Low-Temperature Dielectric Anomalies at the Mott Insulator-Metal Transition
A. Pustogow, R. Rösslhuber, Y. Tan, E. Uykur, M. Wenzel, A. Böhme, A. Löhle, R. Hübner, Y. Saito, A. Kawamoto, J. A. Schlueter, V. Dobrosavljević, M. Dressel
The correlation-driven Mott transition is commonly characterized by a drop in resistivity across the insulator-metal phase boundary; yet, the complex permittivity provides a deeper insight into the microscopic nature. We investigate the frequency- and temperature-dependent dielectric response of the Mott insulator $κ$-(BEDT-TTF)$_{2}$-Cu$_2$(CN)$_3$ when tuning from a quantum spin liquid into the Fermi-liquid state by applying external pressure and chemical substitution of the donor molecules. At low temperatures the coexistence region at the first-order transition leads to a strong enhancement of the quasi-static dielectric constant $ε_1$ when the effective correlations are tuned through the critical value. Several dynamical regimes are identified around the Mott point and vividly mapped through pronounced permittivity crossovers. All experimental trends are captured by dynamical mean-field theory of the single-band Hubbard model supplemented by percolation theory.
Coupling of charge and lattice degrees of freedom in materials can produce intriguing electronic phenomena, such as conventional superconductivity where the electrons are mediated by lattice for creating supercurrent. The Mott transition, which is a source for many fascinating emergent behaviors, is originally thought to be driven solely by correlated electrons with an Ising criticality. Recent studies on the known Mott systems have shown that the lattice degree of freedom is also at play, giving rise to either Landau or unconventional criticality. However, the underlying coupling mechanism of…
Anomalous dielectric response in the dimer Mott insulator $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$
我们测量和分析了二聚体莫特绝缘体$κ-$（BEDT-TTF）$_2$ Cu $_2$（CN）$_3$的介电常数，它被称为自旋液体状态的操场。最出乎意料的是，这种特定的有机盐具有非平凡的电荷自由度，其特征在于低于约60 K的偶极玻璃样介电弛豫。这归因于二聚体内的电荷分离，由于位间库仑排斥。在这方面，这里观察到的偶极子是纯粹的量子力学，从而表现出意想不到的响应。
We report a study of the Mott insulator La$_2$O$_3$Fe$_2$Se$_2$ by means of $^{139}$La nuclear quadrupole resonance (NQR). The NQR spectra evidence a single La site in the paramagnetic phase and two inequivalent La sites, La1 and La2, in the antiferromagnetically-ordered phase (Néel temperature $T_{\rm{N}} \simeq 90$ K). These two sites are characterized by different quadrupole couplings, indicative of distinct lattice and/or charge configurations likely segregated in domains. The dependence of the quadrupole coupling for La2 on temperature suggests that the magnetic order parameter drives the…
Low temperature dielectric anomalies in HoMnO_3: The complex phase diagram
在低温（1.8 K< T <10 K）和外加磁场作用下，多铁性六方HoMnO_3的介电常数呈现出前所未有的异常多样性，这与Ho磁矩、Mn自旋和铁电极化耦合系统中的磁相变有关.推导出的相图是复杂得多，比以前假设的，包括重入相，相变与不同的热和场滞后，以及几个多临界点。磁弹性相互作用在磁相变处引入晶格异常。对HoMnO_3的T-H相图的重新评价是必要的。
利用密度矩阵重整化群计算了具有次近邻跳跃t'的半满Hubbard链对外电场的响应，研究了Mott跃迁随相互作用强度的变化.当从绝缘侧接近临界点时，电极化率χ发散。我们证明了表征这种跃迁的相关长度xi与极化涨落成正比，并且chi ~ xi ^[2]。临界行为表明，对于所有的t '，无论是否存在自旋能隙，跃迁都是无限阶的，并且超标度成立。
The dimer Mott insulator $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ can be tuned into a metallic and superconducting state upon applying pressure of 1.5 kbar and more. We have performed dielectric spectroscopy measurements (7 kHz to 5 MHz) on $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ single crystals as a function of temperature (down to $T=8$ K) and pressure (up to $p=4$ kbar). At ambient conditions, a relaxor-like dielectric behavior develops below 50 K that shifts toward lower temperatures as the crystal is pressurized. Interestingly, a second peak emerges in $\varepsilon_{1}(T)$ around $T=15$ K, which becomes…
Resistive switching induced by electronic avalanche breakdown in GaTa$_4$Se$_{8-x}$Te$_x$ narrow gap Mott Insulators
由强电场引起的Mott跃迁正受到越来越多的关注。最近的理论建议集中在莫特绝缘体的齐纳介质击穿，但实验研究仍然太少，得出结论的机制。在这里，我们报告的研究，在窄间隙莫特绝缘体GaTa $_4$ Se $_{8-x}$ Te $_x$的介电击穿。我们发现，I-V特性和阈值电场（E $_{th}$）的大小不对应于齐纳击穿，而是雪崩击穿。E $_{th}$随Mott Hubbard能隙（E $_g$）的幂律增加，与半导体雪崩击穿的普遍规律E $_{th}$ $\propto$ E $_g$ $^{2.5}$惊人地一致。然而，我们在莫特绝缘体中观察到的雪崩延迟时间比传统半导体长三个数量级以上。我们的研究结果表明，电场诱导局部绝缘体到金属莫特转变，产生导电域，其生长以形成贯穿样品的导电路径。
By combining single crystal x-ray and neutron diffraction, and the magnetodielectric measurements on single crystal Fe4Nb2O9, we present the magnetic structure and the symmetry-allowed magnetoelectric coupling in Fe4Nb2O9. It undergoes an antiferromagnetic transition at TN=93 K, followed by a displacive transition at TS=70 K. The temperature-dependent dielectric constant of Fe4Nb2O9 is strongly anisotropic with the first anomaly at 93 K due to the exchange striction as a result of the long range spin order, and the second one at 70 K emanating from the structural phase transition primarily dri…
We report the magnetic, heat-capacity, dielectric and magnetodielectric (MDE) behaviour of a Haldane spin-chain compound containing light rare-earth ion, Nd2BaNiO5, in detail, as a function of temperature (T) and magnetic field (H) down to 2 K. In addition to the well-known long range antiferromagnetic order setting in at (T_N=) 48 K as indicated in dc magnetization (M), we have observed another magnetic transition near 10 K; this transition appears to be of a glassy-type which vanishes with a marginal application of external magnetic field (even H= 100 Oe). There are corresponding anomalies i…
The dielectric behavior of the single crystals of the spin-chain system Ca3Co2O6, undergoing geometrically frustrated antiferromagnetic ordering below 25 K, has been investigated as a function of temperature and magnetic field (H) and compared with magnetization (M) behavior. The results provide evidence for anisotropic magnetodielectric (MDE) coupling in this compound. Ac susceptibility exhibits a strong frequency dependence for H parallel c with changes of this feature with the application of external dc magnetic field. No feature in ac susceptibility could be observed for H perpendicular c,…
The electronic properties of molecular conductors can be readily varied via physical or chemical pressure as it increases the bandwidth W; this enables crossing the Mott insulator-to-metal phase transition by reducing electronic correlations U/W. Here we introduce an alternative path by increasing the molecular orbitals when partially replacing sulfur by selenium in the constituting bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) molecules of the title compound. We characterize the tuning of the insulating quantum spin liquid state via a Mott transition to the metallic Fermi-liquid state by…
Dirac and Weyl semimetals with linearly crossing bands are the focus of much recent interest in condensed matter physics. Although they host fascinating phenomena, their physics can be understood in terms of weakly interacting electrons. In contrast, more than 40 years ago, Abrikosov pointed out that quadratic band touchings are generically strongly interacting. We have performed terahertz spectroscopy on films of the conducting pyrochlore Pr$_2$Ir$_2$O$_7$, which has been shown to host a quadratic band touching. A dielectric constant as large as $\tilde{\varepsilon }/ε_0 \sim 180 $ is observe…
Using non-perturbative theoretical method, we address the problem of strong correlations in twisted bilayer-layer graphene at the magic angle. We concentrate on the solution without symmetry breaking, where conventional Mott insulating state is expected for all integer fillings. At Coulomb repulsion corresponding to dielectric constant $\varepsilon\approx 5$ and several integer fillings we find a Mott-semiconducting state, which simultaneously hosts the Mott state, and inside the Mott gap, a second much smaller semiconduting gap. The presence of these Mott-ingap states, which are located at th…
Mott-Hubbard and Hund electron correlations have been realized thus far in separate classes of materials. Here, we show that a single moiré homobilayer encompasses both kinds of physics in a controllable manner. We develop a microscopic multiband model that we solve by dynamical mean-field theory to nonperturbatively address the local many-body correlations. We demonstrate how tuning with twist angle, dielectric screening, and hole density allows us to switch between Mott-Hubbard and Hund correlated states in a twisted WSe$_2$ bilayer. The underlying mechanism is based on controlling Coulomb-i…
We describe the critical behavior of electric field-driven (dynamic) Mott insulator-to-metal transitions in dissipative Fermi and Bose systems in terms of non-Hermitian Hamiltonians invariant under simultaneous parity (P) and time-reversal (T) operations. The dynamic Mott transition is identified as a PT symmetry-breaking phase transition, with the Mott insulating state corresponding to the regime of unbroken PT symmetry with a real energy spectrum. We establish that the imaginary part of the Hamiltonian arises from the combined effects of the driving field and inherent dissipation. We derive …
We consider Mott insulators driven by periodic coherent laser radiation, using both single orbital and multi-orbital models, noting that the latter is of more interest in solid state systems. We derive general expressions for the resulting periodically driven spin models and spin-orbital models using time-dependent perturbation theory. First, we show that the effective exchange interactions of the Floquet Hamiltonians are highly tunable by the frequency, amplitude, and polarization of the laser. Second, we take the effect of finite bandwidth of excitations into account and study possible heati…
In this paper we critically discuss several examples of two-dimensional electronic systems displaying interaction-driven metal-insulator transitions of the Mott (or Wigner--Mott) type, including dilute two-dimension electron gases (2DEG) in semiconductors, Mott organic materials, as well as the recently discovered transition-metal dichalcogenide (TMD) moiré bilayers. Remarkably similar behavior is found in all these systems, which is starting to paint a robust picture of Mott criticality. Most notable, on the metallic side a resistivity maximum is observed whose temperature scale vanishes at t…
Recent experiments on cuprates show that as a function of doping, the normal-state specific heat sharply peaks at the doping $δ^*$, where the pseudogap ends at low temperature. This finding is taken as the thermodynamic signature of a quantum critical point, whose nature has not yet been identified. Here we present calculations for the two-dimensional Hubbard model in the doped Mott insulator regime, which indicate that the specific heat anomaly can arise from the finite temperature critical endpoint of a first-order transition between a pseudogap phase with dominant singlet correlations and a…
Raman scattering experiments on stoichiometric, Mott-insulating LaTiO$_3$ over a wide range of excitation energies reveal a broad electronic continuum which is featureless in the paramagnetic state, but develops a gap of $\sim 800$ cm$^{-1}$ upon cooling below the Néel temperature $T_N = 146$ K. In the antiferromagnetic state, the spectral weight below the gap is transferred to well-defined spectral features due to spin and orbital excitations. Low-energy phonons exhibit pronounced Fano anomalies indicative of strong interaction with the electron system for $T > T_N$, but become sharp and symm…
$α$-RuCl$_3$ is a Mott insulator with a honeycomb lattice with strong spin-orbit coupling. We report dielectric measurements on $α$-RuCl$_3$ single crystals under field. At zero field, the dielectric constant, $ε$, drops rapidly when cooled through the magnetic transition temperature T$_N$. With increasing field, the onset of the drop in $ε$ tracks the T$_N$. Such behavior is absent with field above a critical value H$_c$ ~ 7.5 T, indicating the onset of a quantum phase transition. Our data suggest that the dielectric constant can be used as a probe of magnetic ordering in $α$-RuCl$_3$, and $α…
Whether or not anomalies in the thermal conductivity in insulating cuprates can be attributed to antiferromagnetic order and magnons in a 2D Mott insulator remains an intriguing open question. To shed light on this issue, we investigate the thermal conductivity $κ$ and its relationship with the specific heat $c_v$ in the half-filled 2D single-band Hubbard model, using the numerically exact determinant quantum Monte Carlo algorithm and maximum entropy analytic continuation. At low temperatures where the charge degrees of freedom are gapped-out and $c_v$ exhibits a clear magnon peak, we observe …
We explore the coexistence region in the vicinity of the Mott critical end point employing a compressible cell spin-$1/2$ Ising-like model. We analyze the case for the spin-liquid candidate $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, where close to the Mott critical end point metallic puddles coexist with an insulating ferroelectric phase. Our results are fourfold: $i$) a universal divergent-like behavior of the Grüneisen parameter upon crossing the first-order transition line; $ii$) based on scaling arguments, we show that within the coexistence region, for $any$ system close to the critical point, the…
We investigate the strong-field dynamics of a paramagnetic two-band Mott insulator using real-time dynamical mean-field theory. A dielectric breakdown occurs due to many-body Landau-Zener tunnelling, with a threshold field determined by the gap. For a large range of fields, however, we predict that the tunnelling currents are small enough to allow the observation of field-induced localization of electrons, which becomes most strikingly evident in atomic-like local spin multiplets determined by the Hund's coupling $J$. This field-induced localization might provide a way of measuring the value o…
