Field-induced Spin Density Wave Phases Research Articles

Unconventional field-induced spin density wave phases in quasi-one-dimensional conductors in high magnetic fields

It is known that the Field-Induced Spin-Density-Wave (FISDW) phases are experimentally observed in the quasi-one-dimensional (Q1D) organic conductors with chemical formula (TMTSF)$_2$X (X=PF$_6$, ClO$_4$, etc.) and some others in moderate magnetic fields. From a theoretical point of view, they appear as a result of "one-dimensionalization" of the Q1D electron spectra due to the orbital electron effect in a magnetic field. We predict that the novel FISDW phases with different physical meaning have to appear in inclined high magnetic fields in Q1D conductors as a result of combination of the spin-splitting and orbital electron effects. We suggest performing the corresponding experiments in the (TMTSF)$_2$X materials.

Role of anion ordering and effective pressure in the field-induced spin-density-wave phase of (TMTSF)2X

Magnetoresistance and Hall resistance measurements were conducted in the field-induced spin-density-wave (FISDW) phase of (TMTSF)2ReO4 above 1.0 GPa, with an anion ordering specified by . The quantized Hall resistance shows the sequence , with decreasing field that is successfully explained by the “extended standard model”. Consequently, we demonstrate that the difference between the chemical and hydrostatic pressures is linked to the appearance of the peculiar FISDW phase of the TMTSF salts with .

Angular and temperature-dependent 77Se NMR in the metallic, SDW, and field-induced spin density wave phases of (TMTSF)2X

We report angular dependent 77Se NMR measurements on the partially quenched, metallic, and magnetic field-induced spin density wave (FISDW) states of (TMTSF)2CIO4. To correlate the NMR data with the FISDW phase diagram, electrical transport was also measured concurrently in some cases. Similar measurements on (TMTSF)2PF6 allow comparison of the behavior of the NMR signal of the spin density wave (SDW) transition with that of the FISDW transition. We present details of the experimental findings including the enhancement behavior, and correlations of the angular dependence of the NMR spectra with the spin-lattice relaxation rate 1/T1. A possible model to describe the behavior of the NMR signal intensity and 1/T1 in terms of a Hebel-Slichter mechanism upon crossing the metal-FISDW transition is presented.

Field-induced SDW phase and superconductivity of (DMET) 2CuCl 2

We report low temperature electronic properties of a newly synthesized organic conductor (DMET) 2CuCl 2, consisting of stacks of DMET molecules constructing a quasi-one-dimensional Fermi surface. Superconductivity transition below 0.8 K is found, which is suppressed by a small magnetic field of 0.1 T. Under magnetic fields, successive step-like anomalies of magnetoresistance and plateau-like Hall resistance with sign reversals are found, indicating the occurrence of field-induced spin density wave (FISDW) transitions. The angular-dependent magnetoresistance and thermopower are measured in order to clarify the quasi-one-dimensional electronic structure.

Effect of the dimerized gap due to anion ordering in the field-induced spin-density-wave of quasi-one dimensional organic conductors

We have measured Hall resistance and magnetic torque in the field-induced spin-density-wave (FISDW) phase of deuterated (TMTSF) 2 ClO 4 for various cooling rates through the anion ordering temperature. The Hall resistance with the magnetic field parallel to c* is not sensitive to cooling rate above 16T. On the other hand, the Hall resistance between 9 and 14 T rapidly and continuously decreases with increasing cooling rate. This result means that the Hall resistance in the semimetallic SDW phase between 9 and 14 T is not quantized in the intermediate cooled states. A new phase transition from the non-quantized phase to the quantized (n = 1) Hall phase exists with hysteresis of the Hall resistance. Moreover, the new phase boundary is shifted towards a lower field when the cooling rate is increased. We have also found that the magnetic torque in the non-quantized phase rapidly decreases with increasing cooling rate. A possible ground state of non-quantized and Hall phase of the FISDW phase of (TMTSF) 2 ClO 4 is discussed from the viewpoint of the peculiar SDW nesting vector.

Cooling rate dependence of rapid oscillations in deuterated (TMTSF)2ClO4 at high magnetic fields

The transverse magnetoresistance of deuterated (TMTSF) 2 ClO 4 for various cooling rates has been measured, with the field up to 28T parallel to the lowest conductivity direction c*. We describe the cooling rate dependence of rapid oscillations (RO) in the magnetoresistance inside the spin-density-wave (SDW) and the field-induced SDW (FISDW) state. We have found that two different RO in the semimetallic FISDW phase of relaxed (TMTSF) 2 ClO 4 became undistinguishable, with increasing cooling rate.

Field-induced SDW phase diagram of (TMTSF)2PF6 at high magnetic fields

Abstract Magnetoresistance measurements have been carried out along the highly conducting a-axis in the field-induced SDW (FISDW) phase of (TMTSF)2PF6 under 1.0 GPa , with the field up to 27 T parallel to the lowest-conductivity direction c ∗ . We have determined the phase boundary between the last semimetallic FISDW phase with n=1 and the FISDW insulating phase with n=0 and found that the semimetallic FISDW phase existed above the transition temperature of the FISDW insulating phase at least up to 24 T . These results suggest the necessity of the theoretical calculation for more realistic quasi-one-dimensional Fermi surface to explain the FISDW phase of (TMTSF)2PF6.

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.

Butterfly spectrum and quantum Hall effect in three-dimensional FISDW

By extending the two-dimensional (2D) model for Bechgaard salts, we explore field-induced spin-density wave (FISDW) in three-dimensional (3D) systems. We show that a series of FISDW phases peculiar to 3D do exist for quasi-1D systems (rather than quasi-2D ones such as Bechgaard salts). The 3D FISDW is characterized by the quantum Hall effect with σ xy and σ zx being quantized and the quasi-particle spectrum that is fractal as in the Hofstadter butterfly. This state may be regarded as one realization of the general theory for the 3D butterfly [Phys. Rev. Lett. 86 (2001) 1062].

Novel phases in the field-induced spin-density-wave state in(TMTSF)2PF6

Magnetoresistance measurements on the quasi-one-dimensional organic conductor $(\mathrm{TMTSF}{)}_{2}{\mathrm{PF}}_{6}$ performed in magnetic fields B up to 16 T, temperatures T down to 0.12 K and under pressures P up to 14 kbar have revealed new phases on its $P\ensuremath{-}B\ensuremath{-}T$ phase diagram. We found a new boundary which subdivides the field induced spin density wave (FISDW) phase diagram into two regions. Whereas a low-temperature region of the FISDW diagram is characterized by a hysteresis behavior typical for the first-order transitions, in a high-temperature region of the FISDW phase diagram, the hysteresis was found to disappear. We also found that the temperature dependence of the resistance (at a constant $B)$ changes sign at about the same boundary. We compare these results with recent theoretical models.

Field-induced SDW and integer quantum Hall effect in anisotropic three-dimensional electron systems

In order to realize Hofstadter's butterfly and Quantum Hall effect (QHE) in 3D systems as proposed previously (Phys. Rev. Lett. 86 (2001) 1062), we consider a system that has the field induced spin density wave (FISDW) phases. By extending the 2D FISDW model for an organic conductor to 3D, we show that there exists a series of FISDW phases peculiar to 3D in the phase diagram against ( B y , B z ). The quasi-particle spectrum is found to be indeed fractal.

Does the “quantized nesting model” properly describe the magnetic-field-induced spin-density-wave transitions?

Theoretical reinvestigation of a so-called field-induced spin-density-wave (FISDW) phase diagram in a magnetic field in quasi-one-dimensional compounds (TMTSF)2X (X=PF6, ClO4, AsF6, etc.) has revealed some novel qualitative features. Among them are (1) the FISDW wave vector is never strictly quantized; and (2) the FISDW phase diagram consists of two regions: (a) “Quantum FISDW,” where there exist jumps of the FISDW wave vectors between different FISDW subphases and (b) “Quasiclassical FISDW,” where the jumps disappear above some critical points and only one FISDW phase (characterized by a wave vector oscillating with a magnetic field) exists. Both these features are due to taking account of the breaking of an electron-hole symmetry. They contradict the previous textbook theoretical results (including the calculations of the “Three Dimensional Quantum Hall Effect”) performed by means of the “Quantized Nesting Model” which explicitly assumes the existence of the electron-hole symmetry. We stress that some effects related to the phenomena described above were experimentally observed but not properly interpreted.

Two rapid oscillations in the magnetoresistance in the field-induced spin-density-wave state of(TMTSF)2ClO4

We report a systematic study of longitudinal and transverse magnetoresistance in the quasi-one-dimensional organic conductor $(\mathrm{TMTSF}{)}_{2}{\mathrm{ClO}}_{4}.$ We found two distinctly different quantum (rapid) oscillations (RO's) at different temperature regions in the field-induced spin-density-wave (FISDW) phase. For temperature sweeps in a fixed field, a resistance peak is observed at ${T}^{*}$ which depends on the field. Surprisingly, the type of RO is correlated with ${T}^{*}(H).$ The difference between two oscillations leads us to establish subphases in the main FISDW phase of $(\mathrm{TMTSF}{)}_{2}{\mathrm{ClO}}_{4}.$ We also found the low-field boundary of McKernan's 3.5 K phase from a sudden increase of resistance. Our results conclude that the separate SDW transition on each pair of the Fermi surfaces is possibly responsible for subphases in the FISDW phase.

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.

Magnetic field dependence of the SDW transition in (TMTSF)2ClO4

The magnetic field dependence of the SDW transition in (TMTSF) 2 ClO 4 for various anion quenching rates has been measured up to 27T , with the field parallel to the lowest conductivity direction c * . The SDW transition temperature T SDW decreases with decreasing quenching rate and the rate of increase with magnetic field increases with decreasing T SDw . For quenched (TMTSF) 2 ClO 4 , T SDW increases from 4.5K in zero field up to 8.4K at 27T. As predicted, a quadratic behavior is observed below 18T, followed by a saturation behavior. These results are consistent with the prediction of the mean-field theory. For the perfect nesting case, T SDW is estimated as T SDWo =13.5K. This means that the SDW phase of quenched (TMTSF) 2 Cl0 4 is strongly suppressed by the two-dimensionality of the system. In the intermediate cooling state in which there is no SDW phase in zero field, T SDW shows a quadratic behavior above 12T and there is no saturation even at 27T, which is different to the field induced SDW phase of the relaxed state. This may be attributed to the difference of the anion gap structure.

1H NMR studies in the regime of the field-induced spin-density wave phases of (TMTSF)2PF6

We report 1 H proton spin-lattice relaxation measurements T l -1 on pressurized (TMTSF) 2 PF 6 in high magnetic fields. Maxima in T l -1 identify the phase transition to the field-induced spin-density wave (FISDW) state. Fluctuation effects are evident at temperatures above the transition, and vanish continuously in strength as the field is lowered to the critical field B c for the onset of the FISDW cascade.

Field-induced spin-density-wave transitions and rapid oscillations observed in the microwave surface impedance of(TMTSF)2ClO4

We report microwave surface impedance measurements performed in the relaxed state of the quasi-one-dimensional organic conductor $(\mathrm{TMTSF}{)}_{2}{\mathrm{ClO}}_{4}$ at 16.5 GHz in a magnetic field applied along the hard axis ${(c}^{*})$ direction. The transition from the metallic to the field-induced spin-density-wave (FISDW) state is not as sharp as it is generally observed in the dc regime, but the phase diagram is in very good agreement with existing data. An enhanced ac conductivity is measured in the FISDW phases, which should reflect the contribution of the collective mode. The rapid oscillations of the microwave magnetoresistivity appear in both the metallic state and/or in the FISDW phase. The deduced frequency (260 T) and the temperature dependence of the rapid oscillation amplitude in the normal and condensed states are consistent with reported dc results. A frequency dampening of the amplitude that is different in the two phases is, however, observed compared to dc data; its possible origin is discussed and confirms the different origin of the two mechanisms.

Temperature and magnetic field dependences of rapid oscillations in field induced spin-density-wave phase for (TMTSF) 2ClO 4

Magnetoresistance has been measured in the field induced spin-density-wave (FISDW) phase for (TMTSF) 2ClO 4. For a well relaxed single crystal, the a axis magnetoresistance is found to have a sharp maximum in the final FISDW phase. The field where the magnetoresistance has a maximum is 7.5T for T=1.6K and increases with increasing temperature. Anomalous temperature and magnetic field dependences of the rapid oscillations near the FISDW transition field are reported.

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 $x$.

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.