Field-induced Spin Density Wave States Research Articles

Helical and collinear spin density wave order in the S=12 one-dimensional frustrated chain compound NaCuMoO4(OH) investigated by neutron scattering

We performed neutron diffraction experiments on a single crystal of an $S=\frac{1}{2}$ frustrated ferromagnetic chain compound ${\mathrm{NaCuMoO}}_{4}$(OH) at zero and finite magnetic fields. Three magnetic Bragg peaks with the propagation vector of $\mathbit{q}=(0,\ensuremath{\delta},0)$ $[\ensuremath{\delta}=0.481(1)]$ were observed at the zero field, which means that an incommensurate helical magnetic order was realized. Using the ratio of the intensities between these magnetic peaks, a proper-screw structure was suggested, where the spiral plane is the crystallographic $ac$ plane. While the $\ensuremath{\delta}$ is unchanged at lower fields, it decreases linearly with the field where ${\ensuremath{\mu}}_{0}H\ensuremath{\ge}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The field dependence is consistent with that of the field-induced spin density wave state that originated from the formation of the bound state of two magnons in the $S=\frac{1}{2}$ frustrated ferromagnetic chain.

Topological phase transition in quasi-one dimensional organic conductors.

We explore topological phase transition, which involves the energy spectra of field-induced spin-density-wave (FISDW) states in quasi-one dimensional (Q1D) organic conductors, using an extended Su-Schrieffer-Heeger (SSH) model. We show that, in presence of half magnetic-flux FISDW state, the system exhibits topologically nontrivial phases, which can be characterized by a nonzero Chern number. The nontrivial evolution of the bulk bands with chemical potential in a topological phase transition is discussed. We show that the system can have a similar phase diagram which is discussed in the Haldane’s model. We suggest that the topological feature should be tested experimentally in this organic system. These studies enrich the theoretical research on topologically nontrivial phases in the Q1D lattice system as compared to the Haldane topological phase appearing in the two-dimensional lattices.

Nesting of the Fermi surface and the wave-vector-dependent susceptibility in quasi-one-dimensional electron systems

The wave-vector dependence of the susceptibility in quasi-one-dimensional systems is studied theoretically without and with external magnetic field in the perpendicular direction. We show that the wave-vector-dependent susceptibility has a two-step plateaulike structure in the absence of the magnetic field, when the warping of the Fermi surface caused by the $\ensuremath{\mp}2{\ensuremath{\tau}}_{3}\text{ }\text{sin}[3({k}_{y}\ensuremath{\mp}\ensuremath{\phi})]$ term, which breaks the reflection symmetry with respect to the most conducting chain, is larger than the critical value. The susceptibility is shown to have the plateaulike maximum in the small region in the wave vector at the edge of the lower plateau. We discuss the importance of the plateaulike maximum for the the field-induced spin density wave states in quasi-one-dimensional systems, such as the organic conductors ${(\text{TMTSF})}_{2}{\text{PF}}_{6}$ and ${(\text{TMTSF})}_{2}{\text{ClO}}_{4}$.

Generalized susceptibility of the quasi-one-dimensional organic conductors with the triclinic lattice symmetry

The instability of the field-induced spin density wave states of the quasi-one-dimensional system is studied by using the model including the triclinic lattice symmetry and multi-transfer integrals. By calculating the magnetic susceptibility χ 0 ( Q ) , we found the asymmetric sweptback peak at q x – q y plane contrary to the symmetric peak for the model with the square lattice symmetry and single transfer integral.

Phase diagram of the field-induced spin density wave state in [formula omitted] deduced from the specific heat measurements

We have measured the specific heat of the quasi-one dimensional organic conductor ( TMTSF ) 2 ClO 4 . The clear phase diagram of the field-induced spin density wave state was obtained from the density plot of the specific heat. We observed the reentrance to the metallic state and the tetracritical behavior quite clearly, though there is no evidence of the ”tree-like” structure.

Sign reversal of the quantum Hall coefficient in the field-induced spin density wave states of quasi-one-dimensional system with periodic potential

The magnetic susceptibility of the quasi-one-dimensional system with the periodic potential such as slow cooled (TMTSF)2ClO4 is studied numerically. The magnitude of the periodic potential is given by V. The imperfect nesting on the Fermi surface originates from higher harmonics (t'b t3, t4) of the tight-binding model. We have found that qx of Q = (2kF +qx, π/b+qy) which gives a maximum of χ0(Q) has a negative value at t3/tb ≥ 0.007, t4/tb ≥ 0.0001, and V < 2t'b — 2t4, where t'b/tb is fixed to be 0.1. It means that the negative quantum Hall coefficient is possible in these parameters. Since the sign reversal of the quantum Hall coefficient is observed in (TMTSF)2ClO4, t3, t4 and V should satisfy the above conditions. We also discuss the periodic oscillation of Hall resistance with sign reversal which has been observed by Uji et al.

Generalized susceptibility of a quasi-one-dimensional system with periodic potential: A model for the organic superconductor(TMTSF)2ClO4

The nesting vector and the magnetic susceptibility of the quasi-one-dimensional system having imperfectly nested Fermi surface are studied analytically and numerically. The magnetic susceptibility has the plateaulike maximum in sweptback region in the momentum space, which is surrounded by $\mathbf{Q}=(2{k}_{F},\ensuremath{\pi})+{\mathbf{q}}_{i}$ (${k}_{F}$ is the Fermi wave number $i=1,3,4$ and ${\mathbf{q}}_{1}$, ${\mathbf{q}}_{3}$, and ${\mathbf{q}}_{4}$ are given in this paper). The best nesting vector, at which the susceptibility ${\ensuremath{\chi}}_{0}(\mathbf{Q})$ has the absolute maximum at $T=0$, is obtained near but not at the inflection point $\mathbf{Q}=(2{k}_{F},\ensuremath{\pi})+{\mathbf{q}}_{4}$. The effect of the periodic potential $V$ on the susceptibility is studied, which is important for the successive transitions of the field-induced spin-density wave in ${(\text{TMTSF})}_{2}{\text{ClO}}_{4}$. We obtain that the sweptback region (surrounded by ${\mathbf{q}}_{2}$, ${\mathbf{q}}_{3}$, and ${\mathbf{q}}_{4}$ when $V&gt;0$) becomes small as $V$ increases and it shrinks to ${\mathbf{q}}_{3}$ for $V\ensuremath{\ge}4{t}_{b}^{\ensuremath{'}}$, where ${t}_{b}^{\ensuremath{'}}$ gives the degree of imperfect nesting of the Fermi surface, i.e., the second harmonics of the warping in the Fermi surface. The occurrence of the sign reversal of the Hall coefficient in the field-induced spin-density wave states is discussed to be possible only when $V&lt;2{t}_{b}^{\ensuremath{'}}\ensuremath{-}2{t}_{4}$, where ${t}_{4}$ is the amplitude of the fourth harmonics of the warping in the Fermi surface. This gives the limitation for the magnitude of $V$.

Field-induced spin density wave state of the quasi-one-dimensional organic conductors under the anion ordering

The periodical oscillation of the Hall resistance as a function of the magnetic field (B) has been observed in the field-induced spin density wave (FISDW) state of (TMTSF)2ClO4in strong magnetic field [Uji et al. Phys. Rev. Lett. 94, 077206 (2005)]. The mechanism of this interesting phenomenon has not been explained. It is known that the periodic potential due to the ordering of the anion plays important role in the FISDWand the Hall conductivity. Although some properties such as the suppression of the FISDW state with even index are explained in the perturbative treatment of the periodic potential, the non-perturbative calculation is indispensable to this system because the periodic potential is thought to be not small. We calculate the susceptibility in the system to obtain the wave vector of the FISDW at strong fields. We also discuss the Hall resistance in the strong periodic potential case.

Magnetotransport and Magnetic-Field-Induced Density Waves in Low-Dimensional Layered Conductors

We discuss a quantum-mechanical picture for interlayer magnetotransport in quasi-one-dimensional layered conductors. We show that the lowest order contribution of interlayer coupling to interlayer conductivity, which originates from a single tunneling process between two neighboring layers, leads all of the fundamental angular effects; Lebed resonance, the Danner–Kang–Chaikin oscillations, and the third angular effect. We also discuss the mean field picture of charge density wave (CDW) and spin density wave (SDW) under magnetic fields. We see that the orbital quantization effect causes the successive transitions of field-induced SDW state. It is also discussed that the CDW state could show complicated phase transitions under magnetic fields as a result of competition between Zeeman effect and orbital effect.

Dissimilarity between metallic-like transport in the dielectric spin density wave and field-induced spin density wave states in (TMTSF)2PF6

We report similarities and differences of the transport features in the spin density wave (SDW) and in the field-induced SDW (FISDW) phases of the quasi-one-dimensional compound (TMTSF)2PF6. As temperature decreases below ≈2 K, the resistance in both phases exhibits a maximum and a subsequent strong drop. However, the characteristic temperature of the R(T) maximum and its scaling behavior in different magnetic fields B are evidence that the nonmonotonic R(T) dependences have different origin in SDW and FISDW regions of the phase diagram. We also found that the borderline T0(B, P) which divides the FISDW region of the P-B-T phase diagram into the hysteresis and nonhysteresis domains terminates in the N=1 subphase; the borderline thus has no extension to the SDW N=0 phase.

Novel phase boundary in the field induced spin density wave state in (TMTSF) 2PF 6

Magnetoresistance measurements on the quasi one-dimensional organic conductor (TMTSF) 2PF 6 performed in magnetic fields B up to 16 T, temperatures T down to 0.12 K, and under pressures P up to 1.4 GPa have revealed new phase boundary on the P– B– T phase diagram. The boundary subdivides the field induced spin density wave (FISDW) phase diagram into two regions. The low-temperature region of the FISDW diagram is characterized by a hysteresis behavior of the resistance R( B) which was observed in earlier studies and is typical for the first-order transitions. In contrast to the common believe, in the high temperature region of the FISDW phase diagram, the hysteresis in R( B) and, hence, the first-order transitions vanish. Nevertheless, sharp changes in d R/d B are observed both in the low and high temperature domains indicating that the transitions between different sub-phases exist over all range of the FISDW state. We compare these results with a theoretical prediction.

Quantized Hall Effect-like behavior in (DMET-TSeF) 2AuI 2

The Hall effect in the FISDW (Field Induced Spin Density Wave) states in the quasi one-dimensional (DMET-TSeF) 2 AuI 2 shows step like hehavior which is similar to the quantized Hall effect observed in the TMTSF salts. The result contrasts with that for the salt (DMET-TSeF) 2 AuCl 2 of the same family where the Hall voltage is not clearly quantized. The Hall voltage for the AuI 2 salt becomes 0 above 24 T which is higher than that observed in the AuCl 2 salt (18T). The absolute values of the Hall voltages at the corresponding phase indices are about one sixth of the values observed in the TMTSF system. Possible explanations are moving FISDWs and different pinning mechanisms peculiar to each salts in the DMET-TSeF family.

Quantum oscillations in(DMET−TSeF)2AuCl2

We report magnetoresistance measurements to 33 T at low temperatures in dimethil(ethylendithio)tetraselenafulvalene ${(\mathrm{D}\mathrm{M}\mathrm{E}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{S}\mathrm{e}\mathrm{F})}_{2}{\mathrm{AuCl}}_{2},$ an organic conductor with a strictly quasi-one-dimensional electronic structure at low temperatures. Magnetic field induced spin-density wave (FISDW) transitions, similar in origin to those observed in the Bechgaard salts, are investigated vs temperature and tilted magnetic field. Rapid oscillations (RO) of about 300 T are detected in this material, but only in the FISDW states. We conclude from this study that the reconstruction of a one-dimensional Fermi surface is a necessary condition for the RO phenomena to occur, and we describe the form of the resulting electronic structure.

Quantum Hall effect and anomalous transport in (TMTSF)2PF6

Under low temperatures and high magnetic fields, quasi-one dimensional organic conductor (TMTSF) 2 PF 6 exhibits a series of transitions to field-induced spin density wave (FISDW). Slightly above the onset of superconductivity in (TMTSF) 2 PF 6 , we observe a series of intervening phases that interrupt the sequence of FISDW that gives rise to the quantum Hall effect. These phases can be identified either as negative quantum numbered FISDW states or a puzzling arboresecent phase. Detailed study of the QHE in (TMTSF) 2 PF 6 reveals that the transport in the FISDW phases is dominated by anomalous longitudinal resistivities pzz and pyy that remain finite at low temperatures. While the quantization of σxy is not adversely affected at high magnetic fields, the transport in the intermediate magnetic field remains complicated. In addition, the conductivity along applied magnetic field, σ zz , cannot be easily understood in terms of three-dimensional QHE and is suggestive of the importance of inter-layer coupling.

Quantum hall effect and edge states in (TMTSF) 2PF 6

Study of high quality (TMTSF) 2PF 6 crystals reveals that the quantum Hall effect seen in the field-induced spin density wave states is accompanied by nearly vanishing longitudinal magnetoresistance. Motivated by these findings, we examine whether there are current carrying edge states associated with the quantum Hall effect in the (TMTSF) 2X Bechgaard salts. Measurements of in-plane and out-plane resistivities, R xx and R zz , and 2 and 4-terminal resistance suggest that the transport in the Bechgaard salts is dominated by the bulk and that it is difficult to separate the contribution from the edge states.

High field Hall effect in (DMET-TSeF) 2AuCl 2

The Hall effect of the quasi one-dimensional system (DMET-TSeF) 2AuCl 2 is reported at high fields where the system is in the FISDW (Field Induced Spin Density Wave) state. The Hall voltage shows similar but different behavior to those observed in the famous TMTSF system. The Hall voltages show step like behavior, but they are not clearly quantized. The voltage disappears above 18 T, and not the inverses of the FISDW boundary fields but the squares of them seem to show linear dependence to the phase indices n. The absolute values of the Hall voltages at the corresponding phase indices are about one tenth of the values observed in TMTSF system. Moving FISDW states have been proposed to understand the differences.

High Field Phase Diagram in (TMTSF) 2ClO 4 under Anion Ordering

We study the ordered states of the field induced spin density wave (FISDW) in the presence of an anion ordering potential at zero and finite temperature self-consistently. We obtain a new FISDW state stabilized in a strong potential region, by which we can explain recently obtained phase diagram of (TMTSF) 2 ClO 4 . The anomalous behavior of the so-called rapid oscillation in the transport and thermodynamics quantities is attributable to the new FISDW state.

Theory of the High Field Phase in (TMTSF)2ClO4under Anion Ordering

Stimulated by a recent experiment on (TMTSF) 2 ClO 4 , reporting a new phase diagram with a first order transition line at the high field, the field induced spin density wave (FISDW) states are reexamined under the anion ordering. It is found that a new type of FISDW state, which differs from that described by the so-called standard theory, is stabilized. We interpret the first order line as arising from a transition between the new state and the standard one when H is varied.

Simultaneous thermodynamic and transport measurements of the field-induced spin-density-wave transitions in (TMTSF)2ClO4.

The standard model for the field-induced spin-density-wave (FISDW) transitions in the Bechgaard salts (TMTSF${)}_{2}$X, where TMTSF is tetramethyltetraselenafulvalene, explains a cascade of phase trasitions with each phase associated with the quantum Hall effect. The ${\mathrm{ClO}}_{4}$ salt is sufficiently different that it has inspired a series of theoretical modifications from the standard model. To test these models we have performed simultaneous ${\mathrm{\ensuremath{\rho}}}_{\mathit{x}\mathit{x}}$, ${\mathrm{\ensuremath{\rho}}}_{\mathit{x}\mathit{y}}$, specific-heat, magnetocaloric effect, and magnetization measurements in the field range from 0--9 T. We find that all of the transport transitions, specifically the Hall resistance jumps, are associated with thermodynamic transitions. We observe the emergence of a new FISDW state characterized by a distinct Hall plateau. It arises from what was originally believed to be a tetracritical point in the phase diagram. We find no evidence for an arborescent phase diagram, but rather the signature of a single pairwise splitting of the phase boundaries. The higher-field transitions are decidedly first order, hysteretic, and ``lossy.'' Anion disorder decreases the number of observed phases and shifts the transition fields.

Quantum Hall effect in the field-induced spin density wave states

The Hall conductance is investigated in the field-induced spin density wave phases of the quasi-one-dimensional system. The role of the order parameters in the quantization of the Hall effect is shown in the framework of the topological number of the wave functions in the momentum space. A possible mechanism of the sign changes of the Hall conductance observed in the experiments is proposed in this framework.