes the determination of the full spin hamiltonian of this exceptional spin-ladder material and shows ways to study anisotropy effects in spin ladders.
The $S=1/2$ spin ladder compound (C$_5$H$_{12}$N)$_2$CuBr$_4$ (BPCB) is studied by means of high-resolution inelastic neutron scattering. In agreement with previous studies we find a band of triplet excitations with a spin gap of $sim0.8$~meV and a b
andwidth of $sim0.6$~meV. In addition, we observe a distinct splitting of the triplet band of $50(1)$~$mu$eV or $40(2)$~$mu$eV at the band minimum or maximum, respectively. By comparison to a strong coupling expansion calculation of the triplet dispersion for a spin ladder with anisotropic exchange, weakly anisotropic leg interactions are identified as the dominant source of magnetic anisotropy in BPCB. Based on these results, we discuss the nature of magnetic exchange anisotropy in BPCB and in related transition-metal insulators.
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 Lutt
inger-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.
We re-examine the thermodynamic properties of the coupled dimer system Cu$_2$(C$_5$H$_{12}$N$_2$)$_2$Cl$_4$ under magnetic field in the light of recent NMR experiments [Clemancey {it et al.}, Phys. Rev. Lett. {bf 97}, 167204 (2006)] suggesting the ex
istence of a finite Dzyaloshinskii-Moriya interaction. We show that including such a spin anisotropy greatly improves the fit of the magnetization curve and gives the correct trend of the insofar unexplained anomalous behavior of the specific heat in magnetic field at low temperature.
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