• Friday, September 13 1:30PM
  Sasha Abanov
  (Department of Physics and Astronomy, Stony Brook University)
  On the probability of ferromagnetic strings in antiferromagnetic spin chains.
  We study the probability of formation of ferromagnetic string in the antiferromagnetic spin-1/2 XXZ chain. We show that in the limit of long strings with weak magnetization per site the bosonization technique can be used to address the problem. At zero temperature the obtained probability is Gaussian as a function of the length of the string. At finite but low temperature there is a crossover from the Gaussian behavior at intermediate lengths of strings to the exponential decay for very long strings. Although the weak magnetization per site is a necessary small parameter justifying our results, the extrapolation of obtained results to the case of maximally ferromagnetic strings is in qualitative agreement with known numerics and exact results. The effect of an external magnetic field on the probability of formation of ferromagnetic strings is also studied.
  Host: Abanov


• Friday, September 27, 2002, 1:30PM
  Igor Beloborodov
  (Bell Laboratories, Lucent Technologies)
  Coulomb Blockade in Metallic Grain at Large Conductance
  I will discuss mesoscopic fluctuations of the Coulomb blockade oscillations in a disordered metallic grain which is coupled to a metallic lead by a tunneling contact with a large conductance $g_T$. We concentrate on the thermodynamic quantities of the grain and study the average and the correlator of the Coulomb blockade oscillations. The correlations decay algebraically with the gate voltage difference. Our results are valid beyond the zero-dimensional limit and describe the crossover between the unitary and orthogonal ensembles.
  Host: Abanov


• Friday, October 18, 2002, 1:30PM
  Natalya Zimbovskaya
  (City College)
  Theory of long-range electron transport through macromolecules
  A theory of electrical transport through molecular wires is used to estimate the electronic factor in the intramolecular electron transfer in porphyrin-nitrobenzene supermolecules, and to analyze its structure. The chosen molecules have complex donor and acceptor configurations, and relatively simple structure of the bridge, which enables us to concentrate our studies on the donor/acceptor coupling to the bridge. We present analytical and numerical results concerning the effect of donor/acceptor coupling to the bridge on the electron transfer process in molecules with complex donor/acceptor subsystems.
  Host: Likharev


• Friday, October 25, 2002, 1:30PM
  Sophie deBrion
  (Grenoble HMFL)
  Phase separation in the charge ordered compounds Nd(Pr)_1-xCa_xMnO₃: an ESR study
  High frequency (9.4 GHz - 475 GHz), high magnetic field ( 0 - 12 T ) Electro Spin Resonance has been used to study the magnetic excitations and possible phase separation effects in the charge ordered manganites Nd(Pr)1-xCaxMnO3. In the Nd compounds, in form of powders, we show [1] that the Nd ions are weakly coupled to the Mn ions via ferromagnetic exchange and are responsible for the peculiar ferromagnetic resonance observed in the FM phase of both compounds ( ground state below 120K for x=0.3, high field state for x=0.5). We also show that there is no trace of the FM state imbedded in the low field, CO phase in Nd 0.5Ca 0.5MnO3. On the contrary, in a 250 nm thick Pr0.5Ca0.5MnO3 film grown on LaAl2O3 substrate, we show evidences for the presence of a FM phase within the CO phase in form of very thin layers, with the ferromagnetic easy axis at 45° from the film plane. The coupling of this FM phase with the CO phase depends on the orientation of the applied magnetic field (parallel or perpendicular to the film plane).
  Host: Mihaly


• Friday, November 1, 2002, 1:30PM
  Ferenc Simon
  (Technical University, Budapest)
  Electron Spin Resonance and Microwave Conductivity studies of MgB₂
  We report the observation of conduction electron spin resonance (CESR) in MgB₂ powder samples using multifrequency (3-225 GHz, 0.1-8 T) ESR technique. The normal state spin-susceptiblity, measured from the CESR signal intensity agrees with band structure calculations confirming that MgB₂ is a weakly correlated metal. The CESR linewidth, that measures the spin-lattice relaxation, follows the normal state resistivity, which also shows a conventional metallic behavior. Below T_c, we observe a peculiar magnetic field dependence of the CESR signal: above 75 GHz (2.7 T Zeeman field) we find that part of the sample is in the normal state in addition to the superconducting material. The normal state fraction grows with magnetic field at the cost of the superconducting one. We conclude that a significant H_c2 anisotropy can explain the observations. Field and temperature dependent magnetization data confirms this explanation [1]. Since our original report on powders, this has been confirmed by measurements on good quality single crystals. We also observe an anomalously large magnetic field induced susceptibility below T_c at low magnetic fields, where only the superconducting phase is present. At the lowest magnetic field (0.14 T, 0.38 GHz), the opening of the gap is observed and the temperature dependence of susceptibility is compatible with the two-gap superconductivity model suggested for MgB₂. For fields above 0.2 T there is an anomalously large field induced susceptibility and microwave conductivity that can not be explained by the current theories.
  [1] F. Simon et al., Phys. Rev. Lett. 87, 047002 (2001).
  Host: Mihaly


• Friday, November 15, 2002, 1:30PM
  Emil Yuzbashyan
  (Princeton University)
  Extracting Hidden Symmetry from the Energy Spectrum
  I will revisit the problem of finding hidden symmetries in quantum mechanical systems. Our interest in this problem was renewed by nontrivial degeneracies of a simple spin Hamiltonian used to model spin relaxation in alkali-metal vapors. We consider this spin Hamiltonian in detail and use this example to outline a general approach to finding symmetries when eigenvalues and eigenstates of the Hamiltonian are known. We extract all nontrivial symmetries responsible for the degeneracy and show that the symmetry group of the Hamiltonian is SU(2). The symmetry operators have a simple meaning which becomes transparent in the limit of large spin. As an additional example we apply the method to the Hydrogen atom.
  Host: Abanov

Send comments to Laszlo Mihaly; created 11/25/2001.