Academic Commons Search Results
http://academiccommons.columbia.edu/catalog.rss?f%5Bsubject_facet%5D%5B%5D=Condensed+matter+physics&q=&rows=500&sort=record_creation_date+desc
Academic Commons Search Resultsen-usMicroscopic theories of excitons and their dynamics
http://academiccommons.columbia.edu/catalog/ac:177103
Berkelbach, Timothy Charleshttp://dx.doi.org/10.7916/D8BP010FFri, 01 Aug 2014 18:19:23 +0000This thesis describes the development and application of microscopically-defined theories of excitons in a wide range of semiconducting materials. In Part I, I consider the topic of singlet exciton fission, an organic photophysical process which generates two spin-triplet excitons from one photoexcited spin-singlet exciton. I construct a theoretical framework that couples a realistic treatment of the static electronic structure with finite-temperature quantum relaxation techniques. This framework is applied separately, but consistently, to the problems of singlet fission in pentacene dimers, crystalline pentacene, and crystalline hexacene. Through this program, I am able to rationalize observed behaviors and make non-trivial predictions, some of which have been confirmed by experiment.
In Part II, I present theoretical developments on the properties of neutral excitons and charged excitons (trions) in atomically thin transition metal dichalcogenides. This work includes an examination of material trends in exciton binding energies via an effective mass approach. I also present an experimental and theoretical collaboration, which links the unconventional disposition of excitons in the Rydberg series to the peculiar screening properties of atomically thin materials. The light-matter coupling in these materials is examined within low-energy models and is shown to give rise to bright and dark exciton states, which can be qualitatively labeled in analogy with the hydrogen series.
In Part III, I explore theories of relaxation dynamics in condensed phase environments, with a focus on methodology development. This work is aimed towards biological processes, including resonant energy transfer in chromophore complexes and electron transfer in donor-bridge-acceptor systems. Specifically, I present a collaborative development of a numerically efficient but highly accurate hybrid approach to reduced dynamics, which exploits a partitioning of environmental degrees of freedom into those that evolve "fast" and "slow," as compared to the internal system dynamics. This method is tested and applied to the spin-boson model, a two-site Frenkel exciton model, and the seven-site Fenna-Matthews-Olson complex. I conclude with a collaborative analysis of a recently developed polaron-transformed quantum master equation, which is shown to accurately interpolate between the well-known Redfield and Forster theories, even in challenging donor-bridge-acceptor arrangements.Physical chemistry, Condensed matter physics, Chemistrytcb2112Chemical Physics, ChemistryDissertationsEffect of Surface Curvature and Chemistry on Protein Stability, Adsorption and Aggregation
http://academiccommons.columbia.edu/catalog/ac:177097
Radhakrishna, Mithunhttp://dx.doi.org/10.7916/D8VM49GHThu, 31 Jul 2014 12:20:46 +0000Enzyme immobilization has been of great industrial importance because of its use in various applications like bio-fuel cells, bio-sensors, drug delivery and bio-catalytic films. Although research on enzyme immobilization dates back to the 1970's, it has been only in the past decade that scientists have started to address the problems involved systematically. Most of the previous works on enzyme immobilization have been retrospective in nature i.e enzymes were immobilized on widely used substrates without a compatibility study between the enzyme and the substrate. Consequently, most of the enzymes lost their activity upon immobilization onto these substrates due to many governing factors like protein-surface and inter-protein interactions. These interactions also play a major role biologically in cell signaling, cell adhesion and inter-protein interactions specifically is believed to be the major cause for neurodegenerative diseases like Alzheimer's and Parkinson's disease. Therefore understanding the role of these forces on proteins is the need of the hour. In my current research, I have mainly focused on two factors a) Surface Curvature b) Surface Chemistry as both of these play a pivotal role in influencing the activity of the enzymes upon immobilization. I study the effect of these factors computationally using a stochastic method known as Monte Carlo simulations.
My research work carried out in the frame work of a Hydrophobic-Polar (HP) lattice model for the protein shows that immobilizing enzymes inside moderately hydrophilic or hydrophobic pores results in an enhancement of the enzymatic activity compared to that in the bulk. Our results also indicate that there is an optimal value of surface curvature and hydrophobicity/hydrophilicity where this enhancement of enzymatic activity is highest. Further, our results also show that immobilization of enzymes inside hydrophobic pores of optimal sizes are most effective in mitigating protein-aggregation. These results provide us a rationale to understand the role of chaperonins in protein folding and disaggregation. Our results indicate that strong protein-surface interactions and confinement inducement stability inside pores makes it best suitable for enzyme immobilization.Chemical engineering, Condensed matter physicsmr2972Chemical EngineeringDissertationsOptical and Electrical Properties of Single-walled Carbon Nanotubes with Known Chiralities
http://academiccommons.columbia.edu/catalog/ac:175522
Zhang, Zhengyihttp://dx.doi.org/10.7916/D8F769PFMon, 07 Jul 2014 11:34:58 +0000Carbon nanotube (CNT) is a hollow structure consisted by one-atom-thick sheet of carbon atoms, which can be considered as a rolled-up graphene sheet. The diameter and rolling angle (chirality) uniquely determines its electronic structure. Over two decades of study, due to the difficulty of synthesizing clean individual CNTs and the limitation of accurate chirality characterization, there are still unveiled questions towards the intrinsic properties of this 1-D material at single molecular level. In this thesis, I will discuss the approaches of fabricating chirality assigned CNT device and the experimental results of its optical and electrical properties.
In the first part, I describe using 'fast heating' chemical vapor deposition (CVD) method to achieve the high quality suspended CNT growth. Combining Rayleigh and Raman spectroscopy, I demonstrate the accurate assignment of chirality for each suspended individual CNT.
With the ability of chirality identification, a series of optical and electrical experiments were conducted on the selected CNTs of interest. In the following part, I first discuss the probe of many-body effect in a semiconducting CNT by observing the elastic scattering (Rayleigh spectra) with electrostatic gating. We found the dominant short-range interaction is reduced to 85% of its intrinsic strength for doping level of ρ=0.4e/nm, demonstrating the possible control of sub-band exciton resonance frequency without rely on Pauli-blocking effect in CNTs.
In order to study the substrate effect in electrical transport of CNTs, I improved the transfer technique to accurately place individual CNT on a specific substrate. With this technique, I've achieved transferring individual CNT on 20µmx20µm thin layer of hexagonal-boron nitride (h-BN) substrate with a ± 5µm error.
The low field electrical transport studies were conducted on both metallic and semiconducting CNTs with known chiralities on h-BN.
Temperature dependent measurement shows the resistivity becomes super-linear around 250K, consistent with the prediction that the surface polar phonon of h-BN couples with electrons in CNT at higher phonon energy than SiO₂. Moreover, the FET devices of CNT on h-BN with graphite local back gate show hysteresis free feature in vacuum, and the subthreshold swing of 118mV/dec is comparable to high κ dielectric HfO₂ based device.Materials science, Condensed matter physics, NanotechnologyMechanical EngineeringDissertationsA Correlated 1-D Monatomic Condensed Matter System: Experiment and Theory
http://academiccommons.columbia.edu/catalog/ac:168505
Zaki, Nader Wasfyhttp://dx.doi.org/10.7916/D89S1P0DMon, 06 Jan 2014 16:48:42 +0000A one-dimensional quantum mechanical system is experimentally synthesized and investigated for physical phenomena that it may inherit due to quantum confinement and electron correlations. The experimentally realized system is a self-assembled array of monatomic cobalt wires that are grown under ultra-high vacuum conditions on a vicinal copper (111) substrate using a recipe developed specifically for this work. This work experimentally demonstrates that this 1-D system undergoes a charge density wave instability, which is a first for such a 1-D phenomenon on a metallic substrate. It is determined experimentally that this 1-D system undergoes an electronic phase transition at a temperature of about 85K, in which the higher temperature electronic phase is itinerant rather than localized. Using ab initio density functional theory, the cause of the measured charge density wave instability is assigned to erromagnetic interactions along the chain. Specifically, it is deduced that the instability is driven by spin -minority pin-exchange interactions predominately in the cobalt dxz/dyz orbitals. Beyond, shedding light on electron correlations in a physically realized quantum mechanical 1-D system, this work demonstrates that this particular system is a new test-case example for advanced theoretical techniques in predicting the correct structural ground phase.Condensed matter physicsnz2137Electrical EngineeringDissertationsThe Effective Field Theory Approach to Fluid Dynamics
http://academiccommons.columbia.edu/catalog/ac:161458
Endlich, Solomonhttp://hdl.handle.net/10022/AC:P:20419Thu, 23 May 2013 11:24:11 +0000In this thesis we initiate a systematic study of fluid dynamics using the effective field theory (EFT) program. We consider the canonical quantization of an ordinary fluid in an attempt to discover if there is some kind of quantum mechanical inconsistency with ordinary fluids at zero temperature. The system exhibits a number of peculiarities associated with the vortex degrees of freedom. We also study the dynamics of a nearly incompressible fluid via (classical) effective field theory. In the kinematical regime corresponding to near incompressibility (small fluid velocities and accelerations), compressional modes are, by definition, difficult to excite, and can be dealt with perturbatively. We systematically outline the corresponding perturbative expansion, which can be thought of as an expansion in the ratio of fluid velocity and speed of sound. This perturbation theory allows us to compute many interesting quantities associated with sound-flow interactions. Additionally, we also improve on the so-called vortex filament model, by providing a local field theory describing the dynamics of vortex-line systems and their interaction with sound, to all orders in perturbation theory. Next, we develop a cosmological model where primordial inflation is driven by a 'solid'. The low energy EFT describing such a system is just a less symmetric version of the action of a fluid---it lacks the volume preserving diffeomorphism. The symmetry breaking pattern of this system differs drastically from that of standard inflationary models: time translations are unbroken. This prevents our model from fitting into the standard effective field theory description of adiabatic perturbations, with crucial consequences for the dynamics of cosmological perturbations. And finally, we introduce dissipative effects in the effective field theory of hydrodynamics. We do this in a model-independent fashion by coupling the long-distance degrees of freedom explicitly kept in the effective field theory to a generic sector that "lives in the fluid'', which corresponds physically to the microscopic constituents of the fluid. At linear order in perturbations, the symmetries, the derivative expansion, and the assumption that this microscopic sector is thermalized, allow us to characterize the leading dissipative effects at low frequencies via three parameters only, which correspond to bulk viscosity, shear viscosity, and---in the presence of a conserved charge---heat conduction. Using our methods we re-derive the Kubo relations for these transport coefficients.Theoretical physics, Physics, Condensed matter physicssge2104PhysicsDissertationsCluster Dynamical Mean-Field Theory: Applications to High-Tc Cuprates and to Quantum Chemistry
http://academiccommons.columbia.edu/catalog/ac:147701
Lin, Nanhttp://hdl.handle.net/10022/AC:P:13443Thu, 07 Jun 2012 15:19:08 +0000In this thesis we use the recently developed dynamical mean-field approximation to study problems in strongly correlated electron systems, including high-Tc cuprate superconductors as well as a few quantum chemical reference systems. We start with an introduction to the background of the interacting electron systems, followed by a brief description on the current understanding of the physics of high-Tc cuprate superconductors. The approximate models that enter the theoretical framework will be discussed afterwards. Some quantum chemical methods for many-body quantum systems are included for review. Next we present the numerical methods employed in our study. The formalism of the dynamical mean-field approximation will be introduced including the single-site and cluster versions, followed by the Exact Diagonalization impurity solver for the solution of the quantum impurity model. Maximum Entropy analytic continuation method is also discussed, which is useful to obtain the physically relevant response functions. Then we apply dynamical mean-field approximation to high-Tc cuprate superconductors. The two-particle response functions, such as Raman scattering intensity and optical conductivity, are computed for the two dimensional Hubbard model. The calculations include the vertex corrections which are essential to obtain physically reasonable results in interacting electron systems. We also study the physics of the pseudogap in cuprates. The suppression of density of states near Fermi surface is present in our calculations, which is in qualitative agreement with the experimental data. Finally we discuss the application of dynamical mean-field theory to quantum chemistry. We extend the formalism of dynamical mean-field approximation to finite systems, and compare its performance in hydrogen clusters with different spatial configurations to other leading quantum chemical approaches. Dynamical mean-field theory involves mapping onto a quantum impurity model. We further examine the quantum impurity model representation of the transition metal dioxide molecules. The conceptual and technical difficulties will be discussed.Condensed matter physicsnl2219Physics, ChemistryDissertationsQuantum transport in graphene heterostructures
http://academiccommons.columbia.edu/catalog/ac:143085
Young, Andrea Franchinihttp://hdl.handle.net/10022/AC:P:12169Tue, 10 Jan 2012 15:00:12 +0000The two dimensional charge carriers in mono- and bilayer graphene are described by massless and massive chiral Dirac Hamiltonians, respectively. This thesis describes low temperature transport experiments designed to probe the consequences of this basic fact. The first part concerns the effect of the lattice pseudospin, an analog of a relativistic electron spin, on the scattering properties of mono- and bilayer graphene. We fabricate graphene devices with an extremely narrow local gates, and study ballistic carrier transport through the resulting barrier. By analyzing the interference of quasiparticles confined to the region beneath the gate, we are able to determine that charge carriers normally incident to the barrier are transmitted perfectly, a solid state analog of the Klein tunneling of relativistic quantum mechanics. The second part of the work describes the development of hexagonal boron nitride (hBN), an insulating isomorph of graphite, as a substrate and gate dielectric for graphene electronics. We use the enhanced mobility of electrons in h-BN supported graphene to investigate the effect of electronic interactions. We find interactions drive spontaneous breaking of the emergent SU(4) symmetry of the graphene Landau levels, leading to a variety of quantum Hall isospin ferromagnetic (QHIFM) states, which we study using tilted field magnetotransport. At yet higher fields, we observe fractional quantum Hall states which show signatures of the unique symmetries and anisotropies of the graphene QHIFM. The final part of the thesis details a proposal and preliminary experiments to probe isospin ordering in bilayer graphene using capacitance measurements.Condensed matter physicsafy2003PhysicsDissertationsSpectroscopy of Two Dimensional Electron Systems Comprising Exotic Quasiparticles
http://academiccommons.columbia.edu/catalog/ac:143043
Rhone, Trevor David Nathanielhttp://hdl.handle.net/10022/AC:P:12155Tue, 10 Jan 2012 12:01:23 +0000In this dissertation I present inelastic and elastic light scattering studies of collective states emerging from interactions in electron systems confined to two dimensions. These studies span the first, second and third Landau levels. I report for the first time, high energy excitations of composite fermions in the quantum fluid at v = 1/3. The high energies discovered represent excitations across multiple composite fermion energy levels, demonstrating the topological robustness of the fractional quantum Hall state at v = 1/3. This study sets the ground work for similar measurements of states in the second Landau level, such as those at v = 5/2. I present the first light scattering studies of low energy excitations of quantum fluids in the second Landau level. The study of low energy excitations of the quantum fluid at 3 ≥ v ≥ 5/2 reveals a rapid loss of spin polarization for v ≤ 3, as monitored by the intensity of the spin wave excitation at the Zeeman energy. The emergence of a continuum of low-lying excitations for v ≤ 3 reveals competing quantum phases in the second Landau level with intriguing roles of spin degrees of freedom and phase inhomogeneity. The first light scattering studies of the electron systems in the third Landau level are reported here. Measurements of low energy excitations and their spin degrees of freedom reveal contrasting behavior of states in the second and third Landau levels. I discuss these measurements in the context of the charge density wave phases, that are believed, by some, to dominate the third Landau level, and suggest ways of verifying this belief using light scattering. Distinct behavior in the dispersion of the spin wave at v = 3 is measured for the first time. The study may highlight differences in the first and second Landau levels that are manifested through the electron wavefunctions. In addition to intra-Landau level measurements, inter-Landau level studies are also reported. The results of which reveal roles of spin degrees of freedom and many body interactions in odd denominator integer quantum Hall states.Condensed matter physics, Optics, Quantum physicstnr2103Physics, Applied Physics and Applied MathematicsDissertationsFingerprinting analysis of non-crystalline pharmaceutical compounds using high energy X-rays and the total scattering pair distribution function
http://academiccommons.columbia.edu/catalog/ac:136566
Davis, Timur D.http://hdl.handle.net/10022/AC:P:10800Tue, 02 Aug 2011 15:00:25 +0000In the development of new medicinal products, poor oral bioavailability, due to the low solubilities of many active pharmaceutical ingredients (APIs), is increasingly a barrier for treatments to be administered using tablet or capsule formulations and one of the main challenges facing the pharmaceutical industry. Non-crystalline phases such as the amorphous and nanostructured states can confer increased solubility to a drug, and therefore, have recently garnered a lot of interest from pharmaceutical researchers. However, little is known about local ordering in non-crystalline pharmaceuticals due to the lack of reliable experimental probes, hindering the clinical application of these compounds. The powerful tools of crystallography begin to lose their potency for structures on the nanoscale; conventional X-ray powder diffraction (XRPD) patterns become broad and featureless in these cases and are not useful for differentiating between different local molecular packing arrangements. In this thesis, we introduce the use of high energy X-rays coupled with total scattering pair distribution function (TSPDF) and fingerprinting analysis to investigate the local structures of non-crystalline pharmaceutical compounds. The high energy X-rays allow us to experimentally collect diffuse scattering intensities, which contain information about a sample's local ordering, in addition to the Bragg scattering available in conventional XRPD experiments, while the TSPDF allows us to view the intra- and inter-molecular correlations in real space. The goal of this study was to address some fundamental problems involving fingerprinting non-crystalline APIs using TSPDF in order to lay the groundwork for the proper use of the technique by the pharmaceutical community. We achieved this by developing the methodology as well as the exploring the scientific implications. On the methodology side, we introduced PDFGetX3, a new software program for calculating TSPDFs that simplifies the procedure and reduces user interaction. We also set a baseline for the minimum X-ray energy that is needed for fingerprinting analysis, which had implications on the type of X-ray diffractometers that can be used. On the science side, we investigated the local structures of nanocrystalline and amorphous materials as well mixtures containing crystalline and amorphous phases. First, we identified a non-crystalline sample of the mood-stabilizing drug carbamazepine as a nanocrystalline version of one of its polymorphs. Next, we found that amorphous forms created by spray drying and cryomilling a proprietary compound have the same local structure. Finally, we quantified the phase fractions of polymorphic and amorphous components in a sample of the antibiotic sulfamerazine that was recrystallizing from a cryomilling-induced amorphous state.Materials science, Condensed matter physics, Pharmaceutical sciencestd2218Applied Physics and Applied MathematicsDissertationsInvestigations of the Band Structure and Morphology of Nanostructured Surfaces
http://academiccommons.columbia.edu/catalog/ac:135348
Knox, Kevin R.http://hdl.handle.net/10022/AC:P:10649Wed, 06 Jul 2011 15:45:10 +0000Two-dimensional electronic systems have long attracted interest in the physics and material science communities due to the exotic physics that arises from low-dimensional confinement. Studying the electronic behavior of 2D systems can provide insight into a variety of phenomena that are important to condensed-matter physics, including epitaxial growth, two-dimensional electron scattering and many-body physics. Correlation effects are strongly influenced by dimensionality, which determines the many-body excitations available to a system. In this dissertation, I examine the electronic structure of two very dierent types of two-dimensional systems: valence band electrons in single layer graphene and electronic states created at the vacuum interface of single crystal copper surfaces.The characteristics of both electronic systems depend intimately on the morphology of the surfaces they inhabit. Thus, in addition to discussing the respective band structures of these systems, a significant portion of this dissertation will be devoted to measurements of the surface morphology of these systems. Free-standing exfoliated monolayer graphene is an ultra-thin flexible membrane and, as such, is known to exhibit large out-of-plane deformation due to substrate and adsorbate interaction as well as thermal vibrations and, possibly, intrinsic buckling. Such crystal deformation is known to limit mobility and increase local chemical reactivity. Additionally, deformations present a measurement challenge to researchers wishing to determine the band structure by angle-resolved photoemission since they limit electron coherence in such measurements. In this dissertation, I present low energy electron microscopy and microprobe diffraction measurements, which are used to image and characterize corrugation in SiO2-supported and suspended exfoliated graphene at nanometer length scales. Diffraction line-shape analysis reveals quantitative differences in surface roughness on length scales below 20 nm which depend on film thickness and interaction with the substrate. Corrugation decreases with increasing film thickness, reflecting the increased stiffness of multilayer films. Specifically, single-layer graphene shows a markedly larger short range roughness than multilayer graphene. Due to the absence of interactions with the substrate, suspended graphene displays a smoother morphology and texture than supported graphene. A specific feature of suspended single-layer films is the dependence of corrugation on both adsorbate load and temperature, which is manifested by variations in the diffraction lineshape. The effects of both intrinsic and extrinsic corrugation factors will be discussed. Through a carefully coordinated study I show how these surface morphology measurements can be combined with angle resolved photoemission measurements to understand the role of surface corrugation in the ARPES measurement process. The measurements described here rely on the development of an analytical formulation for relating the crystal corrugation to the photoemission linewidth. I present ARPES measurements that show that, despite signicant deviation from planarity of the crystal, the electronic structure of exfoliated suspended graphene is nearly that of ideal, undoped graphene; the Dirac point is measured to be within 25 meV of EF . Further, I show that suspended graphene behaves as a marginal Fermi-liquid, with a quasiparticle lifetime which scales as (E - EF)-1; comparison with other graphene and graphite data is discussed. Image and surface states formed at the vacuum interface of a single crystal provide another example of a two dimensional electronic system. As with graphene, the surface quality and morphology strongly inuence the physics in this 2D electronic system. However, in contrast to graphene, which must be treated as a flexible membrane with continuous height variation, roughness in clean single crystal surfaces arises from lattice dislocations, which introduce discrete height variations. Such height variations can be exploited to generate a self assembled nano-structured surface. In particular, by making a vicinal cut on a single crystal surface, a nanoscale step array can be formed. A model system for such nanoscale self assembly is Cu(111). Cu(775) is formed by making an 8.5° viscinal cut of Cu(111) along the [11 -2] axis. The electronic states formed on the surface of this system, with a nanoscale step array of 14 Å terraces, shows markedly different behavior those formed on Cu(111). In this dissertation, I show that the tunability of a femtosecond optical parametric oscillator, combined with its high-repetition rate and short pulse length, provides a powerful tool for resonant band mapping of the sp surface and image states on flat and vicinal Cu(111)- Cu (775) surfaces, over the photon energy range from 3.9 to 5 eV. Since the time scale for excitation of the metal image state from the Cu surface state is comparable with the electron-electron equilibration time scale, sharp features are measured due to resonant excitation in the photoelectron energy distribution curves. In addition, I explore the range of photon energies and optical intensities which may be used for this approach and show that, despite the relatively high pump intensity, the 250 kHz repetition rate of this laser ameliorates the space-charge broadening and electron-energy shifting even for photon energies close to the vacuum edge. The strong excitation conditions generated by a femtosecond laser pulse applied to a Cu surface also allow the excitation and observation of a recently measured bulk state. In this dissertation I show that angle-resolved, tunable, two-photon photoemission (2PPE) can be used to map a bulk unoccupied band, viz. the Cu sp band 0 to 1 eV below the vacuum level, in the vicinity of the L point. This short-lived bulk band can be accessed using our setup due to the strong optical pump rate. I describe how photoemission from this state can be distinguished from photoemission from 2D states which is also present in the data. In particular, the variation of the initial-state energy with photon energy has a measured slope of ~ 1.64 in contrast with values of 1 or 2 observed for 2PPE from two-dimensional (2D) states. This unique variation illustrates the significant role of the perpendicular momentum of initial and final states in interpreting 2PPE data.Condensed matter physicskrk19PhysicsDissertations