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Beckmann, D.; Wanka, S.; Wosnitza, J.; Schweitzer, D.; Strunz, W.
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7 Citations
We report de Haasvan Alphen (dHvA) measurements of the highT_{c} modification (T_{c} = 7  8 K) and Shubnikovde Haas results of the lowT_{c} modification (Tc ≈ 1:4 K) of the organic superconductor β(BEDTTTF)_{2}I_{3}. In the highT_{c} phase we find a single closed twodimensional orbit with a dHvA frequency F_{0} = (3815±10) T and a 1/cosθ dependence as expected for a quasi twodimensional system (where θ is the angle between the magnetic field and the direction normal to the conducting planes). The effective cyclotron mass is m_{c} = (4:2±0:2)m_{e}. The dHvA signal shows a beating pattern caused by the threedimensional warping of the Fermi cylinder. The interplane transfer integral estimated from the maximum beating frequency ΔF is t_{⊥}/εF ≈ 1/175. At the angles where the beating disappears, i.e., ΔF ≈ 0, the oscillations have a harmonic content which is much larger than expected from the LifshitzKosevich formula. In the lowT_{c} phase weak SdH oscillations with a frequency of F_{SdH} ≈ 110 T and an effective cyclotron mass m_{c} ≈ 1:0m_{e} is found. These results suggest a possible reconstruction of the Fermi surface in the lowT_{c} phase.
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Weiss, Ulrich; Sassetti, Maura; Negele, Thomas; Wollensak, Matthias
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30 Citations
We investigate the dynamics of a quantum particle moving in a tightbinding lattice and coupled to a heat bath environment. Using the FeynmanVernon influence functional method, we obtain an exact series representation in powers of the tunneling matrix for the generating functional of moments of the probability distribution which is valid for arbitrary temperatures and linear dissipation. We prove that the Einstein relation between the linear mobility and the diffusion coefficient holds to any order of the expansion for Ohmic, and for a restricted region of superOhmic dissipation. We also compute in the Ohmic case the mobility in certain regions of the parameter space. In particular, we find that the low temperature correction to the zero temperature mobility behaves asT^{2}, and we also determine the prefactor. Finally, the exact solution of the dynamics for any times, temperatures and bias is presented for a particular value of the damping strength in the case of strict Ohmic dissipation.
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Dmitriev, D.V.; Krivnov, V.Ya.; Ovchinnikov, A.A.
7 Citations
A class of the frustrated quantum s = ½ models with nearest and next nearest neighbor couplings is investigated. An exact wave function of the singlet ground state at the transition point from the ferromagnetic to the singlet state is presented. The recurrence technics of expectation value calculations is developed and the simple expressions for spincorrelation function at N → ∞ are obtained. A long range doublespiral ordering is demonstrated. We show that in one particular case the model reduces to the effective spin1 model and the exact singlet ground state wave function is presented for this model. The behavior of the system in the vicinity of the transition point is investigated.
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By
Liebmann, Rainer
3 Citations
The transition temperature obtained from recent Monte Carlo calculations for the Quartet Ising model on the fcc lattice deviated by 17% from the exact transition temperatureT_{c}^{SD}
required by selfduality which we have proven afterwards. Here we use Monte Carlo results of the internal energy, which agree well with low and high temperature series, to determine entropy and free energy and obtain aT_{c} in excellent agreement (±0.1%) with the exact value. The Quartet model on the hcp lattice is shown to be selfdual too; the rapidly converging series for the fcc and the hcp lattice differ only in higher order.
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Lupascu, D.; Bartos, A.; Lieb, K. P.; Uhrmacher, M.
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22 Citations
The structure of Cform Ho_{2}O_{3} and Er_{2}O_{3} single crystals and powder samples was investigated by the electric quadrupole hyperfine interaction of^{111}In(EC)^{111}Cd probe ions using the perturbed γγ angular correlation method (PAC). The resulting set of refined atomic coordinates is compared to Xray data and used to calculate the orientations of the electric field gradients (EFG) which are reproduced by the PAC measurements in single crystals. The temperature dependence of the coordinates was measured for both substances.
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By
Schwarz, K.; Ripplinger, H.; Neckel, A.
52 Citations
Selfconsistent augmented plane wave calculations are performed for ZrC and ZrN. The charge distributions resulting from valence bands in different regions of the unit cell are discussed and compared with related compounds, where the transition metal is varied between a 3d and 4d element of group IV and V. The limitations of the rigid band model are illustrated and general trends in the chemical bonding are discussed. In connection with Xray emission spectra (XES) matrix element effects are shown for the three spectra ZrL_{III},M_{III}, andN_{III}. Further, comparison between theoretical and experimental XES is made with all data available for both ZrC and ZrN.
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By
Niksch, M.; Hoenig, H. E.
Experimental techniques for tunneling into bulk superconductors have been tested for the investigation of crystal field effects in LaAl_{2}, LaSn_{3} and LaPb_{3} doped with Tb, Nd, or Pr. The evaporation of counterelectrodes onto cleaved and oxidized crystals failed in most cases, but Schottky contacts with GaAs electrodes could be prepared reproducibly by mechanical adjustment of the junction resistance at mK temperature. Problems remain resulting from the specific mechanical and structural properties of the metallic compounds under investigation.
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By
Zhang, Jiyue; Haken, H.
2 Citations
We consider nonlinear optical processes in a cavity. The processes are driven by an external coherent pump. We predict selfpulsing instabilities in the generation of higher harmonics. We also derive formulas for the threshold pump strength and critical selfpulsing frequency.
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