Klizni valovi spinske gustoce: proučavanje strujnog šuma, 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.Proučavali smo karakteristike napon – struja materijala (TMTSF)2PF6 u stanju valova spinske gustoće (SDW). Za oscilatorni dio nelinearnog naponskog odziva na istosmjerno električno polje, osnovna frekventna raspodjela (kao funkcija tog polja) i nelinearan odnos frekvencije i SDW struje pokazuju rast usporednih kanala vođenja označenih manjim brzinama i većim udarnim presjecima. Broj osnovnih frekvencija, njihove amplitude i razina niskofrekventnog šuma, kao i otkočno ponašanje sustavno pokazuju na nehomogenosti uzorka i pridružene varijacije lokalnog polja, i mogu se shvatiti u okviru modela klizne faze. Rast praga električnog polja s magnetskim poljem može se protumačiti Bjeliš-Makijevom teorijom, ako se uzme u obzir ugniježdenje. Konačno, nalazimo da je ovisnost Hallovog otpora o električnom polju u skladu s kliznim mehanizmom SDW vođenja struje

Sliding spin-density waves: studies of conduction noise, magnetic field dependence and Hall resistivity

We have studied the current-voltage characteristics of the (TMTSF)_2PF_6 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

Field-induced spin density wave in (TMTSF)2_2NO3_3

Interlayer magnetoresistance of the Bechgaard salt (TMTSF)$_2$NO$_3$ is investigated up to 50 teslas under pressures of a few kilobars. This compound, the Fermi surface of which is quasi two-dimensional at low temperature, is a semi metal under pressure. Nevertheless, a field-induced spin density wave is evidenced at 8.5 kbar above $\sim$ 20 T. This state is characterized by a drastically different spectrum of the quantum oscillations compared to the low pressure spin density wave state.Comment: to be published in Phys. Rev. B 71 (2005

Klizni valovi spinske gustoce: proučavanje strujnog šuma, 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.Proučavali smo karakteristike napon – struja materijala (TMTSF)2PF6 u stanju valova spinske gustoće (SDW). Za oscilatorni dio nelinearnog naponskog odziva na istosmjerno električno polje, osnovna frekventna raspodjela (kao funkcija tog polja) i nelinearan odnos frekvencije i SDW struje pokazuju rast usporednih kanala vođenja označenih manjim brzinama i većim udarnim presjecima. Broj osnovnih frekvencija, njihove amplitude i razina niskofrekventnog šuma, kao i otkočno ponašanje sustavno pokazuju na nehomogenosti uzorka i pridružene varijacije lokalnog polja, i mogu se shvatiti u okviru modela klizne faze. Rast praga električnog polja s magnetskim poljem može se protumačiti Bjeliš-Makijevom teorijom, ako se uzme u obzir ugniježdenje. Konačno, nalazimo da je ovisnost Hallovog otpora o električnom polju u skladu s kliznim mehanizmom SDW vođenja struje

Atomic-resolution studies of epitaxial strain release mechanisms in La1.85Sr0.15CuO4/La0.67Ca0.33MnO3\mathrm{L}{\mathrm{a}}_{1.85}\mathrm{S}{\mathrm{r}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}/\mathrm{L}{\mathrm{a}}_{0.67}\mathrm{C}{\mathrm{a}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3} superlattices

In this paper we present an atomic-resolution electron microscopy study of superlattices (SLs) where the colossal magnetoresistant manganite La0.67Ca0.33MnO3 (LCMO) and the high critical temperature superconducting cuprate La1.85Sr0.15CuO4 (LSCO) are combined. Although good quality epitaxial growth can be achieved, both the choice of substrate and the relatively large lattice mismatch between these materials (around 2%) have a significant impact on the system properties [Phys. C 468, 991 (2008); Nature (London) 394, 453 (1998)]. Our samples, grown by pulsed laser deposition, are epitaxial and exhibit high structural quality. By means of cutting-edge electron microscopy and spectroscopy techniques we still find that the epitaxial strain is accommodated by a combination of defects, such as interface steps and antiphase boundaries in the manganite. These defects result in inhomogeneous strain fields through the samples. Also, some chemical inhomogeneities are detected, up to the point that novel phases nucleate. For example, at the LCMO/LSCO interface the ABO3-type manganite adopts a tetragonal LSCO-like structure forming localized layers that locally resemble the composition of La2/3Ca4/3MnO4. Structural distortions are detected in the cuprate as well, which may extend over lateral distances of several unit cells. Finally, we also analyze the influence of the substrate-induced strain by examining superlattices grown on two different substrates: (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) and LaSrAlO4 (LSAO). We observe that SLs grown on LSAT, which are nonsuperconducting, present reduced values of the c axis compared to superlattices grown on LSAO (which are fully superconducting). This finding points to the fact that the proper distance between copper planes in LSCO is essential in obtaining superconductivity in cuprates

Spin-density-wave transition of (TMTSF)2_2PF6_6 at high magnetic fields

The transverse magnetoresistance of the Bechgaard salt (TMTSF)$_2$PF$_6$ has been measured for various pressures, with the field up to 24 T parallel to the lowest conductivity direction c$^{\ast}$. A quadratic behavior is observed in the magnetic field dependence of the spin-density-wave (SDW) transition temperature $T_{\rm {SDW}}$. With increasing pressure, $T_{\rm {SDW}}$ decreases and the coefficient of the quadratic term increases. These results are consistent with the prediction of the mean-field theory based on the nesting of the quasi one-dimensional Fermi surface. Using a mean field theory, $T_{\rm {SDW}}$ for the perfect nesting case is estimated as about 16 K. This means that even at ambient pressure where $T_{\rm {SDW}}$ is 12 K, the SDW phase of (TMTSF)$_2$PF$_6$ is substantially suppressed by the two-dimensionality of the system.Comment: 11pages,6figures(EPS), accepted for publication in PR

Superconductivity and charge-carrier localization in ultrathin La1.85Sr0.15CuO4/La2CuO4{\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}/{\mathrm{La}}_{2}{\mathrm{CuO}}_{4} bilayers

La1.85Sr0.15CuO4/La2CuO4 (LSCO15/LCO) bilayers with a precisely controlled thickness of N unit cells (UCs) of the former and M UCs of the latter ([LSCO15_N/LCO_M]) were grown on (001)-oriented SrLaAlO4 (SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and reciprocal space map (RSM) studies confirmed the epitaxial growth of the bilayers and showed that a [LSCO15_2/LCO_2] bilayer is fully strained, whereas a [LSCO15_2/LCO_7] bilayer is already partially relaxed. The in situ monitoring of the growth with reflection high energy electron diffraction (RHEED) revealed that the gas environment during deposition has a surprisingly strong effect on the growth mode and thus on the amount of disorder in the first UC of LSCO15 (or the first two monolayers of LSCO15 containing one CuO2 plane each). For samples grown in pure N2O gas (growth type B), the first LSCO15 UC next to the SLAO substrate is strongly disordered. This disorder is strongly reduced if the growth is performed in a mixture of N2O and O2 gas (growth type A). Electric transport measurements confirmed that the first UC of LSCO15 next to the SLAO substrate is highly resistive and shows no sign of superconductivity for growth type B, whereas it is superconducting for growth type A. Furthermore, we found, rather surprisingly, that the conductivity of the LSCO15 UC next to the LCO capping layer strongly depends on the thickness of the latter. A LCO capping layer with 7 UCs leads to a strong localization of the charge carriers in the adjacent LSCO15 UC and suppresses superconductivity. The magnetotransport data suggest a similarity with the case of weakly hole doped LSCO single crystals that are in a so-called ‘cluster-spin-glass state.’ We discussed several mechanisms that could lead to such a localization of holes that are embedded in a short-range ordered antiferromagnetic background for the case of a thick LCO capping layer with M=7 but not for a thin one with M=2

Sliding spin-density waves: studies of conduction noise, magnetic field dependence and Hall resistivity

We have studied the current-voltage characteristics of the (TMTSF)_2PF_6 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

Controlling the strength of ferromagnetic order in YBa2Cu3O7/La2/3Ca1/3MnO3 multilayers

With dc magnetization and polarized neutron reflectometry we studied the ferromagnetic response of YBa2Cu3O7/La2/3Ca1/3MnO3 (YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found that whereas for certain growth conditions (denoted as A type) the ferromagnetic moment of the LCMO layer is strongly dependent on the structural details of the YBCO layer on which it is deposited, for others (B type) the ferromagnetism of LCMO is much more robust. Both kinds of multilayers are of similar structural quality, but electron energy-loss spectroscopy studies with a scanning transmission electron microscope reveal an enhanced average Mn oxidation state of +3.5 for the A-type as opposed to the B-type samples, for which it is close to the nominal value of +3.33. The related, additional hole doping of the A-type LCMO layers, which likely originates from La and/or Mn vacancies, can explain their fragile ferromagnetic order, since it places them close to the boundary of the ferromagnetic order at which even weak perturbations can induce an antiferromagnetic or glassy state. On the other hand, we show that the B-type samples allow one to obtain YBCO/LCMO heterostructures with very thick YBCO layers and, yet, strongly ferromagnetic LCMO layers