Search

Resistance and magnetic torque measurements are reported in a layered organic superconductor, $\ensuremath{\beta}''\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{4}[({\mathrm{H}}_{3}\mathrm{O})\mathrm{Ga}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}]{\mathrm{C}}_{6}{\mathrm{H}}_{5}{\mathrm{NO}}_{2}$ with ${T}_{c}=4.8$ K, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. Because of the large anion between the BEDT-TTF conducting layers, the superconductivity of this salt is highly anisotropic. In magnetic fields parallel to the conducting layers for $T=0.4$ K, the magnetic torque shows a large diamagnetic signal associated with hysteresis up to $\ensuremath{\sim}21$ T, suggesting the upper critical field ${H}_{c2}\ensuremath{\gtrsim}21$ T at 0.4 K. The large reduction of the diamagnetic signal is observed above 16 T, which shows a Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) phase transition. For $T=0.5$ K, the interlayer resistance has nonzero value in a wide field region up to ${H}_{c2}$, arising from the Josephson vortex dynamics. Successive dips in the second derivative curves of the resistance are observed between 16 T and ${H}_{c2}$, which are ascribed to the commensurability effect between the Josephson vortex lattice and the order parameter oscillation of the FFLO phase. The commensurability effect is observed only in nearly parallel fields, showing that the FFLO phase is stable in a very limited field angle region. The temperature-field phase diagram is determined.

We experimentally demonstrate a mechanism for a new kind of magnetic resonance, the Fermi-surface traversal resonance (FTR). This is caused by the periodic traversal of carriers across quasi-one-dimensional (Q1D) sections of Fermi surface (FS) in an external magnetic field. Owing to the warping of the Q1D Fermi sheets, the real space velocities of the carriers oscillate as they cross the FS, generating resonances in the high frequency conductivity of the material. The results contain information about the FS, including the direction and harmonic content of the warping components. Using a rotating resonant-cavity system, FTRs have been observed in $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{KHg}(\mathrm{SCN}{)}_{4}$.

We report on an experimental observation of self-organized criticality in {sup 4}He very close to its superfluid transition. A constant temperature gradient, independent of the heat flux Q through the sample, is created along a vertical column of {sup 4}He by applying heat to the top of the column. This constant temperature gradient equals the gravity-induced gradient in the superfluid transition temperature, indicating that the thermal conductivity of the sample has self-organized. The closeness to criticality in this state is the same throughout most of the sample, and it depends only on Q. {copyright} {ital 1997} {ital The American Physical Society}

The magnetoabsorption of millimeter-wave radiation by single crystals of the organic metals \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$TlHg(SCN${)}_{4}$ and \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$KHg(SCN${)}_{4}$ has been studied in the frequency range 30--120 GHz. The experiments reveal two dominant contributions to the magnetoabsorption spectra. The first is interpreted as the cyclotron resonance of two-dimensional carriers, and is characterized by broad lines (linewidth/magnetic field \ensuremath{\Delta}B/B\ensuremath{\sim}0.5--1). In addition to a resonance exhibiting a cyclotron mass ${\mathit{m}}_{\mathit{c}}$\ensuremath{\sim}2.8${\mathit{m}}_{0}$, there are two further lines corresponding to frequency-dependent cyclotron masses in the ranges ${\mathit{m}}_{\mathit{c}}$\ensuremath{\sim}(1--1.5)${\mathit{m}}_{0}$ and ${\mathit{m}}_{\mathit{c}}$\ensuremath{\sim}(0.5--0.8)${\mathit{m}}_{0}$. This frequency dependence is believed to result from many-body effects. The second contribution to the magnetoabsorption is formed by a series of narrow lines with \ensuremath{\Delta}B/B\ensuremath{\sim}0.03--0.1 and amplitudes 5--10 times smaller than the features interpreted as cyclotron resonances. These narrow lines are attributed to a superposition of modes due to antiferromagnetic resonance and conduction-electron-spin resonance (ESR). The feature characteristic of antiferromagnetic resonance is the presence of a mode with a frequency that decreases with increasing magnetic field. The magnetoabsorption structure attributed to ESR consists of a relatively broad maximum upon which a sharp dip is superimposed. This behavior is believed to be analogous to the resonant transparency observed in thin metallic films undergoing ESR. \textcopyright{} 1996 The American Physical Society.

Magnetoresistance measurements carried out in pulsed magnetic fields of up to 60 T and at temperatures down to 350 mK and angle-dependent magnetoresistance experiments performed in quasistatic fields have been used to establish that the charge-transfer salt beta ''-(BEDT-TTF)(2)AuBr2 undergoes a change in electronic structure at similar to 10 T. We propose that this arises due to a field-induced transition between two different spin-density-wave states. Furthermore, at the highest magnetic fields, both the background magnetoresistance and effective masses of the quasiparticles were found to increase, possibly as a result of an enhancement of the density of states. It is found that frequency mixing effects at very high magnetic fields, observed in the Fourier spectra of the magnetoresistance, cannot be explained by the Shoenberg magnetic interaction, but are instead probably caused by oscillations in the chemical potential which become important as the extreme quantum limit is approached.

The de Haas-van Alphen effect has been measured in the organic conductor alpha-(BEDT-TTF)(2)KHg(SCN)(4) in pulsed magnetic fields of up to 54 T, temperatures down to 350 mK, and at three different angles. Analysis of the amplitude of the quantum oscillations within the framework of a two-dimensional Lifshitz-Kosevich model reveals that this material appears to be only fully transformed into the high-field state at fields above similar to 27 T, where the quasiparticle scattering rate is observed to be low. On entering the low-field state, the scattering rate is observed to increase dramatically. We also observe an apparent increase in the effective mass from similar to 1.5m(e) in the low-field state to similar to 2.7m(e) in the high-field state. The data at low fields are in accord with previous studies, but the nature of the higher harmonics of the de Haas-van Alphen oscillations at high fields appears to signify a departure from conventional Lifshitz-Kosevich behavior.

High-field magnetoresistance measurements have been performed on single-crystal samples of the charge-transfer salt \ensuremath{\alpha}-${\mathrm{ET}}_{2}$KHg (SCN${)}_{4}$ [where ET is bis(ethylenedithio)tetrathiafulvalene] using fields of up to 30 T. We examine the character of the Shubnikov--de Haas oscillations for a range of samples, temperatures, and field regimes to understand the role played by spin splitting and magnetic breakdown effects. Angle-dependent magnetoresistance oscillations (AMRO) in magnetic fields below the kink transition (22 T) indicate that the low-magnetic-field band structure is dominated by a one-dimensional (1D) open section of Fermi surface (FS) inclined at \ensuremath{\sim}21\ifmmode^\circ\else\textdegree\fi{} to the crystallographic ${\mathit{b}}^{\mathrm{*}}$c plane; above the kink, the AMRO change in character, indicating that the FS now possesses a 2D closed section in the form of a distorted cylinder. The data are discussed in terms of a recent model of the low-field band structure and give definitive proof that the kink is the signature of a magnetic-field-induced change in the FS.

We present high-resolution heat-capacity data for methane adsorbed on graphite for nominal coverages of 0.87 to 7 layers, from T = 70 to 120 K. For films thicker than 1.1 layers, we find capillary condensate coexisting with the film. We have performed heat-capacity scans on films formed by both adsorption and desorption. By comparing the locations of the phase transitions in the chemical potential mu vs T plane, we find that there is no significant interaction between the film and the capillary condensate. The heat-capacity signals from the films map out an unexpectedly rich set of phenomena for the second, third, and fourth layers, including a two-dimensional triple point and a liquid-gas coexistence region for each layer. The fourth-layer critical temperature we find is lower than previous values found by vapor-pressure isotherms.

We present a proposed design for a pair-breaking photodetector for far-infrared and submillimeter radiation. Antenna-coupled radiation generates quasiparticles in a superconducting absorber, the density of which are measured using a single Cooper-pair box. Readout is performed using an electromagnetic oscillator or a microwave resonator, which is well suited for frequency multiplexing in large arrays. Theoretical limits to detector sensitivity are discussed and modeled, with predicted sensitivities on the order of ${10}^{\ensuremath{-}21}\text{ }\text{W}/\sqrt{\text{Hz}}$. We anticipate that this detector can be used to address key scientific goals in far-infrared and submillimeter astronomy.

We have examined time-averaged charges of the molecular donors around the phase boundary between the insulating, superconducting, and metal phases for ${\ensuremath{\beta}}^{\ensuremath{''}}$-type ET [$\mathrm{ET}=\text{bis}$-ethelenedithio-tetrathiafulvalene] salts with one (two) hole(s) per two (three) molecules by using vibrational spectroscopy. Around the phase boundary, the site charges are neither those expected for a well-developed charge-ordered (CO) state nor a homogeneous distribution. The molecular charges exhibit an inhomogeneous distribution just above the insulator-superconductor transition temperature. We have analyzed the distribution of the site charges from the viewpoint of the alternation of intersite Coulomb interactions along the stacking direction. The closeness in the energy of several CO patterns mostly contributes to the inhomogeneous distribution, whereas a large difference (no difference) in the energy levels contributes to the CO (homogeneous) state. Our observations indicate that the instability due to the closeness in the energy of several distributions correlates with the insulator-superconductor transition in nondimerized or weakly dimerized molecular conductors.

We report magnetotransport measurements on the quasi-two-dimensional charge-transfer salts β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y, with Y=C6H5NO2 and C6H5CN using magnetic fields of up to 45 T and temperatures down to 0.5 K. A surprisingly robust superconducting state with an in-plane upper critical field Bc2 33T, comparable to the highest critical field of any BEDT-TTF superconductor, and critical temperature Tcâ 7K is observed when M=Ga and Y=C6H5NO2. The presence of magnetic M ions reduces the in-plane upper critical field to 18T for M=Cr and Y=C6H5NO2 and M=Fe and Y=C6H5CN. Prominent Shubnikov-de Haas oscillations are observed at low temperatures and high magnetic fields, showing that the superconducting salts possess Fermi surfaces with one or two small quasi-two-dimensional pockets, their total area comprising 6% of the room-temperature Brillouin zone; the quasiparticle effective masses were found to be enhanced when the ion M was magnetic (Fe or Cr). The low effective masses and quasiparticle densities, and the systematic variation of the properties of the β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y salts with unit-cell volume points to the possibility of a superconducting groundstate with a charge-fluctuation-mediated superconductivity mechanism such as that proposed by Merino and McKenzie [Phys. Rev. Lett. 87, 237002 (2001)], rather than the spin-fluctuation mechanism appropriate for the κ-(BEDT-TTF)2X salts. © 2005 The American Physical Society.

We report high-field magnetotransport measurements on ${\ensuremath{\beta}}^{\ensuremath{''}}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{4}[({\mathrm{H}}_{3}\mathrm{O})M({\mathrm{C}}_{2}{\mathrm{O}}_{4}{)}_{3}]Y,$ where $M=\mathrm{Ga},$ Cr, and Fe and $Y={\mathrm{C}}_{5}{\mathrm{H}}_{5}\mathrm{N}.$ We observe similar Shubnikov--de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area $\ensuremath{\approx}8.5%$ of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are $\ensuremath{\sim}{m}_{\mathrm{e}},$ rather small compared to those of other BEDT-TTF salts. Apart from the case of the smallest Fermi-surface pocket, varying the M ion seems to have little effect on the overall Fermi-surface topology or on the effective masses. Despite the fact that all samples show quantum oscillations at low temperatures, indicative of Fermi liquid behavior, the sample and temperature dependence of the interlayer resistivity suggest that these systems are intrinsically inhomogeneous. It is thought that intrinsic tendency to disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules leads to the coexistence of insulating and metallic states at low temperatures. A notional phase diagram is given for the general family of ${\ensuremath{\beta}}^{\ensuremath{''}}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{4}[({\mathrm{H}}_{3}\mathrm{O})M({\mathrm{C}}_{2}{\mathrm{O}}_{4}{)}_{3}]Y$ salts.

Short-range antiferromagnetic correlations have been studied in the layered compounds (PPh 4 ) [Fe I I Fe I I I (ox) 3 ] and (NBu 4 ) [Fe I I Fe I I I (ox) 3 ] by neutron polarization analysis and Mossbauer spectroscopy. Polarized neutron diffraction profiles obtained between 2 and 50 K on (d 2 0 -PPh 4 ) [Fe I I Fe I I I (ox) 3 ] show no magnetic Bragg scattering; the lack of such scattering indicates the absence of long-range magnetic order. However, a broad asymmetric feature observed at a Q of ca. 0.8 A - 1 is attributed to two-dimensional short-range magnetic correlations, which are described by a Warren function. The correlation length is ca. 50 A between 2 and 30 K and then decreases to ca. 20 A at 50 K. The Mossbauer spectra of (PPh 4 ) [Fe I I Fe I I I (ox) 3 ] and (NBu 4 ) [Fe I I Fe I I I (ox) 3 ] have been measured between 1.9 and 293 K and 1.9 and 315 K, respectively, and are very similar. The paramagnetic spectra exhibit both high-spin Fe I I and Fe I I I doublets with relative areas which indicate a 5% and 2% excess, respectively, of Fe I I I . The coexistence in (PPh 4 ) [Fe I I Fe I I I (ox) 3 ] between 10 and 30 K of broad sextets and doublets in the Mossbauer spectra and the paramagnetic scattering observed in the polarized neutron measurements indicate the coexistence of spin-correlated and spin-uncorrelated regions in the layers of this compound. The polarized neutron scattering profiles and the Mossbauer spectra yield the magnetic exchange correlation length and lifetime, respectively, and the combined results are best understood in terms of layers composed of random frozen, but exchange correlated domains of ca. 50 A diameter at the lowest temperatures, of spin-correlated domains and spin-uncorrelated regions at intermediate temperatures, and of largely spin-uncorrelated regions above the Neel temperature as determined from magnetometry. The similarity of the Mossbauer spectra of (PPh 4 ) [Fe I I Fe I I I (ox) 3 ] and (NBu 4 ) [Fe I I Fe I I I (ox) 3 ] leads to the conclusion that similar magnetic exchange correlations are present in the latter compound.

The generalized phonon density of states (DOS) has been measured for the ceramic superconductor La/sub 1.85/Sr/sub 0.15/CuO/sub 4/ (T/sub 0/ = 38 K) and antiferromagnetic (T/sub N/ = 250 K) La/sub 2/CuO/sub 4/ from 0.1--100 meV at temperatures between 250 and 30 K. For the superconductor the phonon spectrum, which was temperature independent over the whole range, contained major peaks at 11.5, 18.5, 23.8, 29.5, 38, 55.2, 78 meV while La/sub 2/CuO/sub 4/ had peaks at 12, 18.5 (24), 29.5, 39.8, 57.2, 80 meV. Increased scattering in the range 5--8 meV, the only feature in the DOS of La/sub 2/CuO/sub 4/ which had no analogue in the superconductor was most apparent at 250 K decreasing with temperature to become insignificant at 30 K.