110 research outputs found
Shot noise detection in hBN-based tunnel junctions
High quality Au/hBN/Au tunnel devices are fabricated using transferred
atomically thin hexagonal boron nitride as the tunneling barrier. All tunnel
junctions show tunneling resistance on the order of several
k/m. Ohmic I-V curves at small bias with no signs of
resonances indicate the sparsity of defects. Tunneling current shot noise is
measured in these devices, and the excess shot noise shows consistency with
theoretical expectations. These results show that atomically thin hBN is an
excellent tunnel barrier, especially for the study of shot noise properties,
and this can enable the study of tunneling density of states and shot noise
spectroscopy in more complex systems.Comment: 20 pages, 4 figure
Nanoscale Electronic Transport Studies of Novel Strongly Correlated Materials
Strongly correlated materials are those in which the electron-electron and electron-lattice interactions play pivotal roles in determining many aspects of observable physical behavior, including the electronic and magnetic properties. In this thesis, I describe electronic transport studies of novel strongly correlated materials at the nanoscale. After introducing some basic concepts, briefly reviewing historical development of the field, and discussing the process of making measurements on small length scales, I detail experimental results from studies of four specific materials: two transition metal oxide systems, and two layered transition metal dichalcogenides with intercalated magnetic moments. The first system is a modified version of a classic strongly correlated material, vanadium dioxide (VO2), which here is doped with hydrogen to suppress its metal-insulator transition and stabilize a poorly metallic phase down to liquid helium temperatures. Doped VO2 nanowires, micron flakes, and thin films display magnetoresistance (MR) consistent with weak localization physics, along with mesoscopic resistance fluctuations over short distances, raising questions about how to model transport in bad-metal correlated systems. A second transition metal oxide system is considered next: Quantum wells in SrTiO3 sandwiched between layers of SmTiO3, in which anomalous voltage fluctuation behavior is observed in etched nanostructures at low temperatures. After well-understood alternative origins are ruled out, an explanation is proposed involving a time-varying thermopower due to two-level fluctuations of etching-induced defects. Next, I shift to the topic of layered itinerant magnetic materials with intercalated moments, starting with Fe0.28TaS2, a hard ferromagnet (FM) with strong spin-orbit coupling. Here, a surprisingly large MR of nearly 70% is observed, an especially striking feature given that the closely related compounds at Fe intercalation fractions of 1/4 or 1/3 have MR nearly two orders of magnitude smaller. In the latter compounds, the Fe atoms are arranged in ordered superlattices, whereas for the 0.28 case, a portion of the Fe moments deviate from ordered arrangement and are relatively easily flipped by an external magnetic field to be anti-aligned with neighboring ordered Fe moments. This situation, combined with strong spin-orbit coupling, results in enhanced charge carrier scattering and greatly increased resistance. The thesis concludes with a study of a second layered magnetic material, V5S8 (structurally equivalent to V0.25VS2), which is found to have a magnetic field driven phase transition at low temperatures, believed to be from antiferromagnetism to paramagnetism. This transition is first order in thick crystals, but becomes second order as the crystal thickness decreases toward 10 nm. Together, the experiments described in this thesis highlight the complexity and diversity of strongly correlated materials, while showcasing the power of nanoscale electronic transport in delivering an improved understanding of these systems
Very large magnetoresistance in FeTaS single crystals
Magnetic moments intercalated into layered transition metal dichalcogenides
are an excellent system for investigating the rich physics associated with
magnetic ordering in a strongly anisotropic, strong spin-orbit coupling
environment. We examine electronic transport and magnetization in
FeTaS, a highly anisotropic ferromagnet with a Curie temperature
K. We find anomalous Hall data confirming a
dominance of spin-orbit coupling in the magnetotransport properties of this
material, and a remarkably large field-perpendicular-to-plane MR exceeding 60%
at 2 K, much larger than the typical MR for bulk metals, and comparable to
state-of-the-art GMR in thin film heterostructures, and smaller only than CMR
in Mn perovskites or high mobility semiconductors. Even within the
FeTaS series, for the current = 0.28 single crystals the MR is
nearly higher than that found previously in the commensurate
compound FeTaS. After considering alternatives, we argue that
the large MR arises from spin disorder scattering in the strong spin-orbit
coupling environment, and suggest that this can be a design principle for
materials with large MR.Comment: 8 pages, 8 figures, accepted in PR
Nanostructure Investigations of Nonlinear Differential Conductance in NdNiO Thin Films
Transport measurements on thin films of NdNiO 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 is decreased below the nominal transition temperature, transport
becomes increasily non-Ohmic, with a model of Landau-Zener breakdown becoming
most suited for describing characteristics as the temperature approaches
2~K.Comment: 18 pages, 6 figures, accepted for publication in PR
Thermally Driven Analog of the Barkhausen Effect at the Metal-Insulator Transition in Vanadium Dioxide
The physics of the metal-insulator transition (MIT) in vanadium dioxide
remains a subject of intense interest. Because of the complicating effects of
elastic strain on the phase transition, there is interest in comparatively
strain-free means of examining VO2 material properties. We report contact-free,
low-strain studies of the MIT through an inductive bridge approach sensitive to
the magnetic response of VO2 powder. Rather than observing the expected
step-like change in susceptibility at the transition, we argue that the
measured response is dominated by an analog of the Barkhausen effect, due to
the extremely sharp jump in the magnetic response of each grain as a function
of time as the material is cycled across the phase boundary. This effect
suggests that future measurements could access the dynamics of this and similar
phase transitions.Comment: 16 pages, 4 figures. Accepted for publication in Appl. Phys. Let
Hydrogen Diffusion and Stabilization in Single-crystal VO2 Micro/nanobeams by Direct Atomic Hydrogenation
We report measurements of the diffusion of atomic hydrogen in single
crystalline VO2 micro/nanobeams by direct exposure to atomic hydrogen, without
catalyst. The atomic hydrogen is generated by a hot filament, and the doping
process takes place at moderate temperature (373 K). Undoped VO2 has a
metal-to-insulator phase transition at ~340 K between a high-temperature,
rutile, metallic phase and a low-temperature, monoclinic, insulating phase with
a resistance exhibiting a semiconductor-like temperature dependence. Atomic
hydrogenation results in stabilization of the metallic phase of VO2
micro/nanobeams down to 2 K, the lowest point we could reach in our measurement
setup. Based on observing the movement of the hydrogen diffusion front in
single crystalline VO2 beams, we estimate the diffusion constant for hydrogen
along the c-axis of the rutile phase to be 6.7 x 10^{-10} cm^2/s at
approximately 373 K, exceeding the value in isostructural TiO2 by ~ 38x.
Moreover, we find that the diffusion constant along the c-axis of the rutile
phase exceeds that along the equivalent a-axis of the monoclinic phase by at
least three orders of magnitude. This remarkable change in kinetics must
originate from the distortion of the "channels" when the unit cell doubles
along this direction upon cooling into the monoclinic structure. Ab initio
calculation results are in good agreement with the experimental trends in the
relative kinetics of the two phases. This raises the possibility of a
switchable membrane for hydrogen transport.Comment: 23 pages, 4 figs + supporting materia
Effective out-of-plane g-factor in strained-Ge/SiGe quantum dots
Recently, lithographic quantum dots in strained-Ge/SiGe have become a
promising candidate for quantum computation, with a remarkably quick
progression from demonstration of a quantum dot to qubit logic demonstrations.
Here we present a measurement of the out-of-plane -factor for single-hole
quantum dots in this material. As this is a single-hole measurement, this is
the first experimental result that avoids the strong orbital effects present in
the out-of-plane configuration. In addition to verifying the expected
-factor anisotropy between in-plane and out-of-plane magnetic ()-fields,
variations in the -factor dependent on the occupation of the quantum dot are
observed. These results are in good agreement with calculations of the
-factor using the heavy- and light-hole spaces of the Luttinger Hamiltonian,
especially the first two holes, showing a strong spin-orbit coupling and
suggesting dramatic -factor tunability through both the -field and the
charge state
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