Spin Density Wave Ground State Research Articles

Organic conductors and superconductors

Most of the existing organic conductors are characterized by a strong anisotropy of the electronic motion. Aside superconductivity, they develop a wide range of spectacular physical properties, especially in a magnetic field, including spin-Peierls, spin or change density wave ground states, non-linear conductivity, several kinds of quantum oscillations, quantum Hall effect. These properties may be related to the 3D, 2D or 1D character of the electronic motion.

Collective excitations and sum rules for the Hubbard model in the spin-density-wave regime.

A variational estimate for the spin-wave velocity of the one-band Hubbard model on a square lattice in the spin-density-wave regime is studied. The estimate is given by the ratio of the f-sum rule to the static structure factor of the transverse-spin response function. The known results for the Heisenberg model are used to obtain results for those quantities in the large-U limit of the Hubbard model. The f-sum rule and static structure factor are calculated using the random-phase approximation (RPA). The spin-wave velocity calculated in the RPA, in the spin-density wave ground state, violates the variational bound.

The electrodynamics of spin density waves

The electrodynamics of the spin density wave ground state is explored by examining optical experiments conducted over a broad spectral range. Both collective mode and single particle excitations are recovered in (TMTSF) 2PF 6 with significant mass enhancement associated with the dynamics of the collective mode. The relation between frequency and field dependent transport is also explored.

Tunneling in a spin-density-wave superconductor.

The differential tunneling conductance of normal-insulator-superconductor (NIS) and superconductor-insulator-superconductor (SIS) junctions is calculated for polycrystalline superconducting films which have either a spiral or linear spin-density-wave (SDW). In the NIS case, the SDW induced-gap anisotropy causes an enhancement of the low-bias tunneling conductance by several orders of magnitude, compared to that predicted by (isotropic) Bardeen-Cooper-Schrieffer theory. In the SIS case, the tunneling conductance is also enhanced, though the effect is less spectacular. The calculations are compared with existing experimental data on Al-Pb junctions. Fairly good agreement is found, which supports the speculation that Pb might have a SDW ground state.

Electrodynamics of single-crystal Eu 2CuO 4 — A boson excitation spectrum

Static and microwave measurements of the electronic response of the planar cuprate Eu 2CuO 4 provide evidence for thermally activated collective excitations. These observations together with microwave and other measurements on superconductors suggest that paired charge-carrying objects may exist both above and below T c in the superconductors. If these objects, which can be related to solitons of a spin density wave ground state, play a role in superconductivity, certain conclusions may be drawn. We discuss here the evidence for such objects and their implications.

Frequency dependent response of charge and spin density wave condensates

The collective mode response is examined in various materials with a charge density wave (CDW) and spin density wave (SDW) ground state. The behavior of CDW condensates is well described in terms of a pinned collective mode with a large effective mass and long relaxation time. A low frequency collective mode response is evident in the SDW state of (TMTSF) 2PF 6, with a relaxation time and effective mass comparable with the parameters which characterize the metallic state.

Shift of the SDW Wave Vector in the Transient Region in the Anisotropic 2D Hubbard Model

A variational procedure shows that two new types of spin density wave (SDW) ground states appear successively in the titled system modeling the Bechgaard salts, when the transverse transfer energy increases around its upper bound imposed for the usual SDW state having the so far known best nesting wave vector. From this value the wave vectors of the new states are appreciably shifted, entailing uncompensated carrier pockets of the size found in the Shubnikov-de Haas experiments. An analytic expression clarifying the situation of the starting-point of the shift shows that the shift is sensitive to details of the system.

Physical properties of (TMTSF) 2TaF 6: Influence of the anion size

The physical properties of (TMTSF) 2TaF 6 are discussed through transport and magnetic measurements. A crossover between two different regimes is observed around 100K and a spin density wave ground state appears below 11 K. The phase diagram under magnetic field is determined. Finally, the properties of this compound are compared with that of the other members of the TMTSF series and the influence of the anion size is discussed.

AC-DC coupling experiments in the spin density wave state of tetramethyltetraselenofulvalene-hexafluorophosphate [(TMTSF) 2PF 6

Experiments performed on (TMTSF) 2PF 6 in the spin density wave (SDW) state in the presence of joint ac and dc excitations do not show evidence for coupling between dc and ac response. Irradiation by microwaves does not lead to strong dc nonlinearity, and attempts to resurrect the ESR signal by applied dc currents were also unsuccessful. While the strongly frequency dependent response is indicative of collective effects, the response of the SDW ground state is fundamentally different from that of a charge density wave condensate.

Hall mobility in quasi one-dimensional conductors with charge density or spin density wave ground state

Abstract High mobilities found for free carriers below the phase transition in quasi one-dimensional crystals such as TTF-TCNQ and (TMTSF)2PF6 indicate that defect scattering is unimportant. We calculate the Hall mobility due to phonon scattering and find good agreement with the measured value of 104cm2/Vsec for (TMTSF)2PF6 at 4K.

Spin-density wave ground state in the one-dimensional conductor (TMTSF) 2PF6: microscopic evidence from 77Se and 1H NMR experiments

The vanishing of the 77Se nuclear resonance signal and the broadening by internal magnetic fields of the proton resonance line establish the existence of spin-density waves in the low temperature semiconducting state of (TMTSF) 2PF6. The thermally activated collective mode of the commensurate spin-density wave contributes significantly to the proton spin-lattice relaxation. It is also suggested that the very small amplitude of the spin-density wave is due to the coexistence of two diverging channels in the conducting state : the SDW and Superconductivity channels.

On spin waves in the spin-density wave ground state

Some remarks about interpretation of spin waves in the spin-density wave ground state are given.

Excitations from the spin-density wave ground state

A derivation is given of the dispersion relation for excitations from the spin-density wave ground state. Working in the random phase approximation, and taking a single tight binding electronic band with a delta function interaction between electrons, the author finds three types of collective mode, of which one is longitudinal and two transverse. Calculation shows that the former has a linear dispersion relation for small wave vectors, and is probably a zero-sound excitation modified by the magnetic ordering. One of the transverse modes is also linear in behaviour, and is identified as a spin wave excitation. The other behaves as omega = omega 0+Aq2 for small q, where omega 0 lies in the energy gap; on examining the approach to the ferromagnetic limit, it is demonstrated that omega 0 approaches zero, and the mode goes over smoothly into a ferromagnetic spin wave.

Quantum field theory of phase transitions in Fermi systems

Abstract The theory of fermion phase transitions is reviewed from a unified field theoretic standpoint, based on the diagrammatic perturbation expansion of a generalized matrix propagator. Transitions from a normal to a condensed phase are characterized by the spontaneous appearance of long-range order and (in the presence of a suitable infinitesimal external field) broken symmetry. This is illustrated by the ferromagnetic, solid, superconducting and spindensity wave ground states. The phenomenon is explained qualitatively as caused by the creation of a long-range internal field, F, due to the interactions between particles. This field establishes long-range order in the system, and is in turn itself established by the long-range order, in a self-consistent fashion. The mechanism here is expressed quantitatively in terms of a self-consistent Dyson equation relating a generalized matrix propagator, G, to a proper self-energy matrix, Σ. The off-diagonal elements of G describe ‘anomalous’ propagation process...

Spin-Density-Wave Mechanisms of Antiferromagnetism

The salient properties of spin-density-wave (SDW) states of an electron gas are reviewed. Antiferromagnetism in metals can arise from SDW's whether or not the metal contains localized magnetic moments (with spin degrees of freedom). If there are no local moments, a SDW ground state of the conduction electron gas is required, containing one or more collective-electron magnetization waves. Large amplitude SDW's are observed in Cr and, very likely, small amplitude SDW's will eventually be observed in a number of other metals, including some of the nontransition elements. Localized d or f electrons can be antiferromagnetically ordered by SDW's even though the conduction electron gas, in which they are imbedded, has of itself a nonmagnetic ground state. Exchange interactions between the localized-electron spins and the spin polarization density of excited SDW's are responsible for excitation of the latter, which in turn orient and order the former. Indirect exchange interactions (Ruderman-Kittel) between localized spins are shown to be equivalent to, and derivable from a multiple SDW excitation of the electron gas. However, the indirect-interaction model is inappropriate for a discussion of long range order, since the SDW excitations are then concentrated in a few discrete waves. Observed effects such as truncation of the Fermi surface by repopulation of k space, the energetic importance of commensurateness between the SDW wavelength and lattice constant, and the occurrence of multiply periodic states of long range order are natural and immediate consequences of SDW theory.