Electronic correlation in the infrared optical properties of the quasi two dimensional κ\kappa-type BEDT-TTF dimer system

The polarized optical reflectance spectra of the quasi two dimensional organic correlated electron system $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]$Y$, $Y =$ Br and Cl are measured in the infrared region. The former shows the superconductivity at $T_{\rm c} \simeq$ 11.6 K and the latter does the antiferromagnetic insulator transition at $T_{\rm N} \simeq$ 28 K. Both the specific molecular vibration mode $\nu_{3}(a_{g})$ of the BEDT-TTF molecule and the optical conductivity hump in the mid-infrared region change correlatively at $T^{*} \simeq$ 38 K of $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br, although no indication of $T^{*}$ but the insulating behaviour below $T_{\rm ins} \simeq$ 50-60 K are found in $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl. The results suggest that the electron-molecular vibration coupling on the $\nu_{3}(a_{g})$ mode becomes weak due to the enhancement of the itinerant nature of the carriers on the dimer of the BEDT-TTF molecules below $T^{*}$, while it does strong below $T_{\rm ins}$ because of the localized carriers on the dimer. These changes are in agreement with the reduction and the enhancement of the mid-infrared conductivity hump below $T^{*}$ and $T_{\rm ins}$, respectively, which originates from the transitions between the upper and lower Mott-Hubbard bands. The present observations demonstrate that two different metallic states of $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br are regarded as {\it a correlated good metal} below $T^{*}$ including the superconducting state and {\it a half filling bad metal} above $T^{*}$. In contrast the insulating state of $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl below $T_{\rm ins}$ is the Mott insulator.Comment: 8 pages, 7 figure

Competition between Charge Ordering and Superconductivity in Layered Organic Conductors α\alpha-(BEDT-TTF)2M_2MHg(SCN)4_4 (M = K, NH4_4)

While the optical properties of the superconducting salt $\alpha$-(BEDT-TTF)$_2$NH$_4$Hg(SCN)$_4$ remain metallic down to 2 K, in the non-superconducting K-analog a pseudogap develops at frequencies of about 200 cm$^{-1}$ for temperatures T < 200 K. Based on exact diagonalisation calculations on an extended Hubbard model at quarter-filling we argue that fluctuations associated with short range charge ordering are responsible for the observed low-frequency feature. The different ground states, including superconductivity, are a consequence of the proximity of these compounds to a quantum phase charge-ordering transition driven by the intermolecular Coulomb repulsion.Comment: 4 pages, 3 figure

On-chain electrodynamics of metallic (TMTSF)_2 X salts: Observation of Tomonaga-Luttinger liquid response

We have measured the electrodynamic response in the metallic state of three highly anisotropic conductors, (TMTSF)_2 X, where X=PF_6, AsF_6, or ClO_4, and TMTSF is the organic molecule tetramethyltetraselenofulvalene. In all three cases we find dramatic deviations from a simple Drude response. The optical conductivity has two features: a narrow mode at zero frequency, with a small spectral weight, and a mode centered around 200 cm^{-1}, with nearly all of the spectral weight expected for the relevant number of carriers and single particle bandmass. We argue that these features are characteristic of a nearly one-dimensional half- or quarter-filled band with Coulomb correlations, and evaluate the finite energy mode in terms of a one-dimensional Mott insulator. At high frequencies (\hbar\omega > t_\perp, the transfer integral perpendicular to the chains), the frequency dependence of the optical conductivity \sigma_1(\omega) is in agreement with calculations based on an interacting Tomonaga-Luttinger liquid, and is different from what is expected for an uncorrelated one-dimensional semiconductor. The zero frequency mode shows deviations from a simple Drude response, and can be adequately described with a frequency dependent mass and relaxation rate.Comment: 12 pages, 7 figures, RevTeX; minor corrections to text and references; To be published in Phys. Rev. B, 15 July 199

Transport properties of strongly correlated metals:a dynamical mean-field approach

The temperature dependence of the transport properties of the metallic phase of a frustrated Hubbard model on the hypercubic lattice at half-filling are calculated. Dynamical mean-field theory, which maps the Hubbard model onto a single impurity Anderson model that is solved self-consistently, and becomes exact in the limit of large dimensionality, is used. As the temperature increases there is a smooth crossover from coherent Fermi liquid excitations at low temperatures to incoherent excitations at high temperatures. This crossover leads to a non-monotonic temperature dependence for the resistance, thermopower, and Hall coefficient, unlike in conventional metals. The resistance smoothly increases from a quadratic temperature dependence at low temperatures to large values which can exceed the Mott-Ioffe-Regel value, hbar a/e^2 (where "a" is a lattice constant) associated with mean-free paths less than a lattice constant. Further signatures of the thermal destruction of quasiparticle excitations are a peak in the thermopower and the absence of a Drude peak in the optical conductivity. The results presented here are relevant to a wide range of strongly correlated metals, including transition metal oxides, strontium ruthenates, and organic metals.Comment: 19 pages, 9 eps figure

Incoherent Interplane Conductivity of kappa-(BEDT-TTF)2Cu[N(CN)2]Br

The interplane optical spectrum of the organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br was investigated in the frequency range from 40 to 40,000 cm-1. The optical conductivity was obtained by Kramers-Kronig analysis of the reflectance. The absence of a Drude peak at low frequency is consistent with incoherent conductivity but in apparent contradiction to the metallic temperature dependence of the DC resistivity. We set an upper limit to the interplane transfer integral of tb = 0.1 meV. A model of defect-assisted interplane transport can account for this discrepancy. We also assign the phonon lines in the conductivity to the asymmetric modes of the ET molecule.Comment: 7 pages with embedded figures, submitted to PR

“Medically unexplained” symptoms and symptom disorders in primary care: prognosis-based recognition and classification

Background: Many patients consult their GP because they experience bodily symptoms. In a substantial proportion of cases, the clinical picture does not meet the existing diagnostic criteria for diseases or disorders. This may be because symptoms are recent and evolving or because symptoms are persistent but, either by their character or the negative results of clinical investigation cannot be attributed to disease: so-called “medically unexplained symptoms” (MUS). MUS are inconsistently recognised, diagnosed and managed in primary care. The specialist classification systems for MUS pose several problems in a primary care setting. The systems generally require great certainty about presence or absence of physical disease, they tend to be mind-body dualistic, and they view symptoms from a narrow specialty determined perspective. We need a new classification of MUS in primary care; a classification that better supports clinical decision-making, creates clearer communication and provides scientific underpinning of research to ensure effective interventions. Discussion: We propose a classification of symptoms that places greater emphasis on prognostic factors. Prognosis-based classification aims to categorise the patient’s risk of ongoing symptoms, complications, increased healthcare use or disability because of the symptoms. Current evidence suggests several factors which may be used: symptom characteristics such as: number, multi-system pattern, frequency, severity. Other factors are: concurrent mental disorders, psychological features and demographic data. We discuss how these characteristics may be used to classify symptoms into three groups: self-limiting symptoms, recurrent and persistent symptoms, and symptom disorders. The middle group is especially relevant in primary care; as these patients generally have reduced quality of life but often go unrecognised and are at risk of iatrogenic harm. The presented characteristics do not contain immediately obvious cut-points, and the assessment of prognosis depends on a combination of several factors. Conclusion: Three criteria (multiple symptoms, multiple systems, multiple times) may support the classification into good, intermediate and poor prognosis when dealing with symptoms in primary care. The proposed new classification specifically targets the patient population in primary care and may provide a rational framework for decision-making in clinical practice and for epidemiologic and clinical research of symptoms

Autonomous Meridian Sensory Response: from Internet subculture to audiovisual therapy

ASMR (Autonomous Sensory Meridian Response) is the name given to a pleasant sensation that can be felt most commonly on the scalp and can be triggered by various gentle sounds (like whispers, crinkles or tapping), smooth and repetitive visual stimuli, personal attention (like the touch of a hairdresser or a masseur) or other events. ASMR is often associated with a general feeling of relaxation and peace. Whilst academic research on the sociological, artistic, sensory and cognitive dimensions is still in its infancy ASMR has grown into a worldwide, cross-disciplinary, inter-cultural, multi-lingual social media sensation. This paper outlines the rise of ASMR as Internet subculture from its inception as ‘whispering community’ on Internet platforms and blogs, to become a truly popular (i.e. made by the people) platform for creative expression, self-made holistic therapy and in some instances true artistic audiovisual endeavours. This paper comments on the reasons behind the rise of the ASMR community as a fertile ground for creative expression. Audiences’ expectations are dictated by the attention-induced nature of the sensory experience, a factor that spawned an exceptionally perceptive viewership if one considers the inherently fragmented essence of ubiquitous streaming media and the impatient scanning and skipping modes of reception it encourages. ‘ASMRtists’ thus enjoy a privileged relationship with audiences who are not impressed with the relentless pour of energy and information from social media platforms and treasure, instead, the slow, the quiet and the subtle. Examples from various ASMR content creators will be analysed from the compositional standpoint, highlighting technical and idiomatic similarities with forms of improvisatory practices and experimental artistic languages such as Musique Concrète. The paper will also illustrate recent audiovisual projects related to ASMR carried out at Keele University and will introduce the audience to planned developments towards ASMR related content delivered through mobile platforms