Field Induced Spin Density Wave Research Articles

Low-Dimensional Organic Conductors under High Maghetic Fields

Novel electronic properties of low-dimensional organic conductors under high magnetic fields are briefly reviewed and some topical problems are discussed. Magnetic field effects to electron spins and electron orbital motions are discussed in the quasi one-dimensional conductors (R 1 R 2 DCNQI) 2 Cu and (TMTSF) 2 X and in the quasi two-dimensional conductor (BEDT-TTF)2XHg(SCN)4. Electronic properties in the anomalous state of (BEDT-TTF)2KHg(SCN)4 are discussed in the light of the angle dependent magnetoresistance oscillations. Properties of field-induced spin-density waves are studied in relation to the superlattice potential made by the anion ordering in (TMTSF) 2 ClO 4 . Possible origins of the rapid oscillations in magnetoresistance of (TMTSF) 2 X are examined. Introduction is made to theoretical ideas of field-induced superconductivity and experimental trial to find it.

Competing Instabilities and the High Field Phases of (TMTSF)2ClO4.

In this Letter we establish the phase diagram of $(\mathrm{TMTSF}{)}_{2}$ ${\mathrm{ClO}}_{4}$ to 30 T and above 0.5 K by transport and magnetization measurements. The high field region is bounded above by a second order transition from metallic to field induced spin density wave state at 5.5 K. Completely contained inside this boundary is a first order line terminating in a critical point. Neither phase boundary shows the large oscillations predicted in recent models. Our results suggest that there are separate transitions on each of the two Fermi surfaces, with only weak coupling between the order parameters.

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.

Quasi-One Dimensional Electrons in Magnetic Field

In the presence of the magnetic field the quasi-one dimensional system with the open fermi surface is essentially one dimensional. In the one-dimensional picture the effective interaction is shown to depend on the magnetic field and the two-dimensional momentum. The amplitude of the effective interaction depends on the magnetic field, resulting in the successive transitions to the field-induced spin density wave. The quantum Hall effect is explained by the momentum dependence of the effective interaction. The effect of the alternating potential is also studied.

FIR-spectroscopy of the FI-SDW excitation spectrum in a quasi-1D organic conductor

The anisotropic quasi one-dimensional conductor (TMTSF) 2X (X = ClO 4, PF 6) exhibits a cascade of field-induced spin density wave (FI-SDW) phase transitions. In this contribution, we present a magneto-optical study of the far infrared excitation spectrum of (TMTSF) 2ClO 4 as a function of magnetic field and temperature covering the low field part of the phase diagram of this system. The observed transition temperatures of the various FI-SDW subphases follow closely the established phase diagram. Our results show that the magnitude of the SDW gap depends neither on magnetic field nor on temperature. In addition we find evidence for the existence of a collective mode excitation in the SDW state, indicating a relatively strong pinning to the lattice. Further indication of a large electron lattice coupling is given by the strong phonon renormalisation in the SDW state.

Superconductivity and field induced states in DMET-TSeF family

Summary form only given. The magnetoresistance and magnetization measurements have been performed in (DMET-TSeF)/sub 2/X (X= AuI/sub 2/, AuBr/sub 2/, AuCl/sub 2/, Au(CN)/sub 2/, and I/sub 3/ .) family. All of them have quasi one-dimensional Fermi surfaces at room temperature. It has been found that two of them(X= AuI/sub 2/ and I/sub 3/) show superconductivity at ambient pressure below I K. It has been also found that AuI/sub 2/, AuBr/sub 2/, and AuCl/sub 2/ salts show the Field Induced Spin Density Wave(FISDW), AuI/sub 2/ and Aul/sub 2/ at ambient pressure and AuBr/sub 2/ under pressure. The observed magnetization oscillations are smaller than those observed in (TMTSF)/sub 2/CIO/sub 4/. We did not observe any magnetoresistance oscillation in superconducting I/sub 3/ salt up to 12 T. The Au(CN)/sub 2/ salt shows Shubnikov-de Haas oscillation indicating closed Fermi surface at the lowest temperature.

Field induced states and hysteresis effect in (DMET-TSeF)2AuI2

Abstract The magnetoresistance in (DMET-TSeF)2AuI2 have been measured up to 30T using a pulsed magnetic field at liquid He temperature. This system shows the field induced spin density wave (FI-SDW) state at ambient pressure. The magnetoresistance shows a field dependence similar to the well-known (TMTSF)2PF6 under pressure. The re-entrant behavior is not observed. The detailed study has shown that the hysteresis effect in this system is very small compared to that observed in TMTSF system.

Magic angles (lebed's and others), and the phase diagram of the bechgaard Salts

We present transport data on the metallic and Field Induced Spin Density Wave phases of the Bechgaard Salts (TMTSF) 2PF 6 and (TMTSF) 2ClO 4. The a and c axis resistivities ( ρ xxand ρ zz) of these compounds have been measured for magnetic field rotations in the b-c planes (Lebed geometry) and striking resistance dips are observed at magic angles. We examine the differences between the two materials and the status of our understanding of the Lebed resonances. We then investigate the resistivities for magnetic fields rotated in the a-c plane. In this case we find a new set of resonances which for the first time allow direct measurements of the Fermi surface parameters of quasi-one-dimensional metals. Extending ρ xxand ρ zz measurements to 30 Tesla we have found that the phase boundary for the FISDW in (TMTSF) 2ClO 4 is most easily investigated by temperature dependent studies of ρ zz at high field. We find a new phase below a nearly temperature independent and we discuss some of the properties of this phase.

Commensurability effects in the magnetoresistance of (TMTSF) 2PF 6

We present a study of the angular dependence of the magnetoresistance of the quasi-one-dimensional organic conductor (TMTSF) 2PF 6. For some special orientations of magnetic field in the least-conducting plane, a commensurability effect in the electronic open-orbit motion is expected. At these special “magic angles”, dips in the resistance along the least-conducting axis in both metallic and field-induced-spin-density-wave (FISDW) states are detected. In the latter states, the dips gradually fade away with the increasing field. Moreover, the commenusurability effect intereferes directly with FISDW transitions: For magic angles, the threshold field goes through a weak minimum and the amplitude of the FISDW magnetoresistance oscillations is diminished.

Fermi surface and physical properties of (TMTSF) 2NO 3

We studied the Fermi surface of the organic conductor (TMTSF) 2NO 3 through the measurements of magnetic quantum oscillations. Well defined Shuvnikov-de Haas(SdH) oscillations with frequency of ca. 60T can be observed below 8K and above 6T at the ambient pressure. Under a pressure of 8kbar, different oscillations with frequency of ca. 180T is observed above 10T at 450mK. Electron structure at low temperature seems strongly modified upon application of pressure which may explain the absence of superconductivity and the field-induced spin-density waves. The angular magnetoresistance could not be fit neither by Yamaji's model nor by Lebed's.

Quantum Hall effect in three-dimensional field-induced spin-density-wave phases with a tilted magnetic field.

The quantum Hall effect in the three-dimensional anisotropic tight-binding electrons is investigated in the field-induced spin density wave phases with a magnetic field tilted to any direction. The Hall conductivity, $\sigma_{xy}$ and $\sigma_{xz}$, are shown to be quantized as a function of the wave vector of FISDW, while $\sigma_{yz}$ stays zero, where $x$ is the most conducting direction and $y$ and $z$ are perpendicular to $x$.

Field-Induced Spin Density Wave Transition

Field-Induced Spin Density Wave Transition

Superconductivity and magnetic field induced spin density waves in the (TMTTF)2X family

We report magnetotransport measurements in the quasi one dimensional (Q-1-D) organic conductor (TMTTF) 2 Br at pressures up to 26 kbar, down to 0.45 K in magnetic fields up to 19 T along the c * direction. It is found that a superconducting ground state is stabilized under 26 kbar at T c =0.8 K. No magnetic field induced spin density wave (FISDW) transitions are observed below 19T unlike other Q-1-D superconductors pertaining to the selenium series. The computed amplitude of the interchain coupling along transverse directions is unable to explain the missing FISDW instability

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.

Magnetoacoustic oscillations in (TMTSF)2ClO4 in high magnetic fields, where TMTSF is tetramethyltetraselenafulvalene.

We have studied the sound velocity in the quasi-one-dimensional organic conductor (TMTSF${)}_{2}$${\mathrm{ClO}}_{4}$, where TMTSF is tetramethyltetraselenafulvalene. The system shows profound magnetoacoustic oscillations at low temperatures and in high magnetic fields. At the reentrant transition from the field-induced spin-density-wave (FISDW) state to the metallic state, the sound velocity shows a discontinuous jump. In the reentrant metallic phase, the amplitude of the oscillations is greatly enhanced and their phase is different by 180\ifmmode^\circ\else\textdegree\fi{} from that in the FISDW state. Above \ensuremath{\sim}20 T the sound velocity shows complex, temperature-dependent structure, which may indicate sharp oscillations of the phase boundary as predicted in some recent theories.

Quantized Hall conductance and its sign reversal in field-induced spin-density waves.

The sign-reversal phenomenon of the quantized Hall conductance as a function of an external magnetic field (H) observed in (TMTSF${)}_{2}$X is investigated theoretically. After giving a general Hall-conductance formula written in terms of order parameters of the field-induced spin-density wave (FISDW), we have done extensive mean-field calculations for a simplified standard model. It is shown that the many competing order parameters ${\mathrm{\ensuremath{\Delta}}}_{\mathit{n}}$ (n=0,\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}2, . . .) which coexist in a FISDW state can make the sign of the Hall constant change particularly near the subphase boundary of the FISDW and that numbering of the integer subphases (N=0,1,2, . . . stabilized in this order from high fields) does not necessarily coincide with the Hall number L defined by L=${\mathrm{\ensuremath{\sigma}}}_{\mathit{x}\mathit{y}}$/(${\mathit{e}}^{2}$/h). We have found that the jumps of the Hall constant are accompanied by the spin-density wave gap closing when a FISDW is continuously evolving as H varies. In order for the sign reversal to occur a FISDW must contain many order parameters ${\mathrm{\ensuremath{\Delta}}}_{\mathit{n}}$ with n being both signs.

Organic conductors in high and tilted magnetic fields

We review some of the unique properties exhibited by the organic superconductors at high magnetic fields with special emphasis on the magnetic field induced spin density waves (FISDWs) and the anomalous behavior of the magnetoresistance at “Magic Angles” where the field drives electron motion commensurate with the reciprocal lattice. We compare and contrast the behaviors of the sister salts (TMTSF) 2PF 6 and (TMTSF) 2ClO 4 at these Lebed magic angle resonances for tilted fields in the b- c plane (perpendicular to the most highly conducing direction) and we report the discovery of a new angular dependent phenomena for fields in the a-c plane. Unlike the purely geometrical resonances previously found, this new effect provides the first way to make direct quantitative measurements of the band structure parameters on the open orbit sheets of the quasi-one-dimensional Fermi surface.

Field-induced spin density wave and quantum Hall effect in (TMTSF) 2X

The sign reversal phenomenon of the quantized Hall conductance observed in (TMTSF) 2 X is explained theoretically in terms of the mean field solution for the field-induced spin density wave (FISDW) characterized by many-order parameters (MOP). The many competing OPs can make the sign of the Hall constant change particularly near the subphase boundary. It is shown that the jump of the Hall constant is accompanied by the energy gap closing of the FISDW.

Pair-breaking effect caused by the anion disorder in the magnetic-field-induced spin-density wave: A calorimetric study.

We have investigated the effect of anion disorder on the spin-density wave induced by the magnetic field (FISDW) in (TMTSF${)}_{2}$${\mathrm{ClO}}_{4}$ (the TMTSF is the tetramethyltetraselenafulvalene molecule). Specific heat and magnetocaloric-effect measurements have been performed under a magnetic field for various cooling rates across the ordering transition of the ${\mathrm{ClO}}_{4}$ anions. The respective variations of the critical temperature and of the jump of the electronic specific heat at the metal-SDW transition are compared with the universal pair-breaking behavior of a Bardeen-Cooper-Schrieffer superconductor in the presence of magnetic impurities. The field-induced spin-density wave (FISDW), in the presence of nonmagnetic anion disorder, qualitatively behaves as the former but exhibits a stronger pair-breaking effect. The anion disorder is also shown to be responsible for the change in the criticality of the metal-SDW phase transition: a peculiar point in the phase diagram is found to evolve from tetracritical to bicritical as the anion disorder is increased. These behaviors are discussed in light of available models for the spin-density wave induced by the magnetic field.

On a Field-Induced Spin-Density Wave in Bechgaard Salts

A new type of field-induced spin-density wave is shown to occur in a weak-coupling layered model which may be relevant for the highly anisotropic electronic structure of the Bechgaard salts. The modulation vector of this instability is perpendicular to the layers and close to the direction of the magnetic field applied perpendicular to the reciprocal layers. The magnitude of the modulation vector, order parameter and critical temperature have an oscillatory dependence on the magnitude of the magnetic field. PACS numbers: 75.30.Fv, 75.50.Rr, 71.45.Lr