arXiv subjects

Patrick S. Kirchmann

Publications and source records attributed to Patrick S. Kirchmann.

At least 19 recordsLinked to original sources

arXiv2026

Collective Magnetic Excitations in a Photo-excited Electron-doped Cuprate Superconductor

Elucidating the microscopic behavior of cuprates under ultrafast photoexcitation offers critical insights into their highly correlated out-of-equilibrium states. Although quasiparticle dynamics have been investigated extensively, the behavior of collective magnetic excitations remains comparatively unexplored. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-edge to track the collective magnetic excitations (paramagnons) in an optimally electron-doped cuprate driven out-of-equilibrium by a femtosecond pump laser pulse. Upon pumping, we observed an anti-Stokes signal associated with paramagnon generation, which modifies the paramagnon dispersion near the zone center, although the bandwidth remained unchanged. Moreover, the spectral weight exhibits a momentum-dependent variation across the Brillouin zone. The light-driven boost of the paramagnon population and the resulting spectral-weight transfer could provide new leverage to manipulate the properties of cuprates.

cond-mat.mtrl-sci↗

arXiv2025

Reconstructing the wavefunction of magnetic topological insulators MnBi2Te4 and MnBi4Te7 using spin-resolved photoemission

Despite their importance for exotic quantum effects, the surface electronic structure of magnetic topological insulators MnBi2Te4 and MnBi4Te7 remains poorly understood. Using high-efficiency spin- and angle-resolved photoemission spectroscopy, we directly image the spin-polarization and orbital character of the surface states in both compounds and map our observations onto a model wavefunction to describe the complex spin-orbital texture, which solidifies our understanding of the surface band structure by establishing the single-band nature of the most prominent states. Most importantly, our analysis reveals a new mechanism for reducing the magnetic gap of the topological surface states based on the orbital composition of the wavefunction.

cond-mat.mtrl-sci↗

arXiv2025

Analysis methodology of coherent oscillations in time- and angle-resolved photoemission spectroscopy

Oscillatory signals from coherently excited phonons are regularly observed in ultrafast pump-probe experiments on condensed matter samples. Electron-phonon coupling implies that coherent phonons also modulate the electronic band structure. These oscillations can be probed with energy and momentum resolution using time- and angle-resolved photoemission spectroscopy (trARPES) which reveals the orbital dependence of the electron-phonon coupling for a specific phonon mode. However, a comprehensive analysis remains challenging when multiple coherent phonon modes couple to multiple electronic bands. Complex spectral line shapes due to strong correlations in quantum materials add to this challenge. In this work, we examine how the frequency domain representation of trARPES data facilitates a quantitative analysis of coherent oscillations of the electronic bands. We investigate the frequency domain representation of the photoemission intensity and \tred{the first moment of the energy distribution curves}. Both quantities provide complimentary information and are able to distinguish oscillations of binding energy, linewidth and intensity.We analyze a representative trARPES dataset of the transition metal dichalcogenide WTe$_2$ and construct composite spectra which intuitively illustrate how much each electronic band is affected by a specific phonon mode. We also show how a linearly chirped probe pulse can generate extrinsic artifacts that are distinct from the intrinsic coherent phonon signal.

cond-mat.mtrl-sci↗

arXiv2024

Band structure of Bi surfaces formed on Bi2Se3 upon exposure to air

Bi$_2$Se$_3$ has been the focus of intense interest over the past decade due to its topological properties. Bi surfaces are known to form on Bi$_2$Se$_3$ upon exposure to atmosphere, but their electronic structure has not been investigated. We report band structure measurements of such Bi surfaces using angle-resolved photoemission spectroscopy. Measured spectra can be well explained by the band structure of a single bilayer of Bi on Bi$_2$Se$_3$, and show that Bi surfaces consistently dominate the photoemission signal for air exposure times of at least 1 hour. These results demonstrate that atmospheric effects should be taken into consideration when identifying two-dimensional transport channels, and when designing surface-sensitive measurements of Bi$_2$Se$_3$, ideally limiting air exposure to no more than a few minutes.

cond-mat.mes-hall↗

arXiv2023

Ultrafast measurements of mode-specific deformation potentials of Bi$_2$Te$_3$ and Bi$_2$Se$_3$

Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, topological superconductivity, and potentially how to manipulate the surface state using a structural degree of freedom. We perform time-resolved x-ray diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on Bi$_2$Te$_3$ and Bi$_2$Se$_3$, following the excitation of coherent A$_{1g}$ optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A$_{1g}$-like displacements in these two materials using the experimentally determined atomic displacements from XRD and electron band shifts from ARPES.We find the coupling in Bi$_2$Te$_3$ and Bi$_2$Se$_3$ to be similar and in general in agreement with expectations from density functional theory. We establish a methodology that quantifies the mode-specific electron-phonon coupling experimentally, allowing detailed comparison to theory. Our results shed light on fundamental processes in topological insulators involving electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast x-ray scattering reveals composite amplitude collective mode in the Weyl charge density wave material (TaSe$_4$)$_2$I

We report ultrafast x-ray scattering experiments of the quasi-1D charge density wave (CDW) material (TaSe$_4$)$_2$I following photoexcitation with femtosecond infrared laser pulses. From the time-dependent diffraction signal at the CDW sidebands we identify an amplitude mode derived primarily from the transverse acoustic component of the CDW static distortion. The dynamics of this acoustic amplitude mode are described well by a model of a displacive excitation, which we interpret as mediated through a coupling to the optical phonon component associated with the tetramerization of the Ta chains.

cond-mat.mtrl-sci↗

arXiv2022

Influence of local symmetry on lattice dynamics coupled to topological surface states

We investigate coupled electron-lattice dynamics in the topological insulator Bi2Te3 with time-resolved photoemission and time-resolved x-ray diffraction. It is well established that coherent phonons can be launched by optical excitation, but selection rules generally restrict these modes to zone-center wavevectors and Raman-active branches. We find that the topological surface state couples to additional modes, including a continuum of surface-projected bulk modes from both Raman- and infrared-branches, with possible contributions from surface-localized modes when they exist. Our calculations show that this surface vibrational spectrum occurs naturally as a consequence of the translational and inversion symmetries broken at the surface, without requiring the splitting-off of surface-localized phonon modes. The generality of this result suggests that coherent phonon spectra are useful by providing unique fingerprints for identifying surface states in more controversial materials. These effects may also expand the phase space for tailoring surface state wavefunctions via ultrafast optical excitation.

cond-mat.mtrl-sci↗

arXiv2021

Expanding the momentum field of view in angle-resolved photoemission systems with hemispherical analyzers

In photoelectron spectroscopy, the measured electron momentum range is intrinsically related to the excitation photon energy. Low photon energies $<10$ eV are commonly encountered in laser-based photoemission and lead to a momentum range that is smaller than the Brillouin zones of most materials. This can become a limiting factor when studying condensed matter with laser-based photoemission. An additional restriction is introduced by widely used hemispherical analyzers that record only electrons photoemitted in a solid angle set by the aperture size at the analyzer entrance. Here, we present an upgrade to increase the effective solid angle that is measured with a hemispherical analyzer. We achieve this by accelerating the photoelectrons towards the analyzer with an electric field that is generated by a bias voltage on the sample. Our experimental geometry is comparable to a parallel plate capacitor and, therefore, we approximate the electric field to be uniform along the photoelectron trajectory. With this assumption, we developed an analytic, parameter-free model that relates the measured angles to the electron momenta in the solid and verify its validity by comparing with experimental results on the charge density wave material TbTe$_3$. By providing a larger field of view in momentum space, our approach using a bias potential considerably expands the flexibility of laser-based photoemission setups.

physics.ins-det↗

arXiv2021

All-optical probe of three-dimensional topological insulators based on high-harmonic generation by circularly-polarized laser fields

We report the observation of a novel nonlinear optical response from the prototypical three-dimensional topological insulator Bi$_2$Se$_3$ through the process of high-order harmonic generation. We find that the generation efficiency increases as the laser polarization is changed from linear to elliptical, and it becomes maximum for circular polarization. With the aid of a microscopic theory and a detailed analysis of the measured spectra, we reveal that such anomalous enhancement encodes the characteristic topology of the band structure that originates from the interplay of strong spin-orbit coupling and time-reversal symmetry protection. Our study reveals a new platform for chiral strong-field physics and presents a novel, contact-free, all-optical approach for the spectroscopy of topological insulators. The implications are in ultrafast probing of topological phase transitions, light-field driven dissipationless electronics, and quantum computation.

cond-mat.mes-hall↗

arXiv2021

Connection between coherent phonons and electron-phonon coupling in Sb (111)

We report time- and angle-resolved photoemission spectroscopy measurements on the Sb(111) surface. We observe band- and momentum-dependent binding-energy oscillations in the bulk and surface bands driven by $A_{1g}$ and $E_{g}$ coherent phonons. While the bulk band shows simultaneous $A_{1g}$ and $E_{g}$ oscillations, the surface bands show either $A_{1g}$ or $E_{g}$ oscillations. The observed behavior is reproduced by frozen-phonon calculations based on density-functional theory. This evidences the connection between electron-phonon coupling and coherent binding energy dynamics.

cond-mat.mtrl-sci↗

arXiv2020

Quantum well states in fractured crystals of the heavy fermion material CeCoIn$_5$

Quantum well states appear in metallic thin films due to the confinement of the wave function by the film interfaces. Using angle-resolved photoemission spectroscopy, we unexpectedly observe quantum well states in fractured single crystals of CeCoIn$_5$. We confirm that confinement occurs by showing that these states' binding energies are photon-energy independent and are well described with a phase accumulation model, commonly applied to quantum well states in thin films. This indicates that atomically flat thin films can be formed by fracturing hard single crystals. For the two samples studied, our observations are explained by free-standing flakes with thicknesses of 206 and 101 Å. We extend our analysis to extract bulk properties of CeCoIn$_5$. Specifically, we obtain the dispersion of a three-dimensional band near the zone center along in-plane and out-of-plane momenta. We establish part of its Fermi surface, which corresponds to a hole pocket centered at $Γ$. We also reveal a change of its dispersion with temperature, a signature that may be caused by the Kondo hybridization.

cond-mat.str-el↗

arXiv2020

Tuning time and energy resolution in time-resolved photoemission spectroscopy with nonlinear crystals

Time- and angle-resolved photoemission spectroscopy is a powerful probe of electronic band structures out of equilibrium. Tuning time and energy resolution to suit a particular scientific question has become an increasingly important experimental consideration. Many instruments use cascaded frequency doubling in nonlinear crystals to generate the required ultraviolet probe pulses. We demonstrate how calculations clarify the relationship between laser bandwidth and nonlinear crystal thickness contributing to experimental resolutions and place intrinsic limits on the achievable time-bandwidth product. Experimentally, we tune time and energy resolution by varying the thickness of nonlinear $β$-BaB$_2$O$_4$ crystals for frequency up-conversion, providing for a flexible experiment design. We achieve time resolutions of 58 to 103 fs and corresponding energy resolutions of 55 to 27 meV.

cond-mat.mtrl-sci↗

arXiv2018

Band resolved imaging of photocurrent in a topological insulator

We study the microscopic origins of photocurrent generation in the topological insulator Bi$_2$Se$_3$ via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.

cond-mat.mtrl-sci↗

arXiv2017

Anomalous Hall Effect in ZrTe5

ZrTe$_5$ has been of recent interest as a potential Dirac/Weyl semimetal material. Here, we report the results of experiments performed via in-situ 3D double-axis rotation to extract the full $4π$ solid angular dependence of the transport properties. A clear anomalous Hall effect (AHE) was detected for every sample, with no magnetic ordering observed in the system to the experimental sensitivity of torque magnetometry. Interestingly, the AHE takes large values when the magnetic field is rotated in-plane, with the values vanishing above $\sim 60$ K where the negative longitudinal magnetoresistance (LMR) also disappears. This suggests a close relation in their origins, which we attribute to Berry curvature generated by the Weyl nodes.

cond-mat.mes-hall↗

arXiv2015

Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO$_{3}$ Films

Ultrathin FeSe films grown on SrTiO$_{3}$ substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. Using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO$_{3}$ films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump-probe delay for 1 unit cell, 3 unit cell, and 60 unit cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00(2) to 5.25(2) THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A$_\textrm{1g}$ phonon. The dominant mechanism for the phonon softening in 1 unit cell thick FeSe films is a substrate-induced lattice strain. Our results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.

cond-mat.mes-hall↗

arXiv2015

Inequivalence of Single-Particle and Population Lifetimes in a Cuprate Superconductor

We study optimally doped Bi-2212 ($T_\textrm{c}=96$~K) using femtosecond time- and angle-resolved photoelectron spectroscopy. Energy-resolved population lifetimes are extracted and compared with single-particle lifetimes measured by equilibrium photoemission. The population lifetimes deviate from the single-particle lifetimes in the low excitation limit by one to two orders of magnitude. Fundamental considerations of electron scattering unveil that these two lifetimes are in general distinct, yet for systems with only electron-phonon scattering they should converge in the low-temperature, low-fluence limit. The qualitative disparity in our data, even in this limit, suggests that scattering channels beyond electron-phonon interactions play a significant role in the electron dynamics of cuprate superconductors.

cond-mat.supr-con↗

arXiv2014

Distinguishing bulk and surface electron-phonon coupling in the topological insulator Bi2Se3 using time-resolved photoemission spectroscopy

We report time- and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi2Se3. We observe oscillatory modulations of the electronic structure of both the bulk and surface states at a frequency of 2.23 THz due to coherent excitation of an A1g phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. DFT calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A1g phonon mode coupling to the surface state in a topological insulator.

cond-mat.str-el↗

arXiv2014

Ultrafast Electron Dynamics in the Topological Insulator Bi2Se3 Studied by Time-Resolved Photoemission Spectroscopy

We characterize the topological insulator Bi$_2$Se$_3$ using time- and angle- resolved photoemission spectroscopy. By employing two-photon photoemission, a complete picture of the unoccupied electronic structure from the Fermi level up to the vacuum level is obtained. We demonstrate that the unoccupied states host a second, Dirac surface state which can be resonantly excited by 1.5 eV photons. We then study the ultrafast relaxation processes following optical excitation. We find that they culminate in a persistent non-equilibrium population of the first Dirac surface state, which is maintained by a meta-stable population of the bulk conduction band. Finally, we perform a temperature-dependent study of the electron-phonon scattering processes in the conduction band, and find the unexpected result that their rates decrease with increasing sample temperature. We develop a model of phonon emission and absorption from a population of electrons, and show that this counter-intuitive trend is the natural consequence of fundamental electron-phonon scattering processes. This analysis serves as an important reminder that the decay rates extracted by time-resolved photoemission are not in general equal to single electron scattering rates, but include contributions from filling and emptying processes from a continuum of states.

cond-mat.mtrl-sci↗