The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin
We investigate the temperature distributions of Joule self-heated graphene nanoribbons (GNRs) with a spatial resolution finer than 100 nm by scanning thermal microscopy (SThM). The SThM probe is calibrated using the Raman G mode Stokes/anti-Stokes intensity ratio as a function of electric power applied to the GNR devices. From a spatial map of the temperature distribution, heat dissipation and transport pathways are investigated. By combining SThM and scanning gate microscopy data from a defected GNR, we observe hot spot formation at well-defined, localized sites.Energy dissipation and heat flow in nanostructured graphene devices are critical issues for understanding charge transport mechanisms and for further optimization of device performance. Local temperature distributions of Joule self-heated graphene devices have been studied by optical methods such as micro-Raman spectroscopy, micro-infra-red and confocal Raman spectroscopy [1][2][3][4][5]. The spatial resolution of these optical techniques, however, is limited by the photon wavelength ∼1 µm, and a new type of the thermal probe is required to investigate microscopic energy dissipation mechanism in graphene nanostructures whose dimensions are often much smaller than this length scale. While scanning thermal microscopy(SThM) [6][7][8][9] has been used for studying thermal dissipation of nanoscaled devices [10-12] with a spatial resolution of 50 nm, due to the complex heat transfer paths involved, this technique requires a nontrivial calibration process for the thermal probe in order to correctly represent the local sample temperature on an absolute scale [13].In this letter, we present a high-resolution study of the spatial distribution of the temperature of graphene nanoribbons (GNRs) under conditions of current flow. The measurements were carried out using SThM, with an absolute calibration of the temperature rise by means of Raman spectroscopy. In this fashion, we were able to probe the thermal contact resistance between a GNR and the underlying substrate.The fabrication process for the GNR devices used in these experiments has been described in previous work [14]. Briefly, single layer graphene samples were deposited by mechanical exfoliation on Si wafers covered with 280 nm thick SiO 2 .Cr/Au electrodes (0.5 nm/40 nm in thickness) were defined by electron beam lithography. A negative tone e-beam resist, hydrogen silsesquioxane (HSQ), was used to form an etch mask for an oxygen plasma etching process which removed the unprotected graphene. Fig. 1(a) shows a schematic diagram for SThM measurements of the Joule self-heated GNR devices. The SThM experiments were carried out with an atomic force microscope (AFM) probe with a high-resolution thermistor installed at the tip (XE-100 with Nano thermal probe, Park Systems Corp.). The measurements were performed in a dry nitrogen environment at room temperature. The resistance change of the thin palladium film resistor at the apex of the probe is monitored using a Wheatstone bridge. The SThM signal is obtained fr...
In complex oxide materials, changes in electronic properties are often associated with changes in crystal structure, raising the question of the relative roles of the electronic and lattice effects in driving the metal-insulator transition. This paper presents a combined theoretical and experimental analysis of the dependence of the metalinsulator transition of NdNiO 3 on crystal structure, specifically comparing properties of bulk materials to one and two layer samples of NdNiO 3 grown between multiple electronically inert NdAlO 3 counterlayers in a superlattice. The comparison amplifies and validates a theoretical approach developed in previous papers and disentangles the electronic and lattice contributions, through an independent variation of each. In bulk NdNiO 3 the correlations are not strong enough to drive a metal-insulator transition by themselves: a lattice distortion is required. Ultra-thin films exhibit two additional electronic effects and one lattice-related effect. The electronic effects are quantum confinement, leading to dimensional reduction of the electronic Hamiltonian, and an increase in electronic bandwidth due to counterlayer induced bond angle changes. We find that the confinement effect is much more important. The lattice effect is an increase in stiffness due to the cost of propagation of the lattice disproportionation into the confining material.transition metal oxide | metal-insulator transition | heterostructure | epitaxial constraint | structural modulation | layer confinement Introduction:. Metal insulator transitions (MIT) in correlated electron materials typically involve changes in both the electronic and atomic structure. The relative importance of the two effects has been the subject of extensive discussion(1-8). In this paper, using a recently developed theoretical approach (3, 8), we argue that comparison of few-layer and bulk materials yields considerable insight into the relative importance of electronic and lattice contributions, essentially because these are affected by heterostructuring in opposite ways. We disentangle these effects by independently changing each. Motivated by recent experimental (9-25) and theoretical (8,13,(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36) results, we focus here on the rare earth nickelate family of materials. The concepts, formalism and findings are applicable to wide classes of materials.
We introduce a set of generalized slave-particle models for extended Hubbard models that treat localized electronic correlations using slave-boson decompositions. Our models automatically include two slave-particle methods of recent interest, the slave-rotor and slave-spin methods, as well as a ladder of new intermediate models where one can choose which of the electronic degrees of freedom (e.g., spin or orbital labels) are treated as correlated degrees of freedom by the slave bosons. In addition, our method removes the aberrant behavior of the slave-rotor model, where it systematically overestimates the importance of electronic correlation effects for weak interaction strength, by removing the contribution of unphysical states from the bosonic Hilbert space. The flexibility of our formalism permits one to separate and isolate the effect of correlations on the key degrees of freedom.
The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures, where one can constrain dimensionality and impose interfacial boundary conditions. Here, we unlock a host of unique, hidden electronic and magnetic phase transitions in NdNiO 3 while approaching the two-dimensional (2D) limit, resulting from the differing influences of dimensional confinement and interfacial coupling. Most notably, we discover a new phase in fully 2D, single layer NdNiO 3 , in which all signatures of the bulk magnetic and charge ordering are found to vanish. In addition, for quasi two-dimensional layers down to a thickness of two unit cells, bulk-type ordering persists but separates from the onset of insulating behavior in a manner distinct from that found in the bulk or thin film nickelates. Using resonant x-ray spectroscopies, first-principles theory, and model calculations, we propose that the single layer phase suppression results from a new mechanism of interfacial electronic reconstruction based on ionicity differences across the interface, while the phase separation in multi-layer NdNiO 3 emerges due to enhanced 2D fluctuations. These findings provide insights into the intertwined mechanisms of charge and spin ordering in strongly correlated systems in reduced dimensions and illustrate the ability to use atomic layering to access hidden phases.
We describe a theoretical approach for finding spontaneously symmetry-broken electronic phases due to strong electronic interactions when using recently developed slave-particle (slave-boson) approaches based on occupation numbers. We describe why, to date, spontaneous symmetry breaking has proven difficult to achieve in such approaches. We then provide a total-energy based approach for introducing auxiliary symmetry breaking fields into the solution of the slave-particle problem that leads to lowered total energies for symmetry broken phases. We point out that not all slaveparticle approaches yield to energy lowering: the slave-particle model being used must explicitly describe the degrees of freedom that break symmetry. Finally, our total energy approach permits us to greatly simplify the formalism used to achieve a self-consistent solution between spinon and slave modes while increasing numerical stability and greatly speeding up the calculations.
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