Fermi Glass Research Articles

Intrinsic instabilities in Fermi glasses

Intrinsic instabilities in Fermi glasses

Bose glass and Fermi glass

It is known that two-dimensional superconducting materials undergo a quantum phase transition from a localized state to superconductivity. When the disordered samples are cooled, bosons (Cooper pairs) are generated from Fermi glass and reach superconductivity through Bose glass. However, there has been no universal expression representing the transition from Fermi glass to Bose glass. Here, we discovered an experimental renormalization group flow from Fermi glass to Bose glass in terms of simple beta-function analysis. To discuss the universality of this flow, we analyzed manifestly different systems, namely a Nd-based two-dimensional layered perovskite and an ultrathin Pb film. We find that all our experimental data for Fermi glass fall beautifully into the conventional self-consistent beta-function. Surprisingly, however, flows perpendicular to the conventional beta-function are observed in the weakly localized regime of both systems, where localization becomes even weaker. Consequently, we propose a universal transition from Bose glass to Fermi glass with the new two-dimensional critical sheet resistance close to R_Box = h/e^{2}.

A fermion phenomenology of low-temperature strongly-noncrystalline solids

Insisting on the relevance of spin-statistics theorem, I propose that anomalous low-energy excitations (LEEs) of strongly-noncrystalline solids (SNSs), observed at low temperatures T<1K, are fermions, which are localized and weakly interacting. This phenomenological theory rationalizes all low-energy quantum many-body states of SNSs as Goldstone bosons-phonons and half-integer-spin fermions. Fermi glass of LEEs, rather than statistical ensemble of isolated two-level systems, appears to be a consistent theory of various thermal and nonlinear response properties of SNSs. A robust consequence of the theory is nearly constant-in-energy density of fermion states near Fermi energy, which in turn implies linear-in-temperature specific heat and dimensionless frequency-independent sound absorption. Consistent parameters of Fermi glass are estimated from theoretical scaling arguments as well as directly from experimental data.

Observation of a marginal Fermi glass

A long-standing open problem in condensed-matter physics is whether or not a strongly disordered interacting insulator can be mapped to a system of effectively non-interacting localized excitations. Using terahertz two-dimensional coherent spectroscopy, we investigate this issue in phosphorus-doped silicon, a classic example of a correlated disordered electron system in three dimensions. Despite the intrinsically disordered nature of these materials, we observe coherent excitations and strong photon echoes that provide us with a powerful method for the study of their decay processes. We extract the energy relaxation and decoherence rates close to the metal–insulator transition. We observe that both rates are linear in excitation frequency with a slope close to unity. The energy relaxation timescale counterintuitively increases with increasing temperature, and the coherence relaxation timescale has little temperature dependence below 25 K, but increases as the material is doped towards the metal–insulator transition. Here we argue that these features imply that the system behaves as a well-isolated electronic system on the timescales of interest, and relaxation is controlled by electron–electron interactions. Our observations constitute a distinct phenomenology, driven by the interplay of strong disorder and strong electron–electron interactions, which we dub the marginal Fermi glass.

Comments on the electronic transport mechanisms in the crystalline state of Ge—Sb—Te phase-change materials

It is known that phase-change materials, such as Ge–Sb—Te ternary system, are promising resistive non-volatile random-access memory applications with ultra-rapid reversible transformations between the crystalline and amorphous phases. This class of electronic transition is categorized to be the metal-insulator transition (MIT). The Anderson-type MIT has been discussed extensively in phase-change materials (PCMs) and isothermal annealing of amorphous PCMs (a-PCMs) which, above a certain temperature, leads to the metallic (crystalline) phase. In the insulator regime near the MI transition, Mott-type variable-range hopping (VRH) and/or Efros-Shkolvskii hopping (ESH) at low temperatures below 20 K (and down to 1 K) in Ge1Sb2Te4 (GST124) have been discussed extensively, however, we criticize the above argument through a detailed discussion of physical parameters that support the VRH mechanism. It is not clear whether or not the density-of-states (DOS) near the Fermi level is localized (like the Fermi glass) in the crystalline phase. It is also suggested that grain boundaries are expected to interfere with the electronic transport in the crystalline state. We should take into account the grain boundary effects on the electronic transport in the crystalline phase of Ge—Sb—Te system.

Influence of hydrostatic pressure on electrical relaxation in non-homogeneous bismuth manganite - Lead titanate ceramics

The composite 0.96 BM–0.04 PT ceramics was sintered from non-polar bismuth manganite (BM) and ferroelectric lead titanate (PT) components. We show influence of small amount of PT on structure ordering of BM. The X-ray pattern analysis showed occurrence of three phases: orthorhombic Pbam related to BiMn2O5, cubic Pn3¯m attributed to δ−Bi2O3, and tetragonal P4¯21c related to unrecognized phase Bi20TiO32 isostructural to β−Bi2O3. Occurrence of these three phases reflected in variation in chemical compositions detected using transmission electron microscopy. Correspondence between nano-structural features of BM-PT and electrical relaxation processes was studied. The DC resistivity temperature dependence exhibited crossover from variable range hopping to nearest-neighbor hopping of small polarons. Furthermore, three electrical relaxations attributed to small polaron hopping were distinguished. Two relaxations corresponded to BM-originated phases. One relaxation exhibited crossover from variable range hopping to nearest-neighbor hopping of small polarons. Second relaxation showed crossover from variable range hopping to Vogel-Fulcher temperature dependence. Third relaxation, related to nearest-neighbor hopping of small polarons, was attributed to Bi20TiO32:Pb phase derived from PT compound. Structural disorder of BM-PT phases reflected in Fermi glass features manifested by variable range hopping of polarons. Moreover, we showed sensitivity of BM-PT ceramics on high hydrostatic pressure. Applied hydrostatic pressure, p = 800 and 1600 MPa, shortened the relaxation times. We propose equation of state for relaxation process related with PT amount doped to BM ceramics. Equation of state of dielectric includes response to varied hydrostatic pressure and temperature, that is to different thermodynamic conditions. Correspondence between effects of applied hydrostatic pressure and presumed chemical pressure was considered as consequence of inter-diffusion of Pb, Bi, Mn, and Ti ions and their mutual substitution.

Hydrostatic pressure influence on electric relaxation response of bismuth manganite ceramics

AbstractElectrical impedance of bismuth manganite ceramics was studied under ambient and high hydrostatic pressure. Local disorder of crystal lattice was confirmed using XRD and DSC. Two relaxation processes were discerned. One was attributed to small polarons, which showed a changeover from variable range hopping features related to Fermi glass and structural disorder to the nearest neighbor hopping behavior in higher temperature range related to uniform distribution of energy levels. Hydrostatic pressure shortened relaxation times that would be interesting for applications. The second process, which was assigned to the nearest neighbor hopping of polarons, also exhibited a changeover to glassy features in a high‐temperature range.

Impedance, dielectric, and magnetic properties study of La2CrMnO6 ceramics

Double perovskite La2CrMnO6 (LCrMO) ceramics was prepared using solid state reaction technique sintered at 1433 K in ambient air. Wide range impedance spectroscopy study conducted in 20 Hz – 1 MHz over 85–440 K range allowed for separation of grain bulk and grain boundary effects. Electrical features corresponded to grain bulk and grain boundary properties were investigated. The features of imaginary part of impedance indicated electrical relaxation phenomenon occurrence in LCrMO. The temperature dependence of most probable relaxation times, τZ, obtained from impedance spectra collected below 120 K were fitted using Arrhenius relation and the activation energy equal to 0.14 eV was estimated. The Kramers-Kronig transform of the dielectric permittivity enabled to distinguish relaxation process over 85–170 K temperature range. The variable range hopping of small polaron model was fitted with better accuracy to the dielectric relaxation times, τdiel, temperature dependence, which indicated occurrence of Fermi glass related to local disorder. AC and DC conductivity relaxation processes, which corresponded to grain bulk and grain boundary contributions, respectively, were discriminated. Magnetic susceptibility of LCrMO ceramics was measured over 5–300 K range. Bifurcation between zero field cooled and field cooled dependence occurred below ~120 K. Magnetization M(H) hysteresis loops were not dependent on temperature and exhibited low values of coercive field, ~108 Oe and remnant magnetization, ~6 emu g−1, over 5–200 K range. The magnetic spin glass features were deduced. We note that coupling between electric and magnetic properties was detected: anomaly around 87 K in magnetization, related to magnetic transition, is accompanied by deviation of dielectric relaxation times discerned from the VRH temperature dependence.

Thermally driven Fermi glass states in warm dense matter: Effects on terahertz and direct-current conductivities

There is a growing interest in the electrical conductivity of warm dense matter from terahertz-frequency alternating current to direct current. Herein, using first-principles molecular dynamics simulations, we show that ionic thermal motion in warm dense matter drives thermal fluctuations in the electronic valence band that produce localized states in Lifshitz tails on the top and bottom of the bands. We predict Fermi glass states when these localized states extend and fill the gap between valence and conduction bands. This significantly affects the ultralow-frequency and direct current conductivity because of the very small but nonzero energy gaps between these localized states. An order parameter is proposed to describe the degree of glassiness of an electron energy band using the local density-of-state distribution. To take into account thermal hopping, we introduce electron energy-level broadening as a thermal correction term in the Kubo–Greenwood equation. The calculated terahertz conductivities of warm dense helium and argon show the differences between the Fermi glass and normal metal states.

Determination of polaronic conductivity in disordered double perovskite La2CrMnO6

Double perovskite La2CrMnO6 ceramics was sintered using standard high temperature route in ambient air. The orthorhombic Pbnm cell characterized by a total disorder between Cr and Mn ions was determined using an XRD powder test. The grain morphology, porosity and chemical composition were determined using scanning electron microscopy. The X-ray photoemission spectroscopy showed several contributions to O 1 s, La 3d, Mn 2p, and Cr 2p core lines related to the occurrence of multiple ionic states. The electrical permittivity, modulus, AC and DC conductivity were measured in ranges of f = 20 Hz – 1 MHz and 76–440 K. The electrical transport mechanism was attributed to the small polarons. The nearest neighbor hopping occurred in the range 170 to 440 K. The variable range hoping, attributed to the Fermi glass features and disorder, was detected in the range 100 to 160 K. The relaxation process related to the temperature-independent activation energy was deduced from the electric modulus scaling.

Correlating the Seebeck coefficient of thermoelectric polymer thin films to their charge transport mechanism

Room temperature flexible heat harvesters based on conducting polymers are ideally suited to cover the energy demands of the modern nomadic society. The optimization of their thermoelectric efficiency is usually sought by tuning the oxidation levels of the conducting polymers, even if such methodology is detrimental to the Seebeck coefficient (S) as both the Seebeck coefficient and the electrical conductivity (σ) are antagonistically related to the carrier concentration. Here we report a concurrent increase of S and σ and we experimentally derive the dependence of Seebeck coefficient on charge carrier mobility for the first time in organic electronics. Through specific control of the conducting polymer synthesis, we enabled the formation of a denser percolation network that facilitated the charge transport and the thermodiffusion of the charge carriers inside the conducting polymer layer, while the material shifted from a Fermi glass towards a semi-metal, as its crystallinity increased. This work sheds light upon the origin of the thermoelectric properties of conducting polymers, but also underlines the importance of enhanced charge carrier mobility for the design of efficient thermoelectric polymers.

Thermoelectric properties of Sr0.61Ba0.39Nb2O6−δ ceramics in different oxygen-reduction conditions**Project supported by the National Basic Research Program of China (Grant No. 2013CB632506) and the National Natural Science Foundation of China (Grant Nos. 51202132 and 51002087).

The thermoelectric properties of Sr0.61Ba0.39Nb2O6−δ ceramics, reduced in different conditions, are investigated in the temperature range from 323 K to 1073 K. The electrical transport behaviors of the samples are dominated by the thermal-activated polaron hopping in the low temperature range, the Fermi glass behavior in the middle temperature range, and the Anderson localized behavior in the high temperature range. The thermal conductivity presents a plateau at high-temperatures, indicating a glass-like thermal conduction behavior. Both the thermoelectric power factor and the thermal conductivity increase with the increase of the degree of oxygen-reduction. Taking these two factors into account, the oxygen-reduction can still contribute to promoting the thermoelectric figure of merit. The highest ZT value is obtained to be ∼0.19 at 1073 K in the heaviest oxygen reduced sample.

Semi-metallic polymers

Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications. We measure the thermoelectric properties of various poly(3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.

Conduction centers in a Ta2O5−δ Fermi glass

We compare our photoemission spectra with the calculated energy spectra of oxygen vacancies in Ta oxide to identify the defects responsible for the formation of a band in the gap of a Ta oxide-based memristor. We have previously explained transitions between high and low resistance states in a memristor conducting channel under bias reversal by accumulation and depletion of oxygen in the channel. Oxidation leads to a higher resistance due to sparser conduction centers for the electrons to hop between. Here, we show that they are likely due to neutral oxygen vacancies located at ‘in-plane’ sites of the Ta–O polyhedra.

From single constituents to correlation: phase transitions in few atom systems

The advancements of nanofabrication, together with the control of implantation of individual atoms at nanometric precision, have opened the experimental study of phase transitions of electronic systems constituted by six electrons and less. Here I review the recent advancements made in the field of phase transitions at the few atoms scale, including the discretized version of the Anderson-Mott quantum phase transition and the melting of electrons arranged in a Wigner-like phase into a Fermi glass realized in a Si : X quantum device where X is a donor element. The rise of collective phenomena is directly probed by controlling the distance between few donor atoms. Four atoms are sufficient to observe emergent phenomena such as Hubbard bands instead of single particle behaviour.

Anderson–Mott transition in arrays of a few dopant atoms in a silicon transistor

Dopant atoms are used to control the properties of semiconductors in most electronic devices. Recent advances such as single-ion implantation have allowed the precise positioning of single dopants in semiconductors as well as the fabrication of single-atom transistors, representing steps forward in the realization of quantum circuits. However, the interactions between dopant atoms have only been studied in systems containing large numbers of dopants, so it has not been possible to explore fundamental phenomena such as the Anderson-Mott transition between conduction by sequential tunnelling through isolated dopant atoms, and conduction through thermally activated impurity Hubbard bands. Here, we observe the Anderson-Mott transition at low temperatures in silicon transistors containing arrays of two, four or six arsenic dopant atoms that have been deterministically implanted along the channel of the device. The transition is induced by controlling the spacing between dopant atoms. Furthermore, at the critical density between tunnelling and band transport regimes, we are able to change the phase of the electron system from a frozen Wigner-like phase to a Fermi glass by increasing the temperature. Our results open up new approaches for the investigation of coherent transport, band engineering and strongly correlated systems in condensed-matter physics.

Electronic structure and transport measurements of amorphous transition-metal oxides: observation of Fermi glass behavior

We characterized the conduction mechanisms in thin sputtered films of three representative binary Me–O (Me=Ta, W, and Nb) systems as a function of oxygen content, by combining in situ chemical state and electronic band structure studies from X-ray photoemission with temperature-dependent transport measurements. Despite certain differences, these amorphous films all displayed Fermi glass behavior following an oxidation-induced transition from metallic to hopping conduction, down to a sub-percolation threshold. The electron localization estimated from the band structure was in good agreement with that from the transport measurements, and the two were used to construct phase diagrams of conduction in the degree of oxidation-conductivity coordinates, which should prove important in the design of resistive switching and other electronic devices.

Fermi Glass Versus High-Temperature Superconductivity Behavior

Very recently BaSrNiO4 was reported to be a Fermi glass (Schilling et al. in J. Phys., Condens. Matter 21:015701, 2009). Its structure is essentially the one of K2NiF4 as is that of La2CuO4, in which the occurrence of high-temperature superconductivity (HTS) upon hole doping was first reported (Bednorz and Muller in Z. Phys. B 64:189, 1986; Adv. Chem. 100:757, 1988). The carriers in both have mainly eg character, and move in a stochastic potential as documented by a number of experiments. The difference of the two behaviors is mainly ascribed to the formation of intersite bipolarons (Kabanov and Mihailovic in J. Supercond. 13:950, 2000) , which is estimated to be up to two orders of magnitude larger in La2CuO4 than in BaSrNiO4. From this it follows that for HTS to occur, a large bipolaron formation energy in layered structures is required.

LaBaNiO4: a Fermi glass

Polycrystalline samples of LaSr1−xBaxNiO4 show a crossover from a state with metallic transport properties forx = 0 to an insulating state as . The end member LaBaNiO4 with a nominal nickel Ni 3d7 configuration might therefore be regarded as a candidate for an antiferromagneticinsulator. However, we do not observe any magnetic ordering inLaBaNiO4 down to 1.5 K, and despite its insulating transport properties several other physical properties ofLaBaNiO4 resemble thoseof metallic LaSrNiO4. Based on an analysis of electrical and thermal-conductivity data as well asmagnetic-susceptibility and low-temperature specific-heat measurements, we suggest thatLaBaNiO4 is a Fermi glass with a finite electron density of states at the Fermi level but these statesare localized.

Electrical Conduction Relaxation in the Bi(Mn1/3Nb2/3)O3 and (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 Ceramics

New Bi(Mn 1/3 Nb 2/3 )O 3 ceramics were obtained. Their pseudo-perovskite symmetry with monoclinic distortion was determined by XRD test. Dispersion in permittivity ε′ (T,f) was observed. Small polaron mechanism of electric conductivity was deduced. Several techniques were applied to evaluate electric conduction relaxation times τ(T,f) from ε′(T,f), ε”(T,f), tan δ(T,f), and M” (T,f) dependences. The relaxation times τ (T,f) temperature dependence was fitted with high accuracy by Fermi glass related term τ (T,f)∼ exp[(T0/T)1/4] corresponding to variable range hopping of small polaron model. Perovskite cubic structure was determined for (Bi 1/9 Na 2/3 )(Mn 1/3 Nb 2/3 )O 3 ceramics. The E a , τ0, T 0, and electric conductivity σ values were changed by Na ions addition.