Charge ordering with structural distortion in quasi-two-dimensional organic conductors θ-(ET)2RbZn(SCN)4 (ET=BEDT-TTF) and α-(ET)2I3 is investigated theoretically. By using the Hartree-Fock approximation for an extended Hubbard model which includes both on-site and intersite Coulomb interactions together with Peierls-type electron-lattice couplings, we examine the role of lattice degrees of freedom on charge order. It is found that the experimentally observed, horizontal charge order is stabilized by lattice distortion in both compounds. In particular, the lattice effect is crucial to the realization of the charge order in θ-(ET)2RbZn(SCN)4, while the peculiar band structure whose symmetry is lower than that of θ-(ET)2RbZn(SCN)4 in the metallic phase is also an important factor in α-(ET)2I3 together with the lattice distortion. For α-(ET)2I3, we obtain a phase transition from a charge-disproportionated metallic phase to the horizontal charge order with lattice modulations, which is consistent with the latest X-ray experimental result.
We performed polarized far-infrared reflectance measurements at different temperatures on $θ$-ET$_2$RbZn(SCN)$_4$ and $θ$-ET$_2$CsZn(SCN)$_4$, two typical charge-transfer salts in a series of isostructural $θ$-ET$_2$MM'(SCN)$_4$. The measurements revealed distinct differences in both electronic and vibronic spectra, which provides a clue towards understanding the different metal-insulator transitions of the two salts.
The temperature-dependent infrared properties of $θ$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ were investigated. In the temperatures above the metal-insulator (M-I) transition, the optical conductivity remains finite in the low energy region, but shows no conventional Drude feature typical of metals. Below the M-I transition temperature, the low-energy spectral weight is significantly reduced. An opening of optical gap at about 300 cm$^{-1}$ is observed clearly in the polarization parallel to the donor-stacking direction. Analysis of both the electronic and vibronic spectra suggests a redistribution of cha…
We report that the sample miniaturization of first-order-phase-transition bulk systems causes a greater degree of supercooling. From a theoretical perspective, the size effects can be rationalized by considering two mechanisms: (i) the nucleation is a rare and stochastic event, and thus, its rate is correlated with the volume and/or surface area of a given sample; (ii) when the sample size decreases, the dominant heterogeneous nucleation sites that play a primary role for relatively large samples are annealed out. We experimentally verified the size effects on the supercooling phenomena for tw…
Charge ordering phenomenon in $θ$-(BEDT-TTF)$_2$X is studied by using 1/4-filled extended Hubbard model on an anisotropic triangular lattice through mean-field approximation. It is found that a metallic charge ordered state with 3-periodicity on lattices (3-fold type charge order) is realized in the realistic parameter region where the nearest neighbor Coulomb interaction V$_{ij}$ is nearly isotropic. This 3-fold state survives up to high temperatures by estimating the free energy. Insulating state with diagonal or vertical-type charge ordering appears as increasing anisotropy of V$_{ij}$. Con…
To elucidate different photoinduced melting dynamics of charge orders observed in quasi-two-dimensional organic conductors $ θ$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ and $ α$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene], we theoretically study photoinduced time evolution of charge and spin correlation functions on the basis of exact many-electron wave functions coupled with classical phonons in extended Peierls-Hubbard models on anisotropic triangular lattices. In both salts, the so-called horizontal-stripe charge order is stabilized by nearest-neighbor repulsive interactions and…
We present systematic magnetic torque studies of the ``magnetic field - temperature'' phase diagram of the layered organic conductor alpha-(BEDT-TTF)2KHg(SCN)4 at fields nearly perpendicular and nearly parallel to the highly conducting plane. The shape of the phase diagram is compared to that predicted for a charge-density-wave system in a broad field range.
The low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 salt is known for its surprising angular dependent magnetoresistance (ADMR), which has been studied intensively in the last decade. However, the nature of the LTP has not been understood until now. Here we analyse theoretically ADMR in unconventional (or nodal) charge density wave (UCDW). In magnetic field the quasiparticle spectrum in UCDW is quantized, which gives rise to spectacular ADMR. The present model accounts for many striking features of ADMR data in alpha-(BEDT-TTF)_2KHg(SCN)_4.
The organic charge-transfer salt $κ$-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Br is a quasi two-dimensional metal with a half-filled conduction band at ambient conditions. When cooled below $T=80$ K it undergoes a pronounced transition to an insulating phase where the resistivity increases many orders of magnitude. In order to elucidate the nature of this metal-insulator transition we have performed comprehensive transport, dielectric and optical investigations. The findings are compared with other dimerized $κ$-(BEDT-TTF) salts, in particular the Cl-analogue, where a charge-order transition takes place a…
We explore the nature of the metal-insulator transition in $β^{\prime\prime}$-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Cl by x-ray, electrical transport, ESR, Raman and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction $a$, setting in already at the crossover temperature of 150K from the metallic to the insulating state. A spin gap $Δ_σ= 47$meV is extracted. From the activated resistivity behavior below 55K a charge gap $Δ_ρ=60$meV is derived. At $T_{\rm CO} = 72$K the C=C vibrational modes reveal the development of a cha…
We have used a low-frequency magneto-thermopower (MTEP) method to probe the high magnetic field ground state behavior of $α$-(BEDT-TTF)$_2$KHg(SCN)$_4$ along all three principal crystallographic axes at low temperatures. The thermopower tensor coefficients ($S_{xx}, S_{yx}$ and $S_{zz}$) have been measured to 30 T, beyond the anomalous low temperature, field-induced transition at 22.5 T. We find a significant anisotropy in the MTEP signal, and also observe large quantum oscillations associated with the de Haas - van Alphen effect. The anisotropy indicates that the ground state properties are c…
To get insight into the nature of the electronic fluid in the frustration-driven charge glasses, we investigate in-plane and out-of-plane charge transport for several quasi-triangular-lattice organic systems, $θ$-(BEDT-TTF)$_2$X [X=RbZn(SCN)$_4$, CsZn(SCN)$_4$ and I$_3$]. These compounds host a charge order, charge glass and Fermi liquid, depending on the strength of charge frustration. We find that the resistivity exhibits extremely two-dimensional (2D) anisotropy and contrasting temperature dependence between in the in-plane and out-of-plane directions in the charge glass phase, qualitativel…
$^{13}$C NMR observation of a nonmagnetic charge-ordered state in the organic conductor $κ$-(ET)$_{2}$Hg(SCN)$_{2}$Cl
University of Tokyo（东京大学）；The Institute for Solid State Physics, University of Tokyo（东京大学固体物理研究所）；Institute of Problems of Chemical Physics RAS（俄罗斯科学院化学物理问题研究所）；The Institute for Quantum Matter and the Department of Physics and Astronomy, The Johns Hopkins University（约翰斯·霍普金斯大学量子物质研究所及物理与天文学系）；Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；University of Stuttgart（斯图加特大学）；Department of Advanced Materials Science, University of Tokyo（东京大学先进材料科学系）
The dielectric constant and ac conductivity have been measured for the layered organic conductor theta-(BEDT-TTF)_2CsZn(SCN)_4 along the out-of-plane direction, which show a relaxation behavior similar to those in the charge-density-wave conductor. Most unexpectedly, they exhibit a large bias dependence with a hysteresis, and changes in magnitude by 100-1000 times at a threshold. These findings are very similar to the collective excitation of the charge density wave. theta-$(BEDT-TTF)_2CsZn(SCN)_4 has collective excitations associated with charge ordering, though it shows no clear indication o…
We report detailed angle-dependent studies of the microwave (f=50 to 90 GHz) interlayer magneto-electrodynamics of a single crystal sample of the organic charge-density-wave (CDW) conductor alpha-(BEDT-TTF)2KHg(SCN)4. Recently developed instrumentation enables both magnetic field (B) sweeps for a fixed sample orientation and, for the first time, angle sweeps at fixed f/B. We observe series' of resonant absorptions which we attribute to periodic orbit resonances (POR) - a phenomenon closely related to cyclotron resonance. The angle dependence of the POR indicate that they are associated with th…
Photoinduced melting of horizontal-stripe charge orders in quasi-two-dimensional organic conductors θ-(BEDT-TTF)2RbZn(SCN)4[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] and α-(BEDT-TTF)2I3 is investigated theoretically. By numerically solving the time-dependent Schrödinger equation, we study the photoinduced dynamics in extended Peierls-Hubbard models on anisotropic triangular lattices within the Hartree-Fock approximation. The melting of the charge order needs more energy for θ-(BEDT-TTF)2RbZn(SCN)4 than for α-(BEDT-TTF)2I3, which is a consequence of the larger stabilization energy in θ-(B…
While the optical properties of the superconducting salt $α$-(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 charg…
The polarized reflectivity of $β^{\prime\prime}$-(BEDO-TTF)$_5$[CsHg(SCN)$_4$]$_2$ is studied in the infrared range between 60 cm-1 and 6000 cm-1 from room temperature down to 10 K. Already at T=300 K a pseudogap in the optical conductivity is present of about 300 cm-1; the corresponding maximum in the spectrum shifts to lower frequencies as the temperature decreases. In contrast to quarter-filled BEDT-TTF-based conductors of the $β^{\prime\prime}$-phase a robust Drude component in the conductivity spectra is observed which we ascribe to the larger fraction of charge carriers associated with t…
Successive magnetic-field-induced charge-density-wave transitions in the molecular conductor $α$-(BEDT-TTF)$_2$KHg(SCN)$_4$ are studied in the hydrostatic pressure regime, in which the zero field charge-density wave (CDW)state is completely suppressed. The orbital effect of the magnetic field is demonstrated to restore the density wave, while the orbital quantization induces different CDW states in different field intervals. In particular, we have found that at certain field orientations the hysteretic first order transitions between field-induced CDW subphases become visible at much higher te…
The critical cooling rate $R_{\rm c}$ above which charge ordering is kinetically avoided upon cooling, which results in charge-glass formation, was investigated for the geometrically frustrated system $θ$-(BEDT-TTF)$_2X$. X-ray diffraction experiments revealed that $θ$-(BEDT-TTF)$_2$TlCo(SCN)$_4$ exhibits a horizontally charge-ordered state, and kinetic avoidance of this state requires rapid cooling of faster than 150 K/min. This value is markedly higher than that reported for two other isostructural $θ$-type compounds, thus demonstrating the lower charge-glass-forming ability of $X$ $=$ TlCo(…
We present an X-ray diffraction study of the quasi-2D conductors a-(BEDTTTF)2MHg(SCN), with M=K and Rb. They exhibit a phase transition of the density wave type at TDW=8-10K and 12-13K respectively, evidenced by magnetoresistivity, specific heat, NMR and Hall constant measurements. The structural study shows the presence of satellite reflections already at ambient temperature. The related modulation is incommensurate with multiple harmonics. For some of the compounds studied, the intensity of the satellite reflections strongly increases below TDW. According to Fermi surface (FS) calculations, …
We present high-resolution measurements of the relative length change as a function of temperature of the organic charge-transfer salt $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl. We identify anomalous features at $T_g \approx\,63$ K which can be assigned to a kinetic glass-like ordering transition. By determining the activation energy $E_A$, this glass-like transition can be related to conformational degrees of freedom of the ethylene endgroups of the organic building block BEDT-TTF. As opposed to other $κ$-(BEDT-TTF)$_2X$ salts, we identify a peculiar ethylene endgroup ordering in the present material i…
The infrared spectra of the quasi-two-dimensional organic conductors $α$-(BEDT-TTF)$_2$$M$Hg(SCN)$_4$ ($M$ = NH$_4$, Rb, Tl) were measured in the range from 50 to 7000 \cm down to low temperatures in order to explore the influence of electronic correlations in quarter-filled metals. The interpretation of electronic spectra was confirmed by measurements of pressure dependant reflectance of $α$-(BEDT-TTF)$_2$KHg(SCN)$_4$ at T=300 K. The signatures of charge order fluctuations become more pronounced when going from the NH$_4$ salt to Rb and further to Tl compounds. On reducing the temperature, th…
The interlayer magnetoresistance of the low-dimensional organic metal α-(BEDT-TTF)2KHg(SCN)4 under pressure shows features which are likely associated with theoretically predicted field-induced charge-density-wave (FICDW) transitions. At ambient pressure, a magnetic field strongly tilted towards the conducting layers induces a series of hysteretic anomalies. We attribute these anomalies to a novel kind of FICDW originating from a superposition of the orbital quantization of the nesting vector and Pauli effect on the charge-density wave.
The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF)$_2$X and the spin density wave phase in (TMTSF)$_2$X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same sp…
The nature of the low-temperature phase of the quasi-two-dimensional conductors $α-(BEDT-TTF)_2{M}Hg(SCN)_4$[{M}=K,Rb,Tl] is considered. It is argued that the magnetic field dependence of the phase diagram is more consistent with a charge-density wave, rather than a spin-density wave ground state. The phase diagram of a charge-density wave in a magnetic field is discussed using a Ginzburg-Landau free energy derived from microscopic theory. New experiments are proposed to test the charge-density-wave hypothesis and detect an additional high field phase.
We present a systematic investigation of the low-frequency charge carrier dynamics in different charge states of the organic conductors $θ$-(BEDT-TTF)$_2$$M$Zn(SCN)$_4$ with $M$=Rb,Tl, which result from quenching or relaxing the charge degrees of freedom on a geometrically frustrated triangular lattice. Due to strong electronic correlations these materials exhibit a charge-ordering transition, which can be kinetically avoided by rapid cooling resulting in a so-called charge-glass state without long-range order. The combination of fluctuation spectroscopy and a heat pulse method allows us to st…
Geometrically frustrated spin systems often do not exhibit long-range ordering, resulting in either quantum-mechanically disordered states, such as quantum spin liquids, or classically disordered states, such as spin ices or spin glasses. Geometric frustration may play a similar role in charge ordering, potentially leading to unconventional electronic states without long-range order; however, there are no previous experimental demonstrations of this phenomenon. Here, we show that a charge-cluster glass evolves upon cooling in the absence of long-range charge ordering for an organic conductor w…
By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, we provide evidence for order-disorder type electronic ferroelectricity which is driven by charge order within the (BEDT-TTF)$_2$ dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and non- or …
We suggest a theory of the Field-Induced Charge-Density-Wave (FICDW) phases, generated by high magnetic fields in quasi-low-dimensional conductors. We demonstrate that, in layered quasi-one-dimensional conductors, the corresponding critical magnetic fields ratios are universal and do not depend on any fitting parameter. In particular, we find that $H_1/H_0 = 0.73, \ H_2/H_0 = 0.59, H_3/H_0 = 0.49, H_4/H_0 = 0.42$, where $H_n$ is a critical field of a phase transition between the FICDW phases with numbers $n$ and $n+1$. The suggested theory is in very good qualitative and quantitative agreement…
We present a combined study of thermal expansion and resistance fluctuation spectroscopy measurements exploring the static and dynamic aspects of the charge-glass formation in the quasi-two-dimensional organic conductors $θ$-(BEDT-TTF)$_2$$MM^\prime$(SCN)$_4$ with $M$ = Cs and $M^\prime$ = Co,Zn. In these materials, the emergence of a novel charge-glass state so far has been interpreted in purely electronic terms by considering the strong frustration of the Coulomb interactions on a triangular lattice. Contrary to this view, we provide comprehensive evidence for the involvement of a \textit{st…
Unconventional density waves (UDW) are one of the ground states in metallic crystalline solids and have been speculated already in 1968. However, more focused studies on UDW started only recently, perhaps after the identification of the low temperature phase in alpha-(BEDT-TTF)_2KHg(SCN)_4 as unconventional charge density wave (UCDW) in 2002. More recently, the metallic phase of Bechgaard salts (TMTSF)_2X with X=PF_6 and ReO_4 under both pressure and magnetic field appears to be unconventional spin density wave (USDW). The pseudogap regime of high T_c superconductors LSCO, YBCO, Bi2212 and the…
Charge ordering accompanied by lattice distortion in quasi-two dimensional organic conductors θ-(ET)2X (ET=BEDT-TTF) is studied by using an extended Hubbard model with Peierls-type electron-lattice couplings within the Hartree-Fock approximation. It is found that the horizontal-stripe charge-ordered state, which is experimentally observed in θ-(ET)2RbZn(SCN)4, is stabilized by the self-consistently determined lattice distortion. Furthermore, in the presence of the anisotropy in nearest-neighbor Coulomb interactions Vij, the horizontal charge order becomes more stable than any other charge patt…
It is shown that interference effects between velocity and density of states, which occur as electrons move along open orbits in the extended Brillouin zone, result in a change of wave functions dimensionality at Magic Angle (MA) directions of a magnetic field. In a particular, we demonstrate that these 1D -> 2D dimensional crossovers result in the appearance of sharp minima in a resistivity component Rzz, perpendicular to conducting layers, which explains the main qualitative features of MA and Angular Magneto-Resistance Oscillations (AMRO) phenomena observed in low-dimensional conductors (TM…
