High-frequency ultrasonic measurements of κ-(BEDT-TTF)2Cu(NCS)2 by devised transducer

Simizu, T.; Yoshimoto, Noriyuki; Nakamura, Mitsuteru; Yoshizawa, Michito

doi:10.1016/s0921-4526(99)01094-7

Cited by 12 publications

(11 citation statements)

References 9 publications

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“…The present experiments were performed in the configuration in which ultrasonic sounds propagated in the in-plane direction. The absolute value of the change seemed to be larger than those in the previous measurements of interlayer ultrasonic responses for the d[0,0] compound [35] and κ-(BEDT-TTF) 2 Cu(NCS) 2 [36], despite the percolative superconductivity originated from the macroscopic phase separation near the Mott transition in the present d [2,2] and d [3,3] compounds. The magnitude of the dip in the inset demonstrated that the volume fraction of the superconducting components changed due to the difference of deuterated numbers and cooling rates.…”

Section: Resultscontrasting

confidence: 85%

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

et al. 2021

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Thermodynamic investigation by calorimetric measurements of the layered organic superconductors, κ-(BEDT-TTF)2Cu[N(CN)2]Br and its partially deuterated compounds of κ-(d[2,2]-BEDT-TTF)2Cu[N(CN)2]Br and κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, performed in a wide temperature range is reported. The latter two compounds were located near the metal–insulator boundary in the dimer-Mott phase diagram. From the comparison of the temperature dependences of their heat capacities, we indicated that lattice heat capacities of the partially deuterated compounds were larger than that of the pristine compound below about 40 K. This feature probably related to the lattice softening was discussed also by the sound velocity measurement, in which the dip-like structures of the Δv/v were observed. We also discussed the variation of the electronic heat capacity under magnetic fields. From the heat capacity data at magnetic fields up to 6 T, we evaluated that the normal-state γ value of the partially deuterated compound, κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, was about 3.1 mJ K−2 mol−1. Under the magnetic fields higher than 3.0 T, we observed that the magnetic-field insulating state was induced due to the instability of the mid-gap electronic state peculiar for the two-dimensional dimer-Mott system. Even though the volume fraction was much reduced, the heat capacity of κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br showed a small hump structure probably related to the strong coupling feature of the superconductivity near the boundary.

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Section: Resultscontrasting

confidence: 85%

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

et al. 2021

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“…At ambient pressure, a pronounced softening of ultrasonic modes has been detected at 38 K and 46 K for -Br and -Cu(NCS) 2 , respectively. 18,73 These sound velocity anomalies have been assigned to a magnetic origin due to their phenomenological relation to the NMR results. 18 However, an alternative interpretation in terms of a structural phase transition could not be excluded.…”

Section: Discussionmentioning

confidence: 95%

“…18 However, an alternative interpretation in terms of a structural phase transition could not be excluded. 73 Owing to the large response of the anomaly in ␣(T), thermal expansion measurements provide a most sensitive tool to investigate these effects in detail. In particular, a comparative study covering various related (ET) 2 X compounds is helpful to find out the relevant structural and/or electronic parameters involved.…”

Section: Discussionmentioning

confidence: 99%

Evidence for structural and electronic instabilities at intermediate temperatures inκ−(BEDT−TTF)2Xfor
Müller
¹
,
Lang
²
,
Steglich
³

et al. 2002
Phys. Rev. B

152

170

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We present high-resolution measurements of the coefficient of thermal expansion ␣(T)ϭ‫ץ‬ ln l(T)/‫ץ‬T of the quasi-two-dimensional ͑quasi-2D͒ salts -(BEDT-TTF) 2 X with XϭCu(NCS) 2 , Cu͓N(CN) 2 ͔Br, and Cu͓N(CN) 2 ͔Cl in the temperature range Tр150 K. Three distinct kinds of anomalies corresponding to different temperature ranges have been identified. These are ͑A͒ phase-transition anomalies into the superconducting "XϭCu(NCS) 2 , Cu͓N(CN) 2 ͔Br… and antiferromagnetic "XϭCu͓N(CN) 2 ͔Cl… ground state, ͑B͒ phasetransition-like anomalies at intermediate temperatures ͑30-50͒ K for the superconducting salts, and ͑C͒ kinetic, glasslike transitions at higher temperatures, i.e., ͑70-80͒ K for all compounds. By a thermodynamic analysis of the discontinuities at the second-order phase transitions that characterize the ground state of system ͑A͒, the uniaxial-pressure coefficients of the respective transition temperatures could be determined. We find that in contrast to what has been frequently assumed, the intraplane-pressure coefficients of T c for this family of quasi-2D superconductors do not reveal a simple form of systematics. This demonstrates that attempts to model these systems by solely considering in-plane electronic parameters are not appropriate. At intermediate temperatures ͑B͒, distinct anomalies reminiscent of second-order phase transitions have been found at T* ϭ38 K and 45 K for the superconducting XϭCu(NCS) 2 and Cu͓N(CN) 2 ͔Br salts, respectively. Most interestingly, we find that the signs of the uniaxial pressure coefficients of T*, ‫ץ‬T*/‫ץ‬p i (iϭa,b,c), are strictly anticorrelated with those of T c . Based on comparative studies including the nonsuperconducting X ϭCu͓N(CN) 2 ͔Cl salt as well as isotopically labeled compounds, we propose that T* marks the transition to a density-wave state forming on minor, quasi-1D parts of the Fermi surface. Our results are compatible with two competing order parameters that form on disjunct portions of the Fermi surface. At elevated temperatures ͑C͒, all compounds show ␣(T) anomalies that can be identified with a kinetic, glasslike transition where, below a characteristic temperature T g , disorder in the orientational degrees of freedom of the terminal ethylene groups becomes frozen in. Our results provide a natural explanation for the unusual time-and cooling-rate dependences of the ground-state properties in the hydrogenated and deuterated Cu͓N(CN) 2 ͔Br salts reported in the literature.

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“…These results could be relevant to understanding recent acoustic experiments. 8,9 The velocity of ultrasonic waves that are propagating perpendicular to the layers was found to have a nonmonotonic temperature dependence. These waves have frequencies of 100 MHz and velocities of about 2000 cm/s.…”

Section: Figmentioning

confidence: 99%

“…[2][3][4][5][6] Recent Raman scattering experiments 7 find that in the metallic state the frequency of certain phonons associated with the BEDT-TTF molecules has a nonmonotonic temperature dependence below 200 K. Acoustic experiments 8,9 find that around 40 K there is a significant softening of the speed of longitudinal sound propagating perpendicular to the layers. This softening, of the order of a few percent, is more than an order of magnitude larger than the softening associated with the superconducting transition.…”

mentioning

confidence: 99%

Phonon anomalies due to strong electronic correlations in layered organic metals

Merino

McKenzie

2000

Phys. Rev. B

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We show how the coupling between the phonons and electrons in a strongly correlated metal can result in phonon frequencies that have a nonmonotonic temperature dependence. Dynamical mean-field theory is used to study the Hubbard-Holstein model that describes the -(BEDT-TTF) 2 X ͓where BEDT-TTF is bis͑ethylenedithia-tetrathiafulvalene͔͒ family of superconducting molecular crystals. The crossover with increasing temperature from a Fermi liquid to a bad metal produces phonon anomalies that are relevant to recent Raman scattering and acoustic experiments.An important problem concerning strongly correlated metals such as heavy fermions, cuprates, and organic superconductors is understanding the interplay of the strong interactions between electrons and the interactions between the electrons and phonons. This is particularly relevant to understanding the question of whether the phonons play any role in superconductivity.1 In this paper, we show how strong electronic correlations can lead to phonon frequencies varying with temperature. Although our calculations focus on explaining recent experiments on a particular family of organic superconductors, the physics involved is relevant to other strongly correlated metals in which there is a significant redistribution of the electronic spectral weight as the temperature is varied.The quasi-two-dimensional organic superconductors, -(BEDT-TTF) 2 X, are strongly correlated electron systems.2-6 Recent Raman scattering experiments 7 find that in the metallic state the frequency of certain phonons associated with the BEDT-TTF molecules has a nonmonotonic temperature dependence below 200 K. Acoustic experiments 8,9 find that around 40 K there is a significant softening of the speed of longitudinal sound propagating perpendicular to the layers. This softening, of the order of a few percent, is more than an order of magnitude larger than the softening associated with the superconducting transition. We will show that such a temperature dependence can arise due to the destruction of Fermi liquid quasiparticles that occur above the coherence temperature, T*. 5,10,11 Anomalies in acoustic phonons in the heavy fermion UPt 3 have also been seen at temperatures of the order of T*. 12The simplest possible strongly correlated electron model for the -(BEDT-TTF) 2 X family is a single-band Hubbard model on an anisotropic triangular lattice at half-filling. 2,4We have found 5 that many of the transport properties of the metallic phase are consistent with the predictions of dynamical mean-field theory ͑DMFT͒, 10 which captures exactly the dynamical fluctuations at each lattice site, but neglects all nonlocal spatial correlations. In order to understand the effect of electronic correlations on the molecular phonon modes, we have considered the relevant Holstein-Hubbard model where the phonon amplitude couples linearly to the local charge density. The phonon self-energy is proportional to the electronic density-density correlation function, which we calculate using DMFT. When the electron-electron...

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High-frequency ultrasonic measurements of κ-(BEDT-TTF)2Cu(NCS)2 by devised transducer

Cited by 12 publications

References 9 publications

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

Evidence for structural and electronic instabilities at intermediate temperatures inκ−(BEDT−TTF)2Xfor
Müller
¹
,
Lang
²
,
Steglich
³

et al. 2002
Phys. Rev. B

Phonon anomalies due to strong electronic correlations in layered organic metals

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High-frequency ultrasonic measurements of κ-(BEDT-TTF)2Cu(NCS)2 by devised transducer

Cited by 12 publications

References 9 publications

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

Evidence for structural and electronic instabilities at intermediate temperatures inκ−(BEDT−TTF)2XforMüller1, Lang2, Steglich3 et al. 2002Phys. Rev. B

Phonon anomalies due to strong electronic correlations in layered organic metals

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Product

Resources

About

Evidence for structural and electronic instabilities at intermediate temperatures inκ−(BEDT−TTF)2Xfor
Müller
¹
,
Lang
²
,
Steglich
³

et al. 2002
Phys. Rev. B