35 research outputs found
Unconventional spin density wave in (TMTSF)2PF6 below T* ~ 4K
The presence of subphases in spin-density wave (SDW) phase of (TMTSF)2PF6
below T* ~ 4K has been suggested by several experiments but the nature of the
new phase is still controversial. We have investigated the temperature
dependence of the angular dependence of the magnetoresistance in the SDW phase
which shows different features for temperatures above and below T*. For T > 4K
the magnetoresistance can be understood in terms of the Landau quantization of
the quasiparticle spectrum in a magnetic field, where the imperfect nesting
plays the crucial role. We propose that below T* ~ 4K the new unconventional
SDW (USDW) appears modifying dramatically the quasiparticle spectrum. Unlike
conventional SDW the order parameter of USDW depends on the quasiparticle
momentum. The present model describes many features of the angular dependence
of magnetoresistance reasonably well. Therefore, we may conclude that the
subphase in (TMTSF)2PF6 below T* ~ 4K is described as SDW plus USDW.Comment: 7 pages, 9 figures, RevTeX4; misprint corrected, references updated,
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Klizni valovi spinske gustoce: prouÄavanje strujnog Å”uma, ovisnosti o magnetskom polju i Hallovog otpora
We have studied the current-voltage characteristics of the (TMTSF)2PF6 in the spin density state (SDW), and in zero and finite external magnetic field. For the oscillating part of the nonlinear voltage response to the applied DC electric field, the fundamental frequency distribution (as a function of this field) and a nonlinear relation between the frequency and the SDW current reveal the growth of parallel conduction channels characterized by lower velocities and larger cross-sections. The number of fundamental frequencies, their amplitude and the level of low-frequency noise as well as the depinning behaviour provide a consistent indication of the sample inhomogeneities and associated local field variations, and might be well understood within the framework of the phase slippage model. The increase of the threshold electric field with the applied magnetic filed can be explained by the Bjeli-Maki theory, if the imperfect nesting is taken into account. Finally, the electric-field dependence of the Hall resistivity is consistent with the sliding mechanism of the SDW conduction.ProuÄavali smo karakteristike napon ā struja materijala (TMTSF)2PF6 u stanju valova spinske gustoÄe (SDW). Za oscilatorni dio nelinearnog naponskog odziva na istosmjerno elektriÄno polje, osnovna frekventna raspodjela (kao funkcija tog polja) i nelinearan odnos frekvencije i SDW struje pokazuju rast usporednih kanala voÄenja oznaÄenih manjim brzinama i veÄim udarnim presjecima. Broj osnovnih frekvencija, njihove amplitude i razina niskofrekventnog Å”uma, kao i otkoÄno ponaÅ”anje sustavno pokazuju na nehomogenosti uzorka i pridružene varijacije lokalnog polja, i mogu se shvatiti u okviru modela klizne faze. Rast praga elektriÄnog polja s magnetskim poljem može se protumaÄiti BjeliÅ”-Makijevom teorijom, ako se uzme u obzir ugniježdenje. KonaÄno, nalazimo da je ovisnost Hallovog otpora o elektriÄnom polju u skladu s kliznim mehanizmom SDW voÄenja struje
Klizni valovi spinske gustoce: prouÄavanje strujnog Å”uma, ovisnosti o magnetskom polju i Hallovog otpora
We have studied the current-voltage characteristics of the (TMTSF)2PF6 in the spin density state (SDW), and in zero and finite external magnetic field. For the oscillating part of the nonlinear voltage response to the applied DC electric field, the fundamental frequency distribution (as a function of this field) and a nonlinear relation between the frequency and the SDW current reveal the growth of parallel conduction channels characterized by lower velocities and larger cross-sections. The number of fundamental frequencies, their amplitude and the level of low-frequency noise as well as the depinning behaviour provide a consistent indication of the sample inhomogeneities and associated local field variations, and might be well understood within the framework of the phase slippage model. The increase of the threshold electric field with the applied magnetic filed can be explained by the Bjeli-Maki theory, if the imperfect nesting is taken into account. Finally, the electric-field dependence of the Hall resistivity is consistent with the sliding mechanism of the SDW conduction.ProuÄavali smo karakteristike napon ā struja materijala (TMTSF)2PF6 u stanju valova spinske gustoÄe (SDW). Za oscilatorni dio nelinearnog naponskog odziva na istosmjerno elektriÄno polje, osnovna frekventna raspodjela (kao funkcija tog polja) i nelinearan odnos frekvencije i SDW struje pokazuju rast usporednih kanala voÄenja oznaÄenih manjim brzinama i veÄim udarnim presjecima. Broj osnovnih frekvencija, njihove amplitude i razina niskofrekventnog Å”uma, kao i otkoÄno ponaÅ”anje sustavno pokazuju na nehomogenosti uzorka i pridružene varijacije lokalnog polja, i mogu se shvatiti u okviru modela klizne faze. Rast praga elektriÄnog polja s magnetskim poljem može se protumaÄiti BjeliÅ”-Makijevom teorijom, ako se uzme u obzir ugniježdenje. KonaÄno, nalazimo da je ovisnost Hallovog otpora o elektriÄnom polju u skladu s kliznim mehanizmom SDW voÄenja struje
Anisotropic Charge Modulation in Ladder Planes of Sr_14-xCa_xCu_24O_41
The charge response of the ladders in Sr_14-xCa_xCu_24O_41 is characterized
by dc resistivity, low frequency dielectric and optical spectroscopy in all
three crystallographic directions. The collective charge-density wave screened
mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to
the mode along the legs. For x=8 and 9, the charge-density-wave response along
the rungs fully vanishes, while the one along the legs persists. The transport
perpendicular to the planes is always dominated by hopping.Comment: 4 pages, 3 figures, submitted to PRB R
The angular dependent magnetoresistance 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.Comment: 6 pages, 4 figure
Influence of doping on the Hall coefficient in Sr_{14-x}Ca_xCu_24O_41
We present Hall-effect measurements of two-leg ladder compounds
Sr_{14-x}Ca_xCu_24O_41 (0 <= x <= 11.5) with the aim to determine the number of
carriers participating in dc transport. Distribution of holes between the
ladder and chain subsystems is one of the crucial questions important for
understanding the physics of these compounds. Our Hall effect and resistivity
measurements show typical semiconducting behavior for x < 11.5. However, for
x=11.5, the results are completely different, and the Hall coefficient and
resistivity behavior is qualitatively similar to that of high temperature
copper-oxide superconductors. We have determined the effective number of
carriers at room temperature and compared it to the number of holes in the
ladders obtained by other experimental techniques. We propose that going from
x=0 to x=11.5 less than 1 hole per formula unit is added to the ladders and is
responsible for a pronounced change in resistivity with Ca doping.Comment: 8 pages, 7 figures, revised versio
The Hall effect in the organic conductor TTF-TCNQ: Choice of geometry for accurate measurements of highly anisotropic system
We have measured the Hall effect on recently synthesized single crystals of
quasi-one-dimensional organic conductor TTF-TCNQ, a well known charge transfer
complex that has two kinds of conductive stacks: the donor (TTF) and the
acceptor (TCNQ) chains. The measurements were performed in the temperature
interval 30 K < T < 300 K and for several different magnetic field and current
directions through the crystal. By applying the equivalent isotropic sample
(EIS) approach, we have demonstrated the importance of the choice of optimal
geometry for accurate Hall effect measurements. Our results show, contrary to
past belief, that the Hall coefficient does not depend on the geometry of
measurements and that the Hall coefficient value is around zero in high
temperature region (T > 150 K), implying that there is no dominance of either
TTF or TCNQ chain. At lower temperatures, our measurements clearly prove that
all three phase transitions of TTF-TCNQ could be identified from Hall effect
measurements.Comment: Revised version; 7 pages, 5 figure
Cooperative dynamics in charge-ordered state of alpha-(BEDT-TTF)2I3
Electric-field-dependent pulse measurements are reported in the
charge-ordered state of alpha-(BEDT-TTF)2I3. At low electric fields up to about
50 V/cm only negligible deviations from Ohmic behavior can be identified with
no threshold field. At larger electric fields and up to about 100 V/cm a
reproducible negative differential resistance is observed with a significant
change in shape of the measured resistivity in time. These changes critically
depend whether constant voltage or constant current is applied to the single
crystal. At high enough electric fields the resistance displays a dramatic drop
down to metallic values and relaxes subsequently in a single-exponential manner
to its low-field steady-state value. We argue that such an electric-field
induced negative differential resistance and switching to transient states are
fingerprints of cooperative domain-wall dynamics inherent to two-dimensional
bond-charge density wave with ferroelectric-like nature.Comment: 6 pages, 4 figures, resubmitted to Phys. Rev.