arXiv subjects

Susanne Stemmer

Publications and source records attributed to Susanne Stemmer.

At least 55 records · Page 3Linked to original sources

arXiv2017

Negative magnetoresistance due to conductivity fluctuations in films of the topological semimetal Cd3As2

Recently discovered Dirac and Weyl semimetals display unusual magnetoresistance phenomena, including a large, non-saturating, linear transverse magnetoresistance and a negative longitudinal magnetoresistance. The latter is often considered as evidence of fermions having a defined chirality. Classical mechanisms, due to disorder or non-uniform current injection, can however, also produce negative longitudinal magnetoresistance. Here, we report on magnetotransport measurements performed on epitaxial thin films of Cd3As2, a three-dimensional Dirac semimetal. Quasi-linear positive transverse magnetoresistance and negative longitudinal magnetoresistance are observed. By evaluating films of different thickness and by correlating the temperature dependence of the carrier density and mobility with the magnetoresistance characteristics, we demonstrate that both the quasi-linear positive and the negative magnetoresistance are caused by conductivity fluctuations. Chiral anomaly is not needed to explain the observed features.

cond-mat.mes-hall↗

arXiv2017

Potential Fluctuations at Low Temperatures in Mesoscopic-Scale SmTiO$_{3}$/SrTiO$_{3}$/SmTiO$_{3}$ Quantum Well Structures

Heterointerfaces of SrTiO$_{3}$ with other transition metal oxides make up an intriguing family of systems with a bounty of coexisting and competing physical orders. Some examples, such as LaAlO$_{3}$/SrTiO$_{3}$, support a high carrier density electron gas at the interface whose electronic properties are determined by a combination of lattice distortions, spin-orbit coupling, defects, and various regimes of magnetic and charge ordering. Here, we study electronic transport in mesoscale devices made with heterostructures of SrTiO$_{3}$ sandwiched between layers of SmTiO$_{3}$, in which the transport properties can be tuned from a regime of Fermi-liquid like resistivity ($ρ\sim T^{2}$) to a non-Fermi liquid ($ρ\sim T^{5/3}$) by controlling the SrTiO$_{3}$ thickness. In mesoscale devices at low temperatures, we find unexpected voltage fluctuations that grow in magnitude as $T$ is decreased below 20 K, are suppressed with increasing contact electrode size, and are independent of the drive current and contact spacing distance. Magnetoresistance fluctuations are also observed, which are reminiscent of universal conductance fluctuations but not entirely consistent with their conventional properties. Candidate explanations are considered, and a mechanism is suggested based on mesoscopic temporal fluctuations of the Seebeck coefficient. An improved understanding of charge transport in these model systems, especially their quantum coherent properties, may lead to insights into the nature of transport in strongly correlated materials that deviate from Fermi liquid theory.

cond-mat.mes-hall↗

arXiv2016

Dichotomy of the transport coefficients of correlated electron liquids in SrTiO3

We discuss the Seebeck coefficient and the Hall mobility of electrons confined in narrow SrTiO3 quantum wells as a function of the three-dimensional carrier density and temperature. The quantum wells contain a fixed sheet carrier density of ~ 7x10^14 cm^-2 and their thickness is varied. At high temperatures, both properties exhibit apparent Fermi liquid behavior. In particular, the Seebeck coefficient increases nearly linearly with temperature (T) when phonon drag contributions are minimized, while the mobility decreases proportional to T^2. Furthermore, the Seebeck coefficient scales inversely with the Fermi energy (decreasing quantum well thickness). In contrast, the transport scattering rate is independent of the Fermi energy, which is inconsistent with a Fermi liquid. At low temperatures, the Seebeck coefficient deviates from the linear temperature dependence for those electron liquids that exhibit a correlation-induced pseudogap, indicating a change in the energy dependence of the scattering rate. The implications for describing transport in strongly correlated materials are discussed.

cond-mat.str-el↗

arXiv2016

Large Electron Concentration Modulation using Capacitance Enhancement in SrTiO3/SmTiO3 FinFETs

Solid-state modulation of 2-dimensional electron gases (2DEGs) with extreme (~3.3 x 1014 cm-2) densities corresponding to 1/2 electron per interface unit cell at complex oxide heterointerfaces (such as SrTiO3/GdTiO3 or SrTiO3/SmTiO3) is challenging because it requires enormous gate capacitances. One way to achieve large gate capacitances is by geometrical capacitance enhancement in fin structures. In this work, we fabricate both Au-gated planar field effect transistors (FETs) and Fin-FETs with varying fin-widths on 60nm SrTiO3/5nm SmTiO3 thin films grown by hybrid molecular beam epitaxy (hMBE). We find that the FinFETs exhibit higher gate capacitance compared to planar FETs. By scaling down the SrTiO3/SmTiO3 fin widths, we demonstrate further gate capacitance enhancement, almost twice compared to the planar FETs. In the FinFETs with narrowest fin-widths, we demonstrate a record 2DEG electron concentration modulation of ~2.4 x 1014 cm-2.

cond-mat.mes-hall↗

arXiv2016

Key role of lattice symmetry in the metal-insulator transition of NdNiO3 films

Bulk NdNiO3 exhibits a metal-to-insulator transition (MIT) as the temperature is lowered that is also seen in tensile strained films. In contrast, films that are under a large compressive strain typically remain metallic at all temperatures. To clarify the microscopic origins of this behavior, we use position averaged convergent beam electron diffraction in scanning transmission electron microscopy to characterize strained NdNiO3 films both above and below the MIT temperature. We show that a symmetry lowering structural change takes place in case of the tensile strained film, which undergoes an MIT, but is absent in the compressively strained film. Using space group symmetry arguments, we show that these results support the bond length disproportionation model of the MIT in the rare-earth nickelates. Furthermore, the results provide insights into the non-Fermi liquid phase that is observed in films for which the MIT is absent.

cond-mat.str-el↗

arXiv2015

Carrier density independent scattering rate in SrTiO3-based electron liquids

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.

cond-mat.str-el↗

arXiv2015

Ferroelectric Transition in Compressively Strained SrTiO3 Thin Films

We report the temperature dependent capacitance-voltage characteristics of Pt/SrTiO3 Schottky diodes fabricated using compressively strained SrTiO3 thin films grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates. The measurements reveal a divergence of the out of plane dielectric constant of SrTiO3 peaked at ~140K, implying a ferroelectric transition. A Curie-Weiss law fit to the zero-bias dielectric constant suggests a Curie temperature of ~56 K. This observation provides experimental confirmation of the theoretical prediction of out of plane ferroelectricity in compressively strained SrTiO3 thin films grown on LSAT substrate. We also discuss the roles of the field-dependent dielectric constant and the interfacial layer in SrTiO3 on the extraction of the Curie temperature.

cond-mat.mtrl-sci↗

arXiv2015

Tuning bad metal and non-Fermi liquid behavior in a Mott material: rare earth nickelate thin films

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.

cond-mat.str-el↗

arXiv2015

Separation of transport lifetimes in SrTiO3-based two-dimensional electron liquids

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.

cond-mat.str-el↗

arXiv2014

Nanostructure Investigations of Nonlinear Differential Conductance in NdNiO$_3$ Thin Films

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.

cond-mat.str-el↗

arXiv2014

Surface Reconstructions in Molecular Beam Epitaxy of SrTiO3

We show that reflection high-energy electron diffraction (RHEED) can be used as a highly sensitive tool to track surface and resulting film stoichiometry in adsorption-limited molecular beam epitaxy of (001) SrTiO3 thin films. Even under growth conditions that yield films with a lattice parameter that is identical to that of stoichiometric bulk crystals within the detection limit of high-resolution x-ray diffraction (XRD), changes in surface reconstruction occur from (1x1) to (2x1) to c(4x4) as the equivalent beam pressure of the Ti metalorganic source is increased. These surface reconstructions are correlated with a shift from mixed SrO/TiO2 termination to pure TiO2 termination. The crossover to TiO2 surface termination is also apparent in a phase shift in RHEED oscillations observed at the beginning of growth. Comparison with prior results for carrier mobilities of doped films shows that the best films are grown under conditions of a TiO2-saturated surface [c(4x4) reconstruction] within the XRD growth window.

cond-mat.mtrl-sci↗

arXiv2014

Au-Gated SrTiO3 Field-Effect Transistors with Large Electron Concentration and Current Modulation

We report the fabrication of low-leakage rectifying Pt and Au Schottky diodes and Au-gated metal-semiconductor field effect transistors (MESFETs) on n-type SrTiO3 thin films grown by hybrid molecular beam epitaxy. In agreement with previous studies, we find that compared to Pt, Au provides a higher Schottky barrier height with SrTiO3. As a result of the large dielectric constant of SrTiO3 and the large Schottky barrier height of Au, the Au-gated MESFETs are able to modulate ~1.6 x 1014 cm-2 electron density, the highest modulation yet achieved using metal gates in any material system. These MESFETs modulate current densities up to ~68 mA/mm, ~20x times larger than the best demonstrated SrTiO3 MESFETs. We also discuss the roles of the interfacial layer, and the field-dependent dielectric constant of SrTiO3 in increasing the pinch off voltage of the MESFET.

cond-mat.mtrl-sci↗

arXiv2014

Intrinsic Mobility Limiting Mechanisms in Lanthanum-doped Strontium Titanate

The temperature dependent Hall mobility data from La-doped SrTiO3 thin films has been analyzed and modeled considering various electron scattering mechanisms. We find that a ~6 meV transverse optical phonon (TO) deformation potential scattering mechanism is necessary to explain the dependence of transport on temperature between 10-200 K. Also, we find that the low temperature electron mobility in intrinsic (nominally undoped) SrTiO3 is limited by acoustic phonon scattering. Adding the above two scattering mechanisms to longitudinal optical phonon (LO) and ionized impurity scattering mechanisms, excellent quantitative agreement between mobility measurement and model is achieved in the whole temperature range (2-300K) and carrier concentrations ranging over a few orders of magnitude (8x1017 cm-3 - 2x1020 cm-3).

cond-mat.mes-hall↗

arXiv2014

Gaps and pseudo-gaps at the Mott quantum Critical point in the perovskite rare earth nickelates

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.

cond-mat.str-el↗

arXiv2014

High-density two-dimensional small polaron gas in a delta-doped Mott insulator

Heterointerfaces in complex oxide systems open new arenas in which to test models of strongly correlated material, explore the role of dimensionality in metal-insulator-transitions (MITs) and small polaron formation. Close to the quantum critical point Mott MITs depend on band filling controlled by random disordered substitutional doping. Delta-doped Mott insulators are potentially free of random disorder and introduce a new arena in which to explore the effect of electron correlations and dimensionality. Epitaxial films of the prototypical Mott insulator GdTiO3 are delta-doped by substituting a single (GdO)+1 plane with a monolayer of charge neutral SrO to produce a two-dimensional system with high planar doping density. Unlike metallic SrTiO3 quantum wells in GdTiO3 the single SrO delta-doped layer exhibits thermally activated DC and optical conductivity that agree in a quantitative manner with predictions of small polaron transport but with an extremely high two-dimensional density of polarons, ~ 7E14 cm-2

cond-mat.str-el↗

arXiv2014

Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO3 quantum wells

The electrical and structural characteristics of SmTiO3/SrTiO3/SmTiO3 and GdTiO3/SrTiO3/GdTiO3 heterostructures are compared. Both types of structures contain narrow SrTiO3 quantum wells, which accommodate a confined, high-density electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO3 quantum wells embedded in GdTiO3 show a metal-to-insulator transition when their thickness is reduced so that they contain only two SrO layers. In contrast, quantum wells embedded in SmTiO3 remain metallic down to a single SrO layer thickness. Symmetry-lowering structural distortions, measured by quantifying the Sr-column displacements, are present in the insulating quantum wells, but are either absent or very weak in all metallic quantum wells, independent of whether they are embedded in SmTiO3 or in GdTiO3. We discuss the role of orthorhombic distortions, orbital ordering, and strong electron correlations in the transition to the insulating state.

cond-mat.str-el↗

arXiv2013

Interface-induced magnetism in perovskite quantum wells

We investigate the angular dependence of the magnetoresistance of thin (< 1 nm), metallic SrTiO3 quantum wells epitaxially embedded in insulating, ferrimagnetic GdTiO3 and insulating, antiferromagnetic SmTiO3, respectively. The SrTiO3 quantum wells contain a high density of mobile electrons (~7x10^14 cm^-2). We show that the longitudinal and transverse magnetoresistance in the structures with GdTiO3 are consistent with anisotropic magnetoresistance, and thus indicative of induced ferromagnetism in the SrTiO3, rather than a nonequilibrium proximity effect. Comparison with the structures with antiferromagnetic SmTiO3 shows that the properties of thin SrTiO3 quantum wells can be tuned to obtain magnetic states that do not exist in the bulk material.

cond-mat.mes-hall↗

arXiv2013

Subband structure of two-dimensional electron gases in SrTiO3

Tunneling between two parallel, two-dimensional electron gases (2DEGs) in a complex oxide heterostructure containing a large, mobile electron density of ~ 3x10^14 cm^-2 is used to probe the subband structure of the 2DEGs. Temperature-dependent current-voltage measurements are performed on SrTiO3/GdTiO3/SrTiO3 junctions, where GdTiO3 serves as the tunnel barrier, and each interface contains a high-density 2DEG. Resonant tunneling features in the conductance and its derivative occur when subbands on either side of the barrier align in energy as the applied bias is changed, and are used to analyze subband energy spacings in the two 2DEGs. We show that the results agree substantially with recent theoretical predictions for such interfaces.

cond-mat.mes-hall↗