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Nanostructure Investigations of Nonlinear Differential Conductance in NdNiO$_3$ Thin Films
Will J. Hardy,Heng Ji,Evgeny Mikheev,Susanne Stemmer,Douglas Natelson
Physics , 2014, DOI: 10.1103/PhysRevB.90.205117
Abstract: Transport measurements on thin films of NdNiO$_3$ reveal a crossover to a regime of pronounced nonlinear conduction below the well-known metal-insulator transition temperature. The evolution of the transport properties at temperatures well below this transition appears consistent with a gradual formation of a gap in the hole-like Fermi surface of this strongly correlated system. As $T$ is decreased below the nominal transition temperature, transport becomes increasily non-Ohmic, with a model of Landau-Zener breakdown becoming most suited for describing $I(V)$ characteristics as the temperature approaches 2~K.
Separation of transport lifetimes in SrTiO3-based two-dimensional electron liquids
Evgeny Mikheev,Christopher R. Freeze,Brandon J. Isaac,Tyler A. Cain,Susanne Stemmer
Physics , 2015, DOI: 10.1103/PhysRevB.91.165125
Abstract: Deviations from Landau Fermi liquid behavior are ubiquitous features of the normal state of unconventional superconductors. Despite several decades of investigation, the underlying mechanisms of these properties are still not completely understood. In this work, we show that two-dimensional electron liquids at SrTiO3/RTiO3 (R = Gd or Sm) interfaces reveal strikingly similar physics. Analysis of Hall and resistivity data show a clear separation of transport and Hall scattering rates, also known as "two-lifetime" behavior. This framework gives a remarkably simple and general description of the temperature dependence of the Hall coefficient. Distinct transport lifetimes accurately describe the transport phenomena irrespective of the nature of incipient magnetic ordering, the degree of disorder, confinement, or the emergence of non-Fermi liquid behavior. The Hall scattering rate diverges at a critical quantum well thickness, coinciding with a quantum phase transition. Collectively, these results introduce new constraints on the existing microscopic theories of lifetime separation and point to the need for unified understanding.
Carrier density independent scattering rate in SrTiO3-based electron liquids
Evgeny Mikheev,Santosh Raghavan,Jack Y. Zhang,Patrick B. Marshall,Adam P. Kajdos,Leon Balents,Susanne Stemmer
Physics , 2015,
Abstract: We examine the carrier density dependence of the scattering rate in two- and three-dimensional electron liquids in SrTiO3 in the regime where it scales with T^n (T is the temperature and n <= 2) in the cases when it is varied by electrostatic control and chemical doping, respectively. It is shown that the scattering rate is independent of the carrier density. This is contrary to the expectations from Landau Fermi liquid theory, where the scattering rate scales inversely with the Fermi energy (E_F). We discuss that the behavior is very similar to systems traditionally identified as non-Fermi liquids (n < 2). This includes the cuprates and other transition metal oxide perovskites, where strikingly similar density-independent scattering rates have been observed. The results indicate that the applicability of Fermi liquid theory should be questioned for a much broader range of correlated materials and point to the need for a unified theory.
Tuning bad metal and non-Fermi liquid behavior in a Mott material: rare earth nickelate thin films
Evgeny Mikheev,Adam J. Hauser,Burak Himmetoglu,Nelson E. Moreno,Anderson Janotti,Chris G. Van de Walle,Susanne Stemmer
Physics , 2015, DOI: 10.1126/sciadv.1500797
Abstract: Resistances that exceed the Mott-Ioffe-Regel limit, known as bad metal behavior, and non-Fermi liquid behavior are ubiquitous features of the normal state of many strongly correlated materials. Here we establish the conditions that lead to bad metal and non-Fermi liquid phases in NdNiO3, which exhibits a prototype, bandwidth-controlled metal-insulator transition. We show that resistance saturation is determined by the magnitude of the Ni eg orbital splitting, which can be tuned by strain in epitaxial films, causing the appearance of bad metal behavior under certain conditions. The results shed light on the nature of a crossover to non-Fermi liquid metal phase and provide a predictive criterion for strong localization. They elucidate a seemingly complex phase behavior as a function of film strain and confinement and provide guidelines for orbital engineering and novel devices.
From surface to random criticality in layered planar Ising models
Lev Mikheev
Physics , 1995,
Abstract: A general case of a spatially nonuniform planar layered Ising model, or an equivalent quantum Ising chain, is analysed with an exact functional real space renormalization group. Various surface, finite size, quasiperiodic and random layer (McCoy-Wu) universality classes are obtained and discussed within a single theoretical framework leading to new insights into the nature of random criticality.
Critical phenomena in superlattices: Reentrant dimensional crossover and anomalous critical amplitudes
Lev Mikheev
Physics , 1995,
Abstract: A crossover from $d$ to $d-1$, and then back to $d$-dimensional critical behavior is argued to be a generic feature characterizing ordering in a $d$-dimensional superlattice composed of atomically {\em thick} films of two ferromagnets. The crossover leads to anomalous changes in the amplitudes of critical singularities. In $d=3$ Heisenberg and $XY$ superlattices large scale critical fluctuations persist over a wide temperature range.
Learning Part-of-Speech Guessing Rules from Lexicon: Extension to Non-Concatenative Operations
Andrei Mikheev
Computer Science , 1996,
Abstract: One of the problems in part-of-speech tagging of real-word texts is that of unknown to the lexicon words. In Mikheev (ACL-96 cmp-lg/9604022), a technique for fully unsupervised statistical acquisition of rules which guess possible parts-of-speech for unknown words was proposed. One of the over-simplification assumed by this learning technique was the acquisition of morphological rules which obey only simple concatenative regularities of the main word with an affix. In this paper we extend this technique to the non-concatenative cases of suffixation and assess the gain in the performance.
Unsupervised Learning of Word-Category Guessing Rules
Andrei Mikheev
Computer Science , 1996,
Abstract: Words unknown to the lexicon present a substantial problem to part-of-speech tagging. In this paper we present a technique for fully unsupervised statistical acquisition of rules which guess possible parts-of-speech for unknown words. Three complementary sets of word-guessing rules are induced from the lexicon and a raw corpus: prefix morphological rules, suffix morphological rules and ending-guessing rules. The learning was performed on the Brown Corpus data and rule-sets, with a highly competitive performance, were produced and compared with the state-of-the-art.
Gaps and pseudo-gaps at the Mott quantum Critical point in the perovskite rare earth nickelates
S. James Allen,Adam J. Hauser,Evgeny Mikheev,Jack Y. Zhang,Nelson E. Moreno,Junwoo Son,Daniel G. Ouellette,James Kally,Alex Kozhanov,Leon Balents,Susanne Stemmer
Physics , 2014,
Abstract: We report on tunneling measurements that reveal for the first time the evolution of the quasi-particle state density across the bandwidth controlled Mott metal to insulator transition in the rare earth perovskite nickelates. In this, a canonical class of transition metal oxides, we study in particular two materials close to the T=0 metal-insulator transition: NdNiO3 , an antiferromagnetic insulator, and LaNiO3, a correlated metal. We measure a sharp gap in NdNiO3, which has an insulating ground state, of ~ 30 meV. Remarkably, metallic LaNiO3 exhibits a pseudogap of the same order that presages the metal insulator transition. The smallness of both the gap and pseudogap suggests they arise from a common origin: proximity to a quantum critical point at or near the T=0 metal-insulator transition. It also supports theoretical models of the quantum phase transition in terms of spin and charge instabilities of an itinerant Fermi surface.
Computer Image Analysis as a Tool for Microbial Viability Assessment: Examples of Use and Prospects  [PDF]
Evgeny Puchkov
Journal of Biosciences and Medicines (JBM) , 2014, DOI: 10.4236/jbm.2014.23001
Abstract: Application of the computer image analysis for improving microbial viability assessment by plate count and fluorescence microscopy was investigated. Yeast cells were used as a model microorganism. The application of the improved methods for the viability assessment of yeast cells after preservation by freezing and freeze-drying was demonstrated.
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