The occurrence of antiferromagnetism in kappa-(ET)_2X can be understood within an effective 1/2-filled band with dimers of ET molecules containing one hole each. We argue that while this effective model can describe the presence of antiferromagnetism, a complete description for these materials requires the correct carrier density of one-half per molecule. For dimerized and strongly frustrated 1/4-filled lattices we show that a singlet-paired state coexisting with charge ordering occurs that we have termed the Paired Electron Crystal (PEC). Here we investigate the 1/4-filled model on a dimerized lattice, showing regions where AFM, PEC, and the Wigner-crystal occur. We point out the need to go beyond quantum spin liquid concepts for highly frustrated materials such as kappa-(ET)_2Cu_2(CN)_3 and beta'-EtMe_3Sb[Pd(dmit)_2]_2 which we believe are PECs at low temperatures.
Mott Transition from a Spin Liquid to a Fermi Liquid in the Spin-Frustrated Organic Conductor kappa-(ET)2Cu2(CN)3
用$^1$ H NMR和电阻率测量研究了三角晶格自旋液体模型体系$κ$ -(ET)$_2$ Cu $_2$(CN)$_3$的压温相图.在压力下,自旋液相在莫特转变为金属或超导相之前是持久的。在Mott转变时,自旋涨落被迅速抑制,自旋-晶格弛豫速率和电阻率随温度的变化具有费米液体特征。相图中Mott边界的特征曲率反映了自旋阻挫对Mott相变的重要影响。
We present a systematic investigation of the low-frequency charge carrier dynamics in different charge states of the organic conductors $θ$-(BEDT-TTF)$_2$$M$Zn(SCN)$_4$ with $M$=Rb,Tl, which result from quenching or relaxing the charge degrees of freedom on a geometrically frustrated triangular lattice. Due to strong electronic correlations these materials exhibit a charge-ordering transition, which can be kinetically avoided by rapid cooling resulting in a so-called charge-glass state without long-range order. The combination of fluctuation spectroscopy and a heat pulse method allows us to st…
Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model
Superconducting Fluctuations and the Reduced Dimensionality of the Organic Superconductor $κ$- (BEDT-TTF)$_{2}$Cu(NCS)$_{2}$ as Observed through Measurements of the de Haas-van Alphen Effect
Magnetic-field-angle-resolved heat capacity of dimer-Mott organic superconductors $κ$-(BEDT-TTF)$_2$$X$ is reported. Temperature and field dependence of heat capacity indicates that the superconductivity has line nodes on the Fermi surface, which is a typical feature of $d$-wave superconductivity. In-plane field-angle dependence exhibits fourfold oscillations as well as twofold oscillations due to anisotropy of superconducting gap functions. From analyses of the fourfold term, the gap symmetry is determined as $d_{x^{2}-y^{2}}$+$s_{\pm}$ for $X$=Cu[N(CN)$_2$]Br and $d_{xy}$ for $X$=Ag(CN)$_2$H…
The electron paramagnetic resonance study for an organic superconductor $β''$-(BEDT-TTF)$_{4}$[(H$_3$O)Ga(C$_2$O$_4$)$_3$]$\cdot$C$_6$H$_5$NO$_2$ reveals that superconductivity coexists uniformly with the charge ordered state in one material. In the charge ordered state, the interplane spin exchange is gapped, while the in-plane conductivity is not significantly modified. This anisotropic behavior is explained by the exotic charge ordered state, in which molecular-site selective carrier localization coexists with conducting carriers on other molecules. Relationship between superconductivity an…
Cyclotron resonance harmonics in the organic superconductor $β^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$; observation of a new kind of effective mass renormalization
在准二维有机超导体$β^{\prime\prime}$ -(BEDT-TTF)$_2$ SF $_5$ CH $_2$ CF $_2$ SO $_3$中观察到了回旋共振及其二次和三次谐波。谐波含量是丰富的角度依赖性,并解释在层间翘曲的费米表面引起的回旋共振。与科恩定理不同,但与最近的理论预测一致,从回旋共振测量中推导出的有效质量大于从磁量子振荡中确定的有效质量。
In this work, we study a biquadratic Heisenberg model with coupled orbital degree of freedom using Monte Carlo simulation in order to investigate the phase transitions in iron-based superconductors. The antiferro-quadrupolar state, which may be related to the magnetism of FeSe [Phys. Rev. Lett. 115, 116401 (2015)], is stabilized by the anisotropic biquadratic interaction induced by a ferro-orbital-ordered state. It is revealed that the orbital and nematic transitions occur at the same temperature for all the cases, supporting the mechanism of the orbital-driven nematicity as revealed in most r…
Intercalation of organic cations in superconducting iron selenide can significantly increase the critical temperature (Tc). We present an electrochemical method using beta-FeSe crystals (Tc ~ 8 K) floating on a mercury cathode to intercalate tetramethylammonium ions (TMA+) quantitatively to obtain bulk samples of (TMA)0.5Fe2Se2 with Tc = 43 K. The layered crystal structure is closely related to the ThCr2Si2-type with disordered TMA+ ions between the FeSe layers. Although the organic ions are not detectable by X-ray diffraction, packing requirements as well as first principle DFT calculations c…
Upper critical field study in the organic superconductor $β''$-(ET)$_{2}$SF$_{5}$CH$_{2}$CF$_{2}$SO$_{3}$ : Possibility of Fulde-Ferrell-Larkin-Ovchinnikov state
本文报道了无金属全有机超导体$β''$ -(ET)$_{2}$ SF $_{5}$ CH $_{2}$ CF $_{2}$ SO $_{3}$的上临界场测量结果。当磁场平行于导体平面时,低温上临界场超过了用半经验方法计算的泡利极限场。此外,我们发现了一个信号,可能是超导涡旋态和Fulde-Ferrell-Larkin-Ovchinnikov态之间的相变,其形式为刚好低于上临界场的扭结,并且仅在低于1.23 K的温度下。
The symmetry of the superconducting order parameter in quasi-two-dimensional BEDT-TTF organic superconductors is a subject of ongoing debate. We report ab initio density functional theory calculations for a number of organic superconductors containing $κ$-type layers. Using projective Wannier functions we derive parameters of a common low-energy Hamiltonian based on individual BEDT-TTF molecular orbitals. In a random phase approximation spin-fluctuation approach we investigate the evolution of the superconducting pairing symmetry within this model and point out a phase-transition between exten…
Shubnikov-de Haas effect and Yamaji oscillations in the antiferromagnetically ordered organic superconductor k-(BETS)2FeBr4: A Fermiology study
在有机超导体$κ$ -(BETS)$_2$ FeBr $_4$中观察到了Shubnikov-De Haas效应(SdH)和角相关磁电阻振荡(AMRO)。与其同构化合物$κ-(BETS)$ _2 $FeCl$ _4 $, SdH oscillations, for fields perpendicular to the conducting planes, reveal three Fermi Surface (FS) closed orbits $ α $, $ β $, and $ γ $ whose cross sectional areas are 19.8 %, 99.9 %, and 2.4 % of the first Brillouin zone, respectively. The conduction electron effective masses were found to be: $ μ_α =(4.7 \pm 0.2)$ m$ _e $, $ μ_β =(8.0 \pm 1.0)$ m$ _e $, and $ μ_γ =(2.0 \pm 0.2)$ m$ _e $. The observation of a $ γ $ orbit is not expected from band structure calculations, $ μ_β $ is …
Towards a consistent picture for quasi-1D organic superconductors
最近在低温下测量了准一维有机超导体(TMTSF)2 PF 6的电阻率,从抑制自旋密度波态所需的临界压力到超导性几乎消失的压力。该数据揭示了在Tc处超导性的开始与正常状态电阻率中的非费米液体线性项的强度之间的直接相关性,在低温下为r(T)= r 0 + AT + BT 2,因此当Tc变为0时,A变为0。在这里,我们表明,低频反铁磁波动的自旋晶格弛豫速率的贡献也与此非费米液体项AT的电阻率。这些相关性表明,异常散射和配对有一个共同的起源,都植根于低频反铁磁波动测量核磁共振。类似的情况也可能在最近发现的铁-磷属元素化物超导体中普遍存在。
The mutual influence of the conduction electron system provided by organic donor layers and magnetic system localized in insulating layers of the molecular charge transfer salt $κ$-(BETS)$_2$FeBr$_4$ has been studied. It is demonstrated that besides the high-field re-entrant superconducting state, the interaction between the two systems plays important role for the low-field superconductivity. The coupling of normal-state charge carriers to the magnetic system is reflected in magnetic quantum oscillations and can be evaluated based on the angle-dependent beating behaviour of the oscillations. …