960 research outputs found
Spectral Density of the Two-Impurity Anderson Model
We investigate static and dynamical ground-state properties of the
two-impurity Anderson model at half filling in the limit of vanishing impurity
separation using the dynamical density-matrix renormalization group method. In
the weak-coupling regime, we find a quantum phase transition as function of
inter-impurity hopping driven by the charge degrees of freedom. For large
values of the local Coulomb repulsion, the transition is driven instead by a
competition between local and non-local magnetic correlations. We find evidence
that, in contrast to the usual phenomenological picture, it seems to be the
bare effective exchange interactions which trigger the observed transition.Comment: 18 pages, 6 figures, submitted to J. Phys.:Condens. Matte
Structural Modification and Metamagnetic Anomaly in the Ordered State of CeOs2Al10
A caged compound CeOs2Al10, crystallizing in the orthorhombic YbFe2Al10-type
structure, undergoes a mysterious phase transition at T_0=29 K. We report the
results of electron diffraction, magnetization, and magnetoresistance for
single crystals. Superlattice reflections characterized by a wave vector q =
(0, -2/3, 2/3) observed at 15 K indicate a structural modification in the
ordered state. Activation-type behavior of the electrical resistivity along the
three principal axes below 50 K suggests gap opening in the conduction band.
The magnetic susceptibility \chi = M/B is highly anisotropic,
\chi_a>\chi_c>\chi_b, all of which sharply decrease on cooling below T_0.
Furthermore, a metamagnetic anomaly in the magnetization and a step in the
magnetoresistance occur at B=6-8 T only when the magnetic field is applied
parallel to the orthorhombic c axis. However, T_0 hardly changes under magnetic
fields up to 14 T, irrespective of the field direction. By using these data, we
present a B-T phase diagram and discuss several scenarios for the mysterious
transition.Comment: 6 pages, 7 figures, accepted for publication in Phys. Rev.
Optical Conductivity of the Trellis-Lattice t-J Model: Charge Fluctuations in NaV_2O_5
Optical conductivity of the trellis lattice t-J model at quarter filling is
calculated by an exact-diagonalization technique on small clusters, whereby the
valence state of V ions of NaV_2O_5 is considered. We show that the
experimental features at \sim 1 eV, including peak positions, presence of
shoulders, and anisotropic spectral weight, can be reproduced in reasonable
range of parameter values, only by assuming that the system is in the charge
disproportionated ground state. Possible reconciliation with experimental data
suggesting the presence of uniform ladders at T>T_c is discussed.Comment: 4 pages, 4 gif figures. Minor revisions have been made. Hardcopies of
figures (or the entire manuscript) can be obtained by e-mail request to
[email protected]
Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations
NaIrO, a honeycomb 5 oxide, has been recently identified as a
potential realization of the Kitaev spin lattice. The basic feature of this
spin model is that for each of the three metal-metal links emerging out of a
metal site, the Kitaev interaction connects only spin components perpendicular
to the plaquette defined by the magnetic ions and two bridging ligands. The
fact that reciprocally orthogonal spin components are coupled along the three
different links leads to strong frustration effects and nontrivial physics.
While the experiments indicate zigzag antiferromagnetic order in NaIrO,
the signs and relative strengths of the Kitaev and Heisenberg interactions are
still under debate. Herein we report results of ab initio many-body electronic
structure calculations and establish that the nearest-neighbor exchange is
strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the
Heisenberg contribution is significantly weaker and antiferromagnetic. The
calculations further reveal a strong sensitivity to tiny structural details
such as the bond angles. In addition to the large spin-orbit interactions, this
strong dependence on distortions of the IrO plaquettes singles out the
honeycomb 5 oxides as a new playground for the realization of
unconventional magnetic ground states and excitations in extended systems.Comment: 13 pages, 2 tables, 3 figures, accepted in NJ
Prominent 5d-orbital contribution to the conduction electrons in gold
We have examined the valence-band electronic structures of gold and silver in
the same column in the periodic table with nominally filled d orbitals by means
of a recently developed polarization-dependent hard x-ray photoemission.
Contrary to a common expectation, it is found that the 5d-orbital electrons
contribute prominently to the conduction electrons in gold while the conduction
electrons in silver are to some extent free-electron-like with negligible 4d
contribution, which could be related to a well-known fact that gold is more
stable than silver in air. The 4d electron correlation effects are found to be
essential for the conduction electron character in silver.Comment: 8 pages, 4 figures, to be appeared in New J. Phys
Brain2Music: Reconstructing Music from Human Brain Activity
The process of reconstructing experiences from human brain activity offers a
unique lens into how the brain interprets and represents the world. In this
paper, we introduce a method for reconstructing music from brain activity,
captured using functional magnetic resonance imaging (fMRI). Our approach uses
either music retrieval or the MusicLM music generation model conditioned on
embeddings derived from fMRI data. The generated music resembles the musical
stimuli that human subjects experienced, with respect to semantic properties
like genre, instrumentation, and mood. We investigate the relationship between
different components of MusicLM and brain activity through a voxel-wise
encoding modeling analysis. Furthermore, we discuss which brain regions
represent information derived from purely textual descriptions of music
stimuli. We provide supplementary material including examples of the
reconstructed music at https://google-research.github.io/seanet/brain2musicComment: Preprint; 21 pages; supplementary material:
https://google-research.github.io/seanet/brain2musi
- …