Alexander (Lex) Kemper

Principal investigator

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Email
akemperobfuscate@ncsu.edu

Dr. Kemper received his PhD in Physics in 2010 from the University of Florida. He spent 2 years as a postdoctoral research at Stanford University and SLAC National Laboratory followed by serving as an Alvarez Fellow at Lawrence Berkeley National Laboratory before joining the faculty at NC State in 2015 as an assistant professor.

During his PhD studies and continuing during his postdoctoral career, he used computational techniques to study the high-Tc cuprate superconductors, and the novel Fe-based superconductors that were discovered in 2006. Using a combination of Density Functional Theory, Quantum Cluster methods and computational modeling he addressed several aspects of the normal and superconducting states in these materials. Of particular note are a study of the gap symmetry in the Fe-based superconductors, as well as a joint experimental-theory paper showing the presence of four-fold symmetry breaking above the ordering temperatures in Co-doped BaFe2As2.

Starting during his postdoctoral career, and continuing now at NC State, he has embarked on a separate research project in non-equilibrium physics of complex materials. Non-equilibrium phenomena are studied experimentally using ultrafast lasers, which perturb the material under study and observing the subsequent dynamics. He has developed a set of software tools that theoretically model the pump-probe process in a variety of systems, which has to the first theoretical examinations of the pump-probe process. This has led to a series of works answering fundamental questions in non-equilibrium physics as well as collaborative works with experimentalists using pump-probe techniques to answer questions in materials physics.

Papers

Band resolved imaging of photocurrent in a topological insulator. Soifer et al., Phys. Rev. Lett. 2019

Nonequilibrium dynamics of spontaneous symmetry breaking into a hidden state of charge-density wave. Zhou et al., arXiv:1904.07120

Coherent Excitonic Quantum Beats in Time-Resolved Photoemission Measurements. Rustagi et al., Phys. Rev. B 2019

Higgs Oscillations in time-resolved Optical Conductivity. A. Kumar et al, arXiv:1902.09549

Direct determination of mode-projected electron-phonon coupling in the time-domain. Na et al., arXiv:1902.05572

Detailed band structure of twinned and detwinned BaFe2As2 studied with angle-resolved photoemission spectroscopy. H. Pfau et al., Phys. Rev. B 2019

Identifying a forward scattering superconductor through pump-probe spectroscopy. A. Kumar et al EPL 124 67002

Dense Electron–Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material Tuning. A.W. Bataller et al., Nano Letters Article ASAP 2019

Nonequilibrium Electron Dynamics in Pump-Probe Spectroscopy; Role of Excited Phonon Populations. O. Abdurazakov et al., Phys. Rev. B 2018

Interfacial Structure of SrZrxTi1−xO3 films on Ge. T Chen et al., Appl. Phys. Lett. 2018

All-optical nonthermal pathway to stabilizing magnetic Weyl semimetals in pyrochlore iridates. G.E. Topp et al., Nature Communications 2018

General principles for the non-equilibrium relaxation of populations in quantum materials. A.F. Kemper et al., Phys. Rev. X 2018

Spectral Evidence for Emergent Order in Ba1-xNaxFe2As2. M. Yi et al., Phys. Rev. Lett. 121, 127001 (2018)

Ultrafast multi-terahertz probes of symmetry breaking in a stripe-phase correlated oxide. G. Coslovich et al., Proc. SPIE 2018

Photoemission signature of excitons. A. Rustagi & A. F. Kemper, Phys. Rev. B 2018

Nonequilibrium electron and lattice dynamics of strongly correlated Bi2Sr2CaCu2O8+\delta single crystals. T. Konstantinova et al., SCIENCE ADVANCES 2018

Dynamics of correlation-frozen antinodal quasiparticles in superconducting cuprates. F. Cilento et al., SCIENCE ADVANCES 2018

Theoretical phase diagram for the room temperature Electron-Hole Liquid in photo-excited quasi-2D monolayer MoS2. A. Rustagi & A. F. Kemper, NANO LETTERS 2017

Ultrafast Dynamics of Vibrational Symmetry Breaking in a Charge-ordered Nickelate. G. Coslovich et al., SCIENCE ADVANCES 2017

Amplitude mode oscillations in pump-probe photoemission spectra from a d-wave superconductor. B. Nosarzewski et al., PHYSICAL REVIEW B 2017

Review of the theoretical description of time-resolved angle-resolved photoemission spectroscopy in electron-phonon mediated superconductors. A. F. Kemper et al., ANNALEN DER PHYSIK 2017

Surface vibrational modes of the topological insulator Bi2Se3 observed by Raman spectroscopy. H. Kung et al., PHYSICAL REVIEW B 2017

Relaxation of nonequilibrium populations after a pump, the breaking of Mathiessens rule. J. K. Freericks et al., ULTRAFAST BANDGAP PHOTONICS II 2017

Creating stable Floquet-Weyl semimetals by laser-driving of 3D Dirac materials. H. Huebener et al., NATURE COMMUNICATIONS 2017

Energy dissipation from a correlated material driven out of equilibrium. J. D. Rameau et al., NATURE COMMUNICATIONS 2016

Spin-Polarized Surface Resonances Accompanying Topological Surface State Formation. C. Jozwiak et al., NATURE COMMUNICATIONS 2016

Relationship between Population Dynamics and the Self-Energy in Driven Non-Equilibrium Systems. A.F. Kemper & J. Freericks, ENTROPY 2016

The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors. J. Hinton et al., SCIENTIFIC REPORTS 2016

Theory of light-enhanced phonon-mediated superconductivity. M. Sentef et al., PHYSICAL REVIEW B 2016

Ultrafast lattice dynamics of the electronically driven, lattice directed charge density wave in TbTe3. R. G. Moore et al., PHYSICAL REVIEW B 2016

Direct observation of Higgs mode oscillations in the pump-probe photoemission spectra of electron-phonon mediated superconductors. A.F. Kemper et al., PHYSICAL REVIEW B 2015

Bandwidth and Electron Correlation-Tuned Superconductivity in Rb0.8Fe2(Se1-zSz)2. M. Yi et al., PHYSICAL REVIEW LETTERS 2015

Scattering bottleneck for spin dynamics in metallic helical antiferromagnetic dysprosium. M. Langner et al., PHYSICAL REVIEW B 2015

Direct characterisation of photo-induced femtosecond lattice dynamics in BaFe2As2. S. Gerber et al., NATURE COMMUNICATIONS 2015

Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO3 Films. S. Yang et al., NANO LETTERS 2015

Classification of Collective Modes in a Charge Density Wave by Momentum-Dependent Modulation of the Electronic Band Structure. D. Leuenberger et al., PHYSICAL REVIEW B 2015

Theory of pump-probe photoemission in graphene; Ultrafast tuning of Floquet bands and local pseudospin textures. M. Sentef et al., NATURE COMMUNICATIONS 2015

First-principles study of the phonon modes in bismuth-sillenites. D. Arenas et al., PHYSICAL REVIEW B 2015

Numerical integration for ab initio many-electron self energy calculations within the GW approximation. F. Liu et al., JOURNAL OF COMPUTATIONAL PHYSICS 2015

Manipulation of gap nodes by uniaxial strain in iron-based superconductors. J. Kang et al., PHYSICAL REVIEW LETTERS 2014

Distinguishing Bulk and Surface Electron-Phonon Coupling in a Photoexcited Topological Insulator. J. Sobota et al., PHYSICAL REVIEW LETTERS 2014

Exact solution for high harmonic generation and the response to an AC driving field for a charge density wave insulator. W. Shen et al., PHYSICAL REVIEW B 2014

Effect of dynamical spectral weight distribution on effective interactions in time-resolved spectroscopy. A.F. Kemper et al., PHYSICAL REVIEW B 2014

Ultrafast electron dynamics in the topological insulator Bi2Se3 studied by time-resolved photoemission spectroscopy. J. Sobota et al., JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 2014

Bandgap closure and reopening in CsAuI3 at high pressure. S. Wang et al., PHYSICAL REVIEW B 2014

Dynamic competition between spin-density wave order and superconductivity in under doped Ba1-xKxFe2As2. M. Yi et al., NATURE COMMUNICATIONS 2014

Examining electron-boson coupling using time-resolved spectroscopy. M. Sentef et al., PHYSICAL REVIEW X 2013

Alternative route to charge density wave formation in multiband systems. H. Eiter et al., Physical Review X 2013

Tunneling spectroscopy for probing orbital anisotropy in iron pnictides. N. Plonka et al., PHYSICAL REVIEW B 2013

Direct optical coupling to an unoccupied Dirac surface state in the topological insulator Bi2Se3. J. Sobota et al., PHYSICAL REVIEW LETTERS 2013

Time-dependent charge-order and spin-order recovery in striped systems. Y. F. Kung et al., PHYSICAL REVIEW B 2013

Electron-Mediated Relaxation Following Ultrafast Pumping of Strongly Correlated Materials; Model Evidence of a Correlation-Tuned Crossover between Thermal and Nonthermal States. B. Moritz et al., PHYSICAL REVIEW LETTERS 2013

Mapping of unoccupied states and relevant bosonic modes via the time dependent momentum distribution. A.F. Kemper et al., PHYSICAL REVIEW B 2013

Infrared study of the electronic structure of the metallic pyrochlore iridate Bi2Ir2O7. Y. S. Lee et al., PHYSICAL REVIEW B 2013

Structure and functionality of bromine doped graphite. R. Hamdan et al., JOURNAL OF CHEMICAL PHYSICS 2013

Hot electron transport in a strongly correlated transition-metal oxide. K. G. Rana et al., SCIENTIFIC REPORTS 2013

Theoretical description of high-order harmonic generation in solids. A.F. Kemper et al., NEW JOURNAL OF PHYSICS 2013

Pulsed high harmonic generation of light due to pumped Bloch oscillations in noninteracting metals. J. Freericks et al., PHYSICA SCRIPTA 2012

Phase fluctuations and the absence of topological defects in a photo-excited charge-ordered nickelate. W. S. Lee et al., NATURE COMMUNICATIONS 2012

Density functional study of gold and iron clusters on perfect and defected graphene. M. K. Srivastava et al., PHYSICAL REVIEW B 2012

Anisotropic quasiparticle lifetimes in Fe-based superconductors. A.F. Kemper et al., PHYSICAL REVIEW B 2011

Symmetry breaking orbital anisotropy on detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition. M. Yi et al., PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 2011

Theory of Two-Magnon Raman Scattering in Iron Pnictides and Chalcogenides. C. Chen et al., PHYSICAL REVIEW LETTERS 2011

Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides. A.F. Kemper et al., NEW JOURNAL OF PHYSICS 2010

Spin fluctuations and superconductivity in a three-dimensional tight-binding model for BaFe2As2. S. Graser et al., PHYSICAL REVIEW B 2010

Effects of cobalt doping and three-dimensionality in BaFe2As2. A.F. Kemper et al., PHYSICAL REVIEW B 2009

Curvature effect on the interaction between folded graphitic surface and silver clusters. A.F. Kemper et al., PHYSICAL REVIEW B 2009

Insensitivity of d-wave pairing to disorder in the high-temperature cuprate superconductors. A.F. Kemper et al., PHYSICAL REVIEW B 2009

Nonequilibrium Green's function study of Pd4-cluster-functionalized carbon nanotubes as hydrogen sensors. C. Cao et al., PHYSICAL REVIEW B 2009