Bad Metal Research Articles

Hints for the metallic phase in Rb4C60 under pressure

The alkali fulleride Rb4C60 was investigated by infrared spectroscopy under high pressure, to study the vibrational and electronic excitations. The material exhibits a bad metal behavior in the measured pressure range (0.8–8 GPa). With increasing pressure the Drude term in the far‐infrared region increases, which signals an increasing metallic character. By following the optical excitations under pressure, we could attribute the underlying mechanism of the pressure‐induced insulator‐to‐metal transition in Rb4C60 to the closing of the charge gap, and the nanoscale phase separation scenario discussed in the literature seems to be unlikely.

Bad metal behaviour in the new Hg-rich amalgam [formula omitted] with polar metallic bonding

The new mercury-rich amalgam KHg6 crystallises with the BaHg6 structure type (orthorhombic, space group Pnma (No. 62), a=13.394(9)Å, b=5.270(3)Å, c=10.463Å). It was prepared by electrolysis of a solution of KI in N,N′-Dimethylformamide at 343K at a reactive Hg cathode. The structure of KHg6 shows motifs of ionic packing, covalent Hg cluster formation and metallic properties. KHg6 decomposes peritectically at 443K. The combination of alkali metals with a noble metal with moderate electron affinity results in the formation of polar metal–metal bonding with considerable but incomplete electron transfer from the electropositive to the electronegative sublattice, resulting in typical “bad metal behaviour”, illustrated by resistance and susceptibility measurements and quantum theoretical calculations.

Conductivity noise study of the insulator-metal transition and phase coexistence in epitaxial samarium nickelate thin films

Interaction between the lattice and the orbital degrees of freedom not only makes rare-earth nickelates unusually "bad metal", but also introduces a temperature driven insulator-metal phase transition. Here we investigate this insulator-metal phase transition in thin films of $\mathrm{SmNiO_3}$ using the slow time dependent fluctuations (noise) in resistivity. The normalized magnitude of noise is found to be extremely large, being nearly eight orders of magnitude higher than thin films of common disordered metallic systems, and indicates electrical conduction via classical percolation in a spatially inhomogeneous medium. The higher order statistics of the fluctuations indicate a strong non-Gaussian component of noise close to the transition, attributing the inhomogeneity to co-existence of the metallic and insulating phases. Our experiment offers a new insight on the impact of lattice-orbital coupling on the microscopic mechanism of electron transport in the rare-earth nickelates.

Pressure suppression of electron correlation in the collapsed tetragonal phase ofCaFe2As2: A DFT-DMFT investigation

Recent studies reveal a pressure induced transition from a paramagnetic tetragonal phase (T) to a collapsed tetragonal phase (CT) in $CaFe_2As_2$, which was found to be superconducting with non-hydrostatic pressure at low temperature. We have investigated the effects of electron correlation and local fluctuating moment in both tetragonal and collapsed tetragonal phases of the paramagnetic $CaFe_2As_2$ using self consistent DFT+DMFT with continuous time quantum Monte Carlo as the impurity solver. From the computed optical conductivity, we find a gain in the optical kinetic energy due to the loss in Hund's rule coupling energy in the CT. We find that the transition from T to CT turns $CaFe_2As_2$ from a bad metal into a good metal. Computed mass enhancement and local moments also show significant decrease in the CT, which confirms the suppression of the electron correlation in the CT phase of $CaFe_2As_2$.

Determination of magnetic form factors for organic charge-transfer salts: A first-principles investigation

Organic charge transfer salts show a variety of complex phases ranging from antiferromagnetic long-range order, spin liquid, bad metal or even superconductivity. A powerful method to investigate magnetism is spin-polarized inelastic neutron scattering. However, such measurements have often been hindered in the past by the small size of available crystals as well as by the fact that the spin in these materials is distributed over molecular rather than atomic orbitals and good estimates for the magnetic form factors are missing. By considering Wannier functions obtained from density functional theory calculations, we derive magnetic form factors for a number of representative organic molecules. Compared to Cu2+, the form factors |F(q)|2 fall off more rapidly as function of q reflecting the fact that the spin density is very extended in real space. Form factors |F(q)|2 for TMTTF, BEDT-TTF and (BEDT-TTF)2 have anisotropic and nonmonotonic structure.

Synthesis particularities, structure and properties of the radical cation salts ω-(BEDT-TTF)5M(SCN)6·C2H5OH, M = Mn, Ni

Electrochemical oxidation of donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) in 1,1,2-trichloroethane-ethanol in the presence of electrolytes including two type of ligands, namely, (Ph4P)2[M(dca)2(SCN)2], M=Mn, Ni; dca=[N(CN)2]−, has been performed and analyzed for the first time. It has been found that in the reaction of electrochemical oxidation of BEDT-TTF, [M(dca)2(SCN)2]2− anion decomposes to form new metal-complex anions, [Mn(dca)3(SCN)2·H2O]3− and [Mn(SCN)6]4− or [Ni(SCN)6]4−, respectively, as well as (dca) and SCN anions. New radical cation salts, (BEDT-TTF)4Mn[(dca)3(SCN)2·H2O](SCN)x, x<1, (I), ω-(BEDT-TTF)5M(SCN)6·C2H5OH, M=Mn (III), Ni (IV), and previously described α″-(BEDT-TTF)2(dca)·2H2O (II) have been synthesized. A solid-phase transformation of crystals I to crystals II have been found. Crystal structure and conducting of the salts, as well as the electronic structure and magnetic properties of III, were analyzed. Salts III and IV have layered structure in which ω-type radical cation layers alternate with insulating anion layers composed of [M(SCN)6]4− anions. Crystals I is a bad metal while two others exhibit a semiconducting behavior of conductivity. No exchange interactions for Mn2+ ions observed in the salt III.

Unveiling the microscopic nature of correlated organic conductors: The case ofκ-(ET)2Cu[N(CN)2]BrxCl1−x

A few organic conductors show a diversity of exciting properties like Mott insulating behavior, spin liquid, antiferromagnetism, bad metal, or unconventional superconductivity controlled by small changes in temperature, pressure, or chemical substitution. While such a behavior can be technologically relevant for functional switches, a full understanding of its microscopic origin is still lacking and poses a challenge in condensed matter physics since these phases may be a manifestation of electronic correlation. Here we determine from first principles the microscopic nature of the electronic phases in the family of organic systems $\ensuremath{\kappa}$-(ET)${}_{2}\mathrm{Cu}$[N(CN)${}_{2}$]Br${}_{x}$Cl${}_{1\ensuremath{-}x}$ by a combination of density functional theory calculations and the dynamical mean field theory approach in a form adapted for organic systems. By computing spectral and optical properties we are able to disentangle the origin of the various optical transitions in these materials and prove that correlations are responsible for relevant features. Remarkably, while some transitions are inherently affected by correlations, others are completely uncorrelated. We discuss the consequences of our findings for the phase diagram in these materials.

Holographic charge transport in 2+1 dimensions at finite N

We study holographic charge transport in (2+1) dimensions at finite N, whose dual gravity background is given by perturbative black hole solution in Einstein theory plus cubic terms of Weyl tensor. We consider the higher derivative corrections to the standard Maxwell action, given by the interacting terms between the Weyl tensor and the field strength. We calculate the DC conductivity by using both the membrane paradigm and the Kubo's formula and find precise agreement. We compute the AC conductivity and find an analog of the crossover from "metal" to "bad metal" in the low frequency limit. Moreover, the conductivity becomes a constant in the large frequency limit. We derive two universal relations for the Green's functions and observe that they are exactly the same as the infinite N counterparts.

Universal thermopower of bad metals

"Bad metals" have a large linear resistivity at high-T that is universally seen in oxides close to the Mott-Hubbard insulating phase. They also have an universal thermopower alpha(T): (i) at very low doping (lightly doped) alpha(T) has a pronounced low-T peak that shifts to higher-T with doping; (ii) at moderate doping (underdoped) alpha(T) has a small low-T peak that shifts to lower-T with doping and has a high-T sign change; and (iii) at the highest doping (overdoped) alpha(T) is negative and depends monotonically on T. Here we show that the simplified Hubbard model provides an easy to understand description of this phenomena due to the universal form for the chemical potential versus T for doped Mott insulators and the applicability of the Kelvin formula for the thermopower.

Origins of bad-metal conductivity and the insulator–metal transition in the rare-earth nickelates

For most metals, increasing temperature (T) or disorder will quicken electron scattering. This hypothesis informs the Drude model of electronic conductivity. However, for so-called bad metals this predicts scattering times so short as to conflict with Heisenberg's uncertainty principle. Here we introduce the rare-earth nickelates (RNiO_3, R = rare earth) as a class of bad metals. We study SmNiO_3 thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni-O bond covalence explains both the bad metal conduction and the insulator-metal transition in the nickelates by shifting spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low \sigma while preserving the Drude model and without violating the uncertainty principle.

Multiorbital effects on thermoelectric properties of strongly correlated materials

The effects of electronic correlations and orbital degeneracy on thermoelectric properties are studied within the context of multi-orbital Hubbard models on different lattices. We use dynamical mean field theory with iterative perturbation theory as a solver to calculate the self-energy of the models in wide range of interaction strengths. The Seebeck coefficient, which measures the voltage drop in response to a temperature gradient across the system, shows a non-monotonic behavior with temperatures in the presence of strong correlations. This anomalous behavior is associated with a crossover from a Fermi liquid metal at low temperatures to a bad metal with incoherent excitations at high temperatures. We find that for interactions comparable to the bandwidth the Seebeck coefficient acquires large values at low temperatures. Moreover, for strongly correlated cases, where the interaction is larger than the band width, the figure of merit is enhanced over a wide range of temperatures because of decreasing electronic contributions to the thermal conductivity. We also find that multi-orbital systems will typically yield larger thermopower compared to single orbital models.

Superconductivity at the border of electron localization and itinerancy

The superconducting state of iron pnictides and chalcogenides exists at the border of anti-ferromagnetic order. Consequently, these materials could provide clues about the relationship between magnetism and unconventional superconductivity. One explanation, motivated by the so-called bad metal behaviour of these materials proposes that magnetism and superconductivity develop out of quasi-localized magnetic moments that are generated by strong electron-electron correlations. Another suggests that these phenomena are the result of weakly interacting electron states that lie on nested Fermi surfaces. Here we address the issue by comparing the newly discovered alkaline iron selenide superconductors, which exhibit no Fermi-surface nesting, to their iron pnictide counterparts. We show that the strong-coupling approach leads to similar pairing amplitudes in these materials, despite their different Fermi surfaces. We also find that the pairing amplitudes are largest at the boundary between electronic localization and itinerancy, suggesting that new superconductors might be found in materials with similar characteristics.

Localized states in the Mott insulatorκ-(BEDT–TTF)2Cu[N(CN)2]Cl as probed by photoluminescence

We compare the photoluminescence spectra of the low-temperature Mott insulator \etcl (T_{MIT}=40 K) with spectra of metallic \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br, which is superconducting below T_c=11.8 K, in the temperature range between 300 and 20 K. In the Mott insulating state of \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl we observe a luminescence band at 1.95 eV due to the recombination of an exciton created by a HOMO-LUMO optical excitation. This luminescence is quenched both in the high-temperature bad metal state of \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl and in metallic \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br. The observation of the luminescence of an exciton provides evidence for the local character of excitations in the Mott insulating state.

Crossover from bad to good metal in BaFe2(As1−xPx)2induced by isovalent P substitution

Bad metallic behavior or incoherent charge dynamics is characteristic of the normal state of high-transition-temperature copper-oxide superconductors. A parent compound of iron-based superconductors, BaFe${}_{2}$As${}_{2}$, is also a bad metal in the high-temperature phase, whereas its phosphorous counterpart, BaFe${}_{2}$P${}_{2}$, is a good metal. Combined measurements of in-plane resistivity and optical conductivity for BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ make it possible to disentangle the coherent channel from the incoherent one in the charge transport. The isovalent P substitution promotes coherent motion of carriers without changing the balance of the electron and hole Fermi-surface volume and hence transforms a bad metal into a good metal.

Finite-temperature crossover and the quantum Widom line near the Mott transition

The experimentally established phase diagram of the half-filled Hubbard model features the existence of three distinct finite-temperature regimes, separated by extended crossover regions. A number of crossover lines can be defined to span those regions, which we explore in quantitative detail within the framework of dynamical mean-field theory. Most significantly, the high-temperature crossover between the bad metal and Mott-insulator regimes displays a number of phenomena marking the gradual development of the Mott insulating state. We discuss the quantum critical scaling behavior found in this regime, and propose methods to facilitate its possible experimental observation. We also introduce the concept of quantum Widom lines and present a detailed discussion that highlights its physical meaning when used in the context of quantum-phase transitions.

Synthesis and thermoelectric properties of Re3As6.6In0.4 with Ir3Ge7 crystal structure

The Re3As7−xInx solid solution was prepared for x ≤ 0.5 by heating the elements in stoichiometric ratios in evacuated silica tubes at 1073 K. It crystallizes with the Ir3Ge7 crystal structure, space group Im−3m, with a unit-cell parameter a ranging from 8.716 to 8.747 Å. The crystal structure and properties were investigated for a composition with x = 0.4. It is shown that indium substitutes arsenic exclusively at one crystallographic site, such that the As–As dumbbells with dAs–As = 2.54 Å remain intact. Re3As6.6In0.4 behaves as a bad metal or heavily doped semiconductor, with electrons being the dominant charge carriers. It possesses high values of Seebeck coefficient and low thermal conductivity, but relatively low electrical conductivity, which leads to rather low values of the thermoelectric figure of merit.

Analytically tractable model of bad metals

We discuss a model Kondo-type Hamiltonian representing an analytically tractable version of the model used by Yin {\it et.al.}, Phys. Rev. B{\bf 86}, 2399 (2012) to explain the non-Fermi liquid behavior of iron chalcogenides and ruthenates in an intermediate energy range. We consider a regime where a complete screening of the local degrees of freedom proceeds in two stages described by two characteristic energy scales $T_K^{orb} >> E_0$. The first scale marks a screening of the orbital degrees of freedom and the second one marks a crossover to the regime with coherent propagation of quasiparticles. We present analytical results for the specific heat and magnetic susceptibility at $T << T_K^{orb}$.

Superconductivity induced by electron doping in La1−xMxOBiS2(M=Ti, Zr, Hf, Th)

We report a strategy to induce superconductivity in the BiS$_2$-based compound LaOBiS$_2$. Instead of substituting F for O, we increase the charge-carrier density (electron dope) via substitution of tetravalent Th$^{+4}$, Hf$^{+4}$, Zr$^{+4}$, and Ti$^{+4}$ for trivalent La$^{+3}$. It is found that both the LaOBiS$_2$ and ThOBiS$_2$ parent compounds are bad metals and that superconductivity is induced by electron doping with \emph{T$_c$} values of up to 2.85 K. The superconducting and normal states were characterized by electrical resistivity, magnetic susceptibility, and heat capacity measurements. We also demonstrate that reducing the charge-carrier density (hole doping) via substitution of divalent Sr$^{+2}$ for La$^{+3}$ does not induce superconductivity.

Thermodynamics of a bad metal-Mott insulator transition in the presence of frustration.

Thermodynamic properties of the Hubbard model on the anisotropic triangular lattice at half filling are calculated by the finite-temperature Lanczos method. The charge susceptibility exhibits clear signatures of a Mott metal-insulator transition. The metallic phase is characterized by a small charge susceptibility, large entropy, large renormalized quasiparticle mass, and large spin susceptibility. The fluctuating local magnetic moment in the metallic phase is large and comparable to that in the insulating phase. These bad metallic characteristics occur above a relatively low coherence temperature, as seen in organic charge transfer salts.

Photoinduced states in a Mott insulator.

We investigate the properties of the metallic state obtained by photodoping carriers into a Mott insulator. In a strongly interacting system, these carriers have a long lifetime, so that they can dissipate their kinetic energy to a phonon bath. In the relaxed state, the scattering rate saturates at a nonzero temperature-independent value, and the momentum-resolved spectral function features broad bands which differ from the well-defined quasiparticle bands of a chemically doped system. Our results indicate that a photodoped Mott insulator behaves as a bad metal, in which strong scattering between doublons and holes inhibits Fermi-liquid behavior down to low temperature.