36 research outputs found
A nanoscale probe of the quasiparticle band structure for two dimensional electron systems
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.Page 138 blank. Cataloged from PDF version of thesis.Includes bibliographical references (pages 121-137).The advent of a broad class of two-dimensional (2D) electronic materials has provided avenues to create and study designer electronic quantum phases. The coexistence of superconductivity, magnetism, density waves, and other ordered phases on the surfaces and interfaces of these 2D materials are governed by interactions which can be experimentally tuned with increasing precision. This motivates the need to develop spectroscopic probes that are sensitive to these tuning parameters, with the objective of studying the electronic properties and emergence of order in these materials. In the first part of this thesis, we report on spectroscopic studies of the topological semimetal antimony (Sb). Our simultaneous observation of Landau quantization and quasiparticle interference phenomena on this material enables their quantitative reconciliation - after two decades of their study on various materials. We use these observations to establish momentum-resolved scanning tunneling microscopy (MR-STM) as a robust nanoscale band structure probe, and reconstruct the multi-component dispersion of Sb(111) surface states. We quantify surface state parameters relevant to spintronics applications, and clarify the relationship between bulk conductivity and surface state robustness. At low momentum, we find a crossover in the single particle behavior from massless Dirac to massive Rashba character - a unique signature of topological surface states. In the second part of this thesis, we report on the spectroscopic study of charge density wave (CDW) order in the dichalcogenide 2H-NbSe2 - a model system for understanding the interplay of coexisting CDW and superconducting phases. We detail the observation of a previously unknown unidirectional (stripe) CDW smoothly interfacing with the familiar triangular CDW on this material. Our low temperature measurements rule out thermal fluctuations and point to local strain as the tuning parameter for this quantum phase transition. The distinct wavelengths and tunneling spectra of the two CDWs, in conjunction with band structure calculations, enable us to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface nesting in the CDW phase of NbSe2. Our observations motivate further spectroscopic studies of the phase evolution of the CDW, and of NbSe 2 as a prototypical strong coupling density wave system in the vicinity of a quantum critical point.by Anjan Soumyanarayanan.Ph.D
Multiple Broken Symmetries in Striped LaBaCuO detected by the Field Symmetric Nernst Effect
We report on a thermoelectric investigation of the stripe and superconducting
phases of the cuprate LaBaCuO near the doping known
to host stable stripes. We use the doping and magnetic field dependence of
field-symmetric Nernst effect features to delineate the phenomenology of these
phases. Our measurements are consistent with prior reports of time-reversal
symmetry breaking signatures above the superconducting , and
crucially detect a sharp, robust, field-invariant peak at the stripe charge
order temperature, . Our observations
suggest the onset of a nontrivial charge ordered phase at , and the subsequent presence of spontaneously
generated vortices over a broad temperature range before the emergence of bulk
superconductivity in LBCO
Stray field signatures of N\'eel textured skyrmions in Ir/Fe/Co/Pt multilayer films
Skyrmions are nanoscale spin configurations with topological properties that
hold great promise for spintronic devices. Here, we establish their N\'eel
texture, helicity, and size in Ir/Fe/Co/Pt multilayer films by constructing a
multipole expansion to model their stray field signatures and applying it to
magnetic force microscopy (MFM) images. Furthermore, the demonstrated
sensitivity to inhomogeneity in skyrmion properties, coupled with a unique
capability to estimate the pinning force governing dynamics, portends broad
applicability in the burgeoning field of topological spin textures.Comment: 6 pages, 4 figures, significantly revised and upgraded. For the
updated supplementary material please contact one of the corresponding
author
Chiral magnetic textures in Ir/Fe/Co/Pt multilayers: Evolution and topological Hall signature
Skyrmions are topologically protected, two-dimensional, localized hedgehogs
and whorls of spin. Originally invented as a concept in field theory for
nuclear interactions, skyrmions are central to a wide range of phenomena in
condensed matter. Their realization at room temperature (RT) in magnetic
multilayers has generated considerable interest, fueled by technological
prospects and the access granted to fundamental questions. The interaction of
skyrmions with charge carriers gives rise to exotic electrodynamics, such as
the topological Hall effect (THE), the Hall response to an emergent magnetic
field, a manifestation of the skyrmion Berry-phase. The proposal that THE can
be used to detect skyrmions needs to be tested quantitatively. For that it is
imperative to develop comprehensive understanding of skyrmions and other chiral
textures, and their electrical fingerprint. Here, using Hall transport and
magnetic imaging, we track the evolution of magnetic textures and their THE
signature in a technologically viable multilayer film as a function of
temperature () and out-of-plane applied magnetic field (). We show that
topological Hall resistivity () scales with the density of
isolated skyrmions () over a wide range of , confirming the
impact of the skyrmion Berry-phase on electronic transport. We find that at
higher skyrmions cluster into worms which carry considerable
topological charge, unlike topologically-trivial spin spirals. While we
establish a qualitative agreement between and areal
density of topological charge , our detailed quantitative
analysis shows a much larger than the prevailing theory
predicts for observed .Comment: Major revision of the original version. The extensive Supplementary
Information is available upon reques
Magnetization dynamics and its scattering mechanism in thin CoFeB films with interfacial anisotropy
Studies of magnetization dynamics have incessantly facilitated the discovery
of fundamentally novel physical phenomena, making steady headway in the
development of magnetic and spintronics devices. The dynamics can be induced
and detected electrically, offering new functionalities in advanced electronics
at the nanoscale. However, its scattering mechanism is still disputed.
Understanding the mechanism in thin films is especially important, because most
spintronics devices are made from stacks of multilayers with nanometer
thickness. The stacks are known to possess interfacial magnetic anisotropy, a
central property for applications, whose influence on the dynamics remains
unknown. Here, we investigate the impact of interfacial anisotropy by adopting
CoFeB/MgO as a model system. Through systematic and complementary measurements
of ferromagnetic resonance (FMR), on a series of thin films, we identify
narrower FMR linewidths at higher temperatures. We explicitly rule out the
temperature dependence of intrinsic damping as a possible cause, and it is also
not expected from existing extrinsic scattering mechanisms for ferromagnets. We
ascribe this observation to motional narrowing, an old concept so far neglected
in the analyses of FMR spectra. The effect is confirmed to originate from
interfacial anisotropy, impacting the practical technology of spin-based
nanodevices up to room temperature.Comment: 23 pages,3 figure
A quantum phase transition from triangular to stripe charge order in NbSe
The competition between proximate electronic phases produces a complex
phenomenology in strongly correlated systems. In particular, fluctuations
associated with periodic charge or spin modulations, known as density waves,
may lead to exotic superconductivity in several correlated materials. However,
density waves have been difficult to isolate in the presence of chemical
disorder, and the suspected causal link between competing density wave orders
and high temperature superconductivity is not understood. Here we use scanning
tunneling microscopy to image a previously unknown unidirectional (stripe)
charge density wave (CDW) smoothly interfacing with the familiar
tri-directional (triangular) CDW on the surface of the stoichiometric
superconductor NbSe. Our low temperature measurements rule out thermal
fluctuations, and point to local strain as the tuning parameter for this
quantum phase transition. We use this discovery to resolve two longstanding
debates about the anomalous spectroscopic gap and the role of Fermi surface
nesting in the CDW phase of NbSe. Our results highlight the importance of
local strain in governing phase transitions and competing phenomena, and
suggest a new direction of inquiry for resolving similarly longstanding debates
in cuprate superconductors and other strongly correlated materials.Comment: PNAS in pres