896 research outputs found
Spin-Peierls and Antiferromagnetic Phases in Cu{1-x}Zn{x}GeO{3}: A Neutron Scattering Study
Comprehensive neutron scattering studies were carried out on a series of
high-quality single crystals of Cu_{1-x}Zn_xGeO_3. The Zn concentration, x, was
determined for each sample using Electron Probe Micro-Analysis. The measured Zn
concentrations were found to be 40-80% lower than the nominal values.
Nevertheless the measured concentrations cover a wide range which enables a
systematic study of the effects due to Zn-doping. We have confirmed the
coexistence of spin-Peierls (SP) and antiferromagnetic (AF) orderings at low
temperatures and the measured phase diagram is presented. Most surprisingly,
long-range AF ordering occurs even in the lowest available Zn concentration,
x=0.42%, which places important constraints on theoretical models of the AF-SP
coexistence. Magnetic excitations are also examined in detail. The AF
excitations are sharp at low energies and show no considerable broadening as x
increases indicating that the AF ordering remains long ranged for x up to 4.7%.
On the other hand, the SP phase exhibits increasing disorder as x increases, as
shown from the broadening of the SP excitations as well as the dimer reflection
peaks.Comment: 17 preprint style pages, 9 postscript files included. Submitted to
Phys. Rev. B. Also available from
http://insti.physics.sunysb.edu/~mmartin/pubs.htm
Possible Pairing Symmetry of Three-dimensional Superconductor UPt -- Analysis Based on a Microscopic Calculation --
Stimulated by the anomalous superconducting properties of UPt, we
investigate the pairing symmetry and the transition temperature in the
two-dimensional(2D) and three-dimensional(3D) hexagonal Hubbard model. We solve
the Eliashberg equation using the third order perturbation theory with respect
to the on-site repulsion . As results of the 2D calculation, we obtain
distinct two types of stable spin-triplet pairing states. One is the
-wave(B) pairing around and in a small region, which is
caused by the ferromagnetic fluctuation. Then, the other is the (or
)-wave(E) pairing in large region far from the half-filling () which is caused by the vertex corrections only. However, we find that the
former -wave pairing is destroyed by introduced 3D dispersion. This is
because the 3D dispersion breaks the favorable structures for the -wave
pairing such as the van Hove singularities and the small pocket structures.
Thus, we conclude that the ferromagnetic fluctuation mediated spin-triplet
state can not explain the superconductivity of UPt. We also study the case
of the pairing symmetry with a polar gap. This -wave(A) is stabilized
by the large hopping integral along c-axis . It is nearly degenerate with
the suppressed (or )-wave(E) in the best fitting parameter region
to UPt (). These two p-wave pairing states exist in
the region far from the half-filling, in which the vertex correction terms play
crucial roles like the case in SrRuO.Comment: 15 pages, 12 figure
Nonmonotonous Magnetic Field Dependence and Scaling of the Thermal Conductivity for Superconductors with Nodes of the Order Parameter
We show that there is a new mechanism for nonmonotonous behavior of magnetic
field dependence of the electronic thermal conductivity of clean
superconductors with nodes of the order parameter on the Fermi surface. In
particular, for unitary scatterers the nonmonotony of relaxation time takes
place. Contribution from the intervortex space turns out to be essential for
this effect even at low temperatures. Our results are in a qualitative
agreement with recent experimental data for superconducting UPt_3. For
E_{2u}-type of pairing we find approximately the scaling of the thermal
conductivity in clean limit with a single parameter x=T/T_c\sqrt{B_{c2}/B} at
low fields and low temperatures, as well as weak low-temperature dependence of
the anisotropy ratio K_{zz}/K_{yy} in zero field. For E_{1g}-type of pairing
deviations from the scaling are more noticeable and the anisotropy ratio is
essentially temperature dependent.Comment: 37 pages, 8 Postscript figures, REVTE
An Effective Theory for Midgap States in Doped Spin Ladder and Spin-Peierls Systems: Liouville Quantum Mechanics
In gapped spin ladder and spin-Peierls systems the introduction of disorder,
for example by doping, leads to the appearance of low energy midgap states. The
fact that these strongly correlated systems can be mapped onto one dimensional
noninteracting fermions provides a rare opportunity to explore systems which
have both strong interactions and disorder. In this paper we show that the
statistics of the zero energy midgap wave functions in these models can be
effectively described by Liouville Quantum Mechanics. This enables us to
calculate the disorder averaged N-point correlation functions of these states
(the explicit calculation is performed for N=2,3). We find that whilst these
midgap states are typically weakly correlated, their disorder averaged
correlation are power law. This discrepancy arises because the correlations are
not self-averaging and averages of the wave functions are dominated by
anomalously strongly correlated configurations.Comment: 13 page latex fil
Reentrant Spin-Peierls Transition in Mg-Doped CuGeO_3
We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_{1-x}Mg_xGeO_3. In a recent paper we have shown that the SP
dimerization attains long-range order only for x < x_c = 0.022(0.001). Here we
report that the SP transition is reentrant in the vicinity of the critical
concentration x_c. This is manifested by broadening of the SP dimerization
superlattice peaks below the reentrance temperature, T_r, which may mean either
the complete loss of the long-range SP order or the development of a
short-range ordered component within the long-range ordered SP state. Marked
hysteresis and very large relaxation times are found in the samples with Mg
concentrations in the vicinity of x_c. The reentrant transition is likely
related to the competing Neel transition which occurs at a temperature similar
to T_r. We argue that impurity-induced competing interchain interactions play
an essential role in these phenomena.Comment: 5 pages, 4 embedded eps figure
On the soliton width in the incommensurate phase of spin-Peierls systems
We study using bosonization techniques the effects of frustration due to
competing interactions and of the interchain elastic couplings on the soliton
width and soliton structure in spin-Peierls systems. We compare the predictions
of this study with numerical results obtained by exact diagonalization of
finite chains. We conclude that frustration produces in general a reduction of
the soliton width while the interchain elastic coupling increases it. We
discuss these results in connection with recent measurements of the soliton
width in the incommensurate phase of CuGeO_3.Comment: 4 pages, latex, 2 figures embedded in the tex
Study of impurities in spin-Peierls systems including lattice relaxation
The effects of magnetic and non-magnetic impurities in spin-Peierls systems
are investigated allowing for lattice relaxation and quantum fluctuations. We
show that, in isolated chains, strong bonds form next to impurities, leading to
the appearance of magneto-elastic solitons. Generically, these solitonic
excitations do not bind to impurities. However, interchain elastic coupling
produces an attractive potential at the impurity site which can lead to the
formation of bound states. In addition, we predict that small enough chain
segments do not carry magnetic moments at the ends
Spin Defects in Spin-Peierls Systems
We examine spin-Peierls systems in the presence of spin defects which are
introduced by replacing magnetic ions with non-magnetic ones
in . By using the action for the bosonized Hamiltonian, it
is shown directly that the antiferromagnetic state induced by the spin defects
coexists with the spin-Peierls states. Further the doping dependences of both
transition temperature of spin-Peierls state and the spin gap have been
calculated. The transition temperature of the present estimation shows good
agreement quantitatively with that observed in Cu_{1-\de} Zn_\de O_3 for the
region of the doping rate, \de<0.02.Comment: jpsj style, 11 pages, 2 figure
The spin-Peierls instability in spin 1/2 XY chain in the non adiabatic limit
The spin-Peierls instability in spin 1/2 XY chain coupled to dispersionless
phonons of frequency has been studied in the nonadiabatic limit. We
have chosen the Lang-Firsov variational wave function for the phonon subsystem
to obtain an effective spin Hamiltonian. The effective spin Hamiltonian is then
solved in the framework of mean-field approximation. We observed a dimerized
phase when g is less than a critical value and an anti-ferromagnetic phase when
it is greater than a critical value . The variation of lattice distortion,
dimerized order parameter and energy gap with spin phonon coupling parameter
has also been investigated here.Comment: 15 pages (Revtex, including 5 .ps figures); Submitted to PR
Electron-spin-resonance in the doped spin-Peierls compound Cu(1-x)Ni(x)GeO3
ESR-study of the Ni-doped spin-Peierls compound CuGeO3 has been performed in
the frequency range 9-75 GHz. At low temperatures the g-factor is smaller than
the value expected for Cu- and Ni-ions. This anomaly is explained by the
formation of magnetic clusters around the Ni-ions within a nonmagnetic
spin-Peierls matrix. The transition into the AFM-state detected earlier by
neutron scattering for doped samples was studied by means of ESR. For x=0.032 a
gap in the magnetic resonance spectrum is found below the Neel temperature and
the spectrum is well described by the theory of antiferromagnetic resonance
based on the molecular field approximation. For x=0.017 the spectrum below the
Neel point remained gapless. The gapless spectrum of the antiferromagnetic
state in weekly doped samples is attributed to the small value of the Neel
order parameter and to the magnetically disordered spin-Peierls background.Comment: 16 pages, LATEX, 12 figures, submitted to Journal of Physics :
Condensed Matte
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