Spin-density Wave Dynamics Research Articles

Semiclassical dynamics of spin density waves

We present a theoretical framework for equilibrium and nonequilibrium dynamical simulation of quantum states with spin-density-wave (SDW) order. Within a semiclassical adiabatic approximation that retains electron degrees of freedom, we demonstrate that the SDW order parameter obeys a generalized Landau-Lifshitz equation. With the aid of an enhanced kernel polynomial method, our linear-scaling quantum Landau-Lifshitz dynamics (QLLD) method enables dynamical SDW simulations with $N \simeq 10^5$ lattice sites. Our real-space formulation can be used to compute dynamical responses, such as dynamical structure factor, of complex and even inhomogeneous SDW configurations at zero or finite temperatures. Applying the QLLD to study the relaxation of a noncoplanar topological SDW under the excitation of a short pulse, we further demonstrate the crucial role of spatial correlations and fluctuations in the SDW dynamics.

Nonlinear transport associated with spin density wave dynamics in Ca3Co4O9

We have carried out the transient nonlinear transport measurements on the layered cobalt oxide Ca$_3$Co$_{4}$O$_9$, in which a spin density wave (SDW) transition is proposed at $T_{\rm SDW} \simeq 30$ K. We find that, below $T_{\rm SDW}$, the electrical conductivity systematically varies with both the applied current and the time, indicating a close relationship between the observed nonlinear conduction and the SDW order in this material. The time dependence of the conductivity is well analyzed by considering the dynamics of SDW which involves a low-field deformation and a sliding motion above a threshold field. We also measure the transport properties of the isovalent Sr-substituted systems to examine an impurity effect on the nonlinear response, and discuss the obtained threshold fields in terms of thermal fluctuations of the SDW order parameter.

Nonlinear Electric Conduction in the SDW Phase of (TMTSF)2PF6 under Pressure

We have measured the nonlinear conductivity with the threshold electric field ET in the spin-density-wave (SDW) phase of (TMTSF)2PF6 under pressure to clarify the dynamics of SDW. The gradual temperature dependence of ET was observed under 0.21 GPa where TSDW is 12.5 K and was explained in the classical strong pinning model. The normalized excess electric conductivity [σ(2ET)-σ(0)]/σ(0) rapidly decreases below about T* ≃ 0.3TSDW. In high electric fields far above ET, the temperature independent electric conduction expressed as the function j = j0 exp (—E0/E) is observed below T*. It is suggested that the sliding of SDW with quantum origin occurs at low temperature. These phenomena are related to a phase transition dividing the SDW phase into two sub-phases in (TMTSF)2PF6. The electric conductivity in the low temperature sub-phase is discussed in terms of the SDW with discommensuration.

Depinning of Spin-Density-Wave in (TMTSF) 2ClO 4

We have performed the electric conductivity measurements in the spin-density-wave (SDW) phase of (TMTSF) 2 ClO 4 to clarify the dynamics of SDW. The temperature dependence of threshold electric fie...

Spin-density wave dynamics in the organic conductor (TMTSF)2X

We have measured current-voltage characteristic in the SDW phase of slightly pressurized (TMTSF) 2 AsF6 in order to investigate SDW dynamics. Using (TMTSF)2AsF6 crystals with various thickness the threshold electric field (ET) for the SDW depinning was determined. In thick samples, ET depends strongly on temperature above 8K and also below 3K. On the other hand, ET in thinner samples is temperature independent and is ∼ 100 mV/cm. It is found that ET at 4.2K increases with decreasing the cross-sectional area (S) when S<10 -4 cm 2 . Similar phenomenon has been reported in charge-density wave systems. In thin samples, the pinning mechanism seems to be different from that for thicker ones. Sliding properties might be also different between thin and thick samples. From measurements of transient voltage oscillations and excess current density, the pinning periodicity in the thinnest sample was found to be about 8A. The periodicity which is 4 times larger in thicker ones is almost the same as that expected in the washboard model. It is likely that the whole SDW slides collectively in thinner samples. The effect of the cross-sectional area for SDW dynamics is discussed.

Size effect on the spin-density wave dynamics in (TMTSF) 2X (X=AsF 6, PF 6)

The threshold electric field (E T) and the pinning lengths for the spin density wave (SDW) sliding were determined for (TMTSF) 2AsF 6 samples of various cross-sectional area (S). Contrary to large S, E T in small S samples is independent of temperature and is as large as 100mV/cm at 4.2K, We obtained the pinning length ~ 8Å in the thinnest sample, that is 4 times larger than that in thicker ones and is equal to that expected in the washboard model. Crystal size effect on the SDW dynamics is discussed.

Suppression of polarization fluctuations in chromium alloys with commensurate spin-density waves.

We compare electrical resistance noise in commensurate and incommensurate phases of the spin-density wave (SDW) in Cr and dilute CrMn alloys. The commensurate phase gives much less polarization fluctuation noise than the incommensurate phase. The incommensurability of the SDW and the lattice in Cr may affect the SDW dynamics through the existence of weak planes in which the induced orbital moment contribution to the SDW is close to zero.

Collective transport of spin density waves in organic salts

The spin density wave (SDW) in organic conductors like (TMTSF) 2PF 6 possesses two distinct collective modes; the spin wave and the sliding mode. Since the spin dynamics of SDW is described by an equation similar to the Leggett equation for superfluid 3He, we predict the longitudinal resonance. On the other hand both the electric charge and the current couple to the sliding SDW mode, which gives rise to a variety of nonlinear phenomena similar to those found in the quasi-one dimensional charge density wave (CDW) like NbSe 3.