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Register a new userX-Ray Study of the Density Wave Instability of alpha-(BEDT-TTF)2MHg(SCN)4 with M=K and Rb
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We present an X-ray diffraction study of the quasi-2D conductors a-(BEDTTTF)2MHg(SCN), with M=K and Rb. They exhibit a phase transition of the density wave type at TDW=8-10K and 12-13K respectively, evidenced by magnetoresistivity, specific heat, NMR and Hall constant measurements. The structural study shows the presence of satellite reflections already at ambient temperature. The related modulation is incommensurate with multiple harmonics. For some of the compounds studied, the intensity of the satellite reflections strongly increases below TDW. According to Fermi surface (FS) calculations, the wave vector of the structural modulation achieves a quite good nesting of the global FS. This suggests a coupling of the modulation with the electronic degrees of freedom leading to a charge density wave ground state.
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Single crystals of the organic charge-transfer salts $alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ have been studied using Hall-potential measurements ($M=$K) and magnetization experiments ($M$ = K, Rb). The data show that two types of screening currents occur within the high-field, low-temperature CDW$_x$ phases of these salts in response to time-dependent magnetic fields. The first, which gives rise to the induced Hall potential, is a free current (${bf j}_{rm free}$), present at the surface of the sample. The time constant for the decay of these currents is much longer than that expected from the sample resistivity. The second component of the current appears to be magnetic (${bf j}_{rm mag}$), in that it is a microscopic, quasi-orbital effect; it is evenly distributed within the bulk of the sample upon saturation. To explain these data, we propose a simple model invoking a new type of quantum fluid comprising a CDW coexisting with a two-dimensional Fermi-surface pocket which describes the two types of current. The model and data are able to account for the body of previous experimental data which had generated apparently contradictory interpretations in terms of the quantum Hall effect or superconductivity.
The low-temperature charge-density-wave (CDW) state in the layered organic metals $alpha $-(BEDT-TTF)$_2$MHg(SCN)$_4$ has been studied by means of the Shubnikov -- de Haas and de Haas -- van Alphen effects. In addition to the dominant alpha-frequency, which is also observed in the normal state, both the magnetoresistance and magnetic torque possess a slowly oscillating component. These slow oscillations provide a firm evidence for the CDW-induced reconstruction of the original cylindrical Fermi surface. The alpha-oscillations of the interlayer magnetoresistance exhibit an anomalous phase inversion in the CDW state, whereas the de Haas -- van Alphen signal maintains the normal phase. We argue that the anomaly may be attributed to the magnetic-breakdown origin of the alpha-oscillations in the CDW state. A theoretical model illustrating the possibility of a phase inversion in the oscillating interlayer conductivity in the presence of a spatially fluctuating magnetic breakdown gap is proposed.
The low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 salt is known for its surprising angular dependent magnetoresistance (ADMR), which has been studied intensively in the last decade. However, the nature of the LTP has not been understood until now. Here we analyse theoretically ADMR in unconventional (or nodal) charge density wave (UCDW). In magnetic field the quasiparticle spectrum in UCDW is quantized, which gives rise to spectacular ADMR. The present model accounts for many striking features of ADMR data in alpha-(BEDT-TTF)_2KHg(SCN)_4.
In spite of extensive experimental studies of the angular dependent magnetoresistance (ADMR) of the low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 about a decade ago, the nature of LTP remains elusive. Here we present a new study of ADMR of LTP in alpha-(ET)_2 salts assuming that LTP is unconventional charge density wave (UCDW). In the presence of magnetic field the quasiparticle spectrum in UCDW is quantized, which gives rise to striking ADMR in UCDW. The present model appears to account for many existing ADMR data of alpha-(BEDT-TTF)_2KHg(SCN)_4 remarkably well.
The infrared spectra of the quasi-two-dimensional organic conductors $alpha$-(BEDT-TTF)$_2$$M$Hg(SCN)$_4$ ($M$ = NH$_4$, Rb, Tl) were measured in the range from 50 to 7000 cm down to low temperatures in order to explore the influence of electronic correlations in quarter-filled metals. The interpretation of electronic spectra was confirmed by measurements of pressure dependant reflectance of $alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$ at T=300 K. The signatures of charge order fluctuations become more pronounced when going from the NH$_4$ salt to Rb and further to Tl compounds. On reducing the temperature, the metallic character of the optical response in the NH$_4$ and Rb salts increases, and the effective mass diminishes. For the Tl compound, clear signatures of charge order are found albeit the metallic properties still dominate. From the temperature dependence of the electronic scattering rate the crossover temperature is estimated below which the coherent charge-carriers response sets in. The observations are in excellent agreement with recent theoretical predictions for a quarter-filled metallic system close to charge order.
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