摘要 :
We report the observation of a jump in the a- and c-lattice constants at the critical field H_(c1) and at temperatures of the order 40 mK in the S = 1/2 spin-gapped compound Cu_2(C_5H_(12)N_2)_2Cl_4 by means of synchrotron x-ray d...
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We report the observation of a jump in the a- and c-lattice constants at the critical field H_(c1) and at temperatures of the order 40 mK in the S = 1/2 spin-gapped compound Cu_2(C_5H_(12)N_2)_2Cl_4 by means of synchrotron x-ray diffraction. This anomaly, characteristic of a first-order phase transition, results from the development and further stabilization of a symmetry breaking magnetic long-range order.
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This work reports on a family of three-dimensional photonic crystals with the symmetry of the pyrochlore lattice that exhibit interesting photonic band structures. It is found that such systems exhibit large complete photonic band...
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This work reports on a family of three-dimensional photonic crystals with the symmetry of the pyrochlore lattice that exhibit interesting photonic band structures. It is found that such systems exhibit large complete photonic band gaps for a wide range of dielectric constants and sizes of the features that define the lattice that renders them amenable of fabrication. The results described below make this family of photonic crystals very promising for potential applications and very interesting from the fundamental point of view.
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Magnetic excitations in the quasi-one-dimensional antiferromagnet IPA-CuCl_3 are studied by cold neutron inelastic scattering. Strongly dispersive gap excitations are observed. Contrary to previously proposed models, the system is...
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Magnetic excitations in the quasi-one-dimensional antiferromagnet IPA-CuCl_3 are studied by cold neutron inelastic scattering. Strongly dispersive gap excitations are observed. Contrary to previously proposed models, the system is best described as an asymmetric quantum spin ladder. The observed spectrum is interpreted in terms of composite Haldane spin chains. The key difference from actual S=1 chains is a sharp cutoff of the single-magnon spectrum at a certain critical wave vector.
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Magnetic susceptibility and heat capacity due to the spin-state transition in LaCoO_3 were calculated by a molecular-field model in which the energy-level diagram of high-spin state reported by Ropka and Radwanski [Phys. Rev. B 67...
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Magnetic susceptibility and heat capacity due to the spin-state transition in LaCoO_3 were calculated by a molecular-field model in which the energy-level diagram of high-spin state reported by Ropka and Radwanski [Phys. Rev. B 67, 172401 (2003)] is assumed for the excited state, and the energy and entropy of mixing of high-spin Co ions and low-spin Co ions are introduced phenomenologically. The experimental data below 300 K were well reproduced by this model, which proposes that the high-spin excited state can be populated even if the energy of high-spin state is much larger than that of low-spin state, because the negatively large energy of mixing reduces the net excitation energy. The stability of each spin state including the intermediate-spin state is discussed based on the present results and other reports.
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摘要 :
The (three-dimensional) pyrochlore lattice antiferromagnet with Heisenberg spins of large spin length S is a highly frustrated model with a macroscopic degeneracy of classical ground states. The zero-point energy of (harmonic-orde...
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The (three-dimensional) pyrochlore lattice antiferromagnet with Heisenberg spins of large spin length S is a highly frustrated model with a macroscopic degeneracy of classical ground states. The zero-point energy of (harmonic-order) spin-wave fluctuations distinguishes a subset of these states. I derive an approximate but illuminating effective Hamiltonian, acting within the subspace of Ising spin configurations representing the collinear ground states. It consists of products of Ising spins around loops, i.e., has the form of a Z_2 lattice gauge theory. The remaining ground-state entropy is still infinite but not extensive, being O(L) for system size O(L~3). All these ground states have unit cells bigger than those considered previously.
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The magnetic ordering and critical behavior of antiferromagnetic Fe(NCS)_2(pyrazine)_2 has been studied by neutron powder diffraction (NPD), inelastic neutron scattering (INS), Moessbauer spectroscopy, and magnetic measurements. T...
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The magnetic ordering and critical behavior of antiferromagnetic Fe(NCS)_2(pyrazine)_2 has been studied by neutron powder diffraction (NPD), inelastic neutron scattering (INS), Moessbauer spectroscopy, and magnetic measurements. The system can be regarded as a two-dimensional (2D) antiferromagnet even in the ordered phase, given that long-range magnetic ordering between the layers simply follows a necessary consequence of the establishment of long-range ordering within the planes. The INS data, which were taken on a cold neutron time-of-flight spectrometer, reveals that when the temperature is lowered towards T_N, the correlation length within the 2D layers increases and ultimately crosses over from two- to three-dimensional (3D) behavior. Indeed, 3D long-range antiferromagnetic order, associated with a propagation vector [l,0, 1/4 + ε], is observed in the NPD data below 6.8 K. Furthermore, in agreement with the behavior of both χ(T) and C_m(T) data, the order parameter follows the exact Osanger solution for a 2D S = 1/2, Ising system.
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摘要 :
Magnetic and dielectric properties have been investigated for a mixed-crystal system Tb_(1-x)Gd_xMnO_3 in between TbMnO_3 with ferroelectric and incommensurate antiferromagnetic (AFM) orders and GdMnO_3 with paraelectric and cante...
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Magnetic and dielectric properties have been investigated for a mixed-crystal system Tb_(1-x)Gd_xMnO_3 in between TbMnO_3 with ferroelectric and incommensurate antiferromagnetic (AFM) orders and GdMnO_3 with paraelectric and canted AFM (weakly ferromagnetic) states, as functions of x, temperature T, and magnetic field B applied along the c axis. A systematic study on the magnetic and electric phase diagrams in the x-T, B-T, and x-B planes has revealed important interplay between Mn d-electron spins and Gd f-electron moments. A clear anticorrelation between the (weak) ferromagnetism and the ferroelectricity is observed near the phase boundaries.
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The analysis of the high-T_C superconductor parent compound La_2CuO_4 phase diagram shows that in the tetragonal phase of this system spontaneous magnetization at any finite temperature equals zero, whereas in the orthorhombic pha...
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The analysis of the high-T_C superconductor parent compound La_2CuO_4 phase diagram shows that in the tetragonal phase of this system spontaneous magnetization at any finite temperature equals zero, whereas in the orthorhombic phase long-range order exists up to a certain temperature greater than zero. In this paper, such behavior is demonstrated exactly for the spin model which describes this compound, by making use of the Bogoliubov's inequality. We may therefore conclude that the results of Mermin and Wagner can also be extended to some 3d-isotropic magnetic lattices. The situation for the YBa_2Cu_3O_6-type model is also discussed.
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摘要 :
We present a systematic analysis of point-contact Andreev reflection (PCAR) spectra for ferromagnetic materials, using both modeling and experimental data. We emphasize the importance of consistent data analysis to avoid possible ...
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We present a systematic analysis of point-contact Andreev reflection (PCAR) spectra for ferromagnetic materials, using both modeling and experimental data. We emphasize the importance of consistent data analysis to avoid possible misinterpretation of the data. We consider the relationship between ballistic and diffusive transport, the effect of different transport regimes on spin polarization measurements, and the importance of unambiguous identification of the type of transport regime. We find that in a realistic parameter range, the analysis of PCAR spectra of purely diffusive character by a ballistic model yield approximately the same (within ～3%) values of the spin polarization and the barrier strength Z larger by ～0.5-0.6. We also consider the dependence of polarization values on Z, and have shown by simple modeling that letting the superconducting gap vary as an adjustable parameter can result in a spurious dependence of the spin-polarization P_c on Z. At the same time we analyzed the effects of finite Z on the apparent value of P_c measured by the PCAR technique, using a large number of examples from both our own measurements and from the literature. We conclude that there is a system-dependent variation in P_c (Z), presumably due to spin-flip scattering at the interface. However, the exact type of this dependence is hard to determine with any statistical certainty.
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We report polarized and unpolarized neutron scattering measurements of the magnetic order in single crystals of Na_(0.5)CoO_2. Our data indicate that below T_N = 88 K the spins form a novel antiferromagnetic pattern within the CoO...
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We report polarized and unpolarized neutron scattering measurements of the magnetic order in single crystals of Na_(0.5)CoO_2. Our data indicate that below T_N = 88 K the spins form a novel antiferromagnetic pattern within the CoO_2 planes, consisting of alternating rows of ordered and nonordered Co ions. The domains of magnetic order appear to be closely coupled to the domains of Na ion order, consistent with such a twofold symmetric spin arrangement. Magnetoresistance and anisotropic susceptibility measurements further support this model for the electronic ground state.
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