arXiv subjects

Paulina Majchrzak

Publications and source records attributed to Paulina Majchrzak.

At least 19 recordsLinked to original sources

arXiv2026

Spectroscopic signatures of persistent exciton condensation in a bulk magnetic topological insulator

Exciton condensates are long-sought correlated quantum states arising from macroscopic coherence of bound electron-hole pairs. Although equilibrium and transient excitonic states have been reported in several material platforms, direct evidence for a light-induced exciton condensate state has been challenging to achieve as an intrinsic property of a bulk quantum material. Here, we use time- and angle-resolved photoemission spectroscopy to investigate long-lived photoexcited carriers in the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ with the chemical potential tuned to the topological surface state by Sb substitution. Following optical excitation, we observe the delayed emergence of a transient state whose formation coincides with depopulation of the bulk conduction band and whose lifetime extends to the microsecond timescale. The quasiparticle dispersion exhibits a pronounced flattening and develops a Mexican-hat-like profile. These spectral signatures are consistent with the formation of a metastable excitonic condensate state. Our results establish magnetic topological insulators as a promising platform for investigating long-lived photoinduced many-body states and their interplay with topology and magnetism.

cond-mat.mes-hall↗

arXiv2026

Selective coupling of high-order phonons in La2-xSrxCuO4

In crystals and molecules with strong coupling between the charge carriers and atomic vibrations, high order harmonics of the vibrational excitations can be observed with intensity following the Franck-Condon envelope. Here, we uncover a new regime of electron-phonon coupling in La2-xSrxCuO4 using resonant inelastic X-ray scattering. In the undoped compound, we find sharp peaks at approximately 85$\pm$5 meV, 180$\pm$7 meV, and 330$\pm$5 meV with no observable dispersion, and an absence of appreciable intensity from other modes observed in Raman spectroscopy in this energy range. The 180 meV and 330 meV excitations have energies approximately consistent with two-phonon and four-phonon in-plane Cu-O bond stretching mode, suggesting that the resonantly excited valence electrons strongly couple to this mode. These excitations exhibit significant doping dependence, eventually becoming unresolvable at 10% hole doping, where the low energy charge excitations are dominated by dispersive plasmons. The observation of selective coupling to even-ordered phonons and stark contrast with the Raman spectra indicate anomalous electron-phonon coupling beyond the Franck-Condon picture. One intriguing possibility is the existence of locally paired quasiparticles in insulating cuprates.

cond-mat.str-el↗

arXiv2026

Direct visualization of gate-tunable flat bands in twisted double bilayer graphene

The symmetry-broken correlated states in twisted double bilayer graphene (TDBG) can be tuned via several external knobs, including twist angle, displacement field, and carrier density. However, a direct, momentum-resolved characterization of how these parameters reshape the flat-band structure remains limited. In this study, we employ micro focused angle-resolved photoemission spectroscopy to investigate the flat-band dispersion of TDBG at a twist angle of 1.6, systematically varying the displacement field and carrier density via electrostatic gating. We directly observe multiple flat moir'e minibands near charge neutrality, including a flat remote valence band residing below the low-energy flat-band manifold. Furthermore, the dominant Coulomb repulsive energy over the flat- band bandwidth suggests favorable conditions for the emergence of interaction-driven correlated phenomena in TDBG. These findings establish that the formation and evolution of flat bands in TDBG arises from the interplay between the electron filling and the displacement field.

cond-mat.mes-hall↗

arXiv2025

Clustering-Enhanced Time- and Angle-Resolved Photoemission Study of LaTe$_3$: Absence of a Photoinduced Secondary CDW in the Electronic Structure

Optical control offers a compelling route for tailoring material properties on an ultrafast time scale. Ordered states such as charge density waves (CDWs) can be transiently melted by an ultrafast light excitation. This is also the case for the rare-earth tritelluride LaTe$_3$, a prototypical CDW compound. For this material it has recently been reported that the suppression of the primary CDW allows the transient formation of a second CDW, whose wave vector is orthogonal to the primary one. This creates the intriguing scenario where light enables switching between two distinct ordered phases of the material. While the second CDW has so far been observed by structural techniques, it remains an open question how the interplay of the two CDW phases is reflected in the material's electronic structure. We investigate this via time- and angle-resolved photoemission measurements of LaTe$_3$. The complex Fermi contour is probed using a FeSuMa analyzer, which records the photoemission intensity of the entire Fermi contour at once. The dynamics revealed by the FeSuMa analyzer are complemented by measurements using a conventional hemispherical electron analyzer. We combine conventional data analysis with $k$-means clustering, an unsupervised machine learning technique, demonstrating its strong potential for disentangling large datasets. While we do not find any features that cannot be explained by the melting and re-establishment of the primary CDW, distinct dynamics and coherent oscillations are observed in the different branches of the Fermi contour.

cond-mat.str-el↗

arXiv2025

Resonantly enhanced photoemission from topological surface states in MnBi$_6$Te$_{10}$

The dispersion of topological surface bands in MnBi$_2$Te$_4$-based magnetic topological insulator heterostructures is strongly affected by band hybridization and is spatially inhomogeneous due to varying surface layer terminations on microscopic length scales. Here, we apply micro-focused angle-resolved photoemission spectroscopy with tunable photon energy from 18 to 30 eV to distinguish bulk valence and conduction bands from surface bands on the three surface terminations of MnBi$_6$Te$_10$. We observe a strong enhancement of photoemission intensity from the topological surface bands at the Bi O4 absorption edge, which is exploited to visualize a gapless Dirac cone on the MnBi$_2$Te$_4$-terminated surface and varying degrees of hybridization effects in the surface bands on the two distinct Bi$_2$Te$_3$-terminated surfaces.

cond-mat.mtrl-sci↗

arXiv2025

Non-monotonic band flattening near the magic angle of twisted bilayer MoTe$_2$

Twisted bilayer MoTe$_2$ (tMoTe$_2$) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemission spectroscopy (ARPES) measurements of tMoTe$_2$, revealing pronounced twist-angle-dependent band reconstruction shaped by orbital character, interlayer coupling, and moiré potential modulation. Density functional theory (DFT) captures the qualitative evolution, yet underestimates key energy scales across twist angles, highlighting the importance of electronic correlations. Notably, the hole effective mass at the K point exhibits a non-monotonic dependence on twist angle, peaking near 2°, consistent with band flattening at the magic angle predicted by continuum models. Via electrostatic gating and surface dosing, we further visualize the evolution of electronic structure versus doping, enabling direct observation of the conduction band minimum and confirm tMoTe$_2$ as a direct band gap semiconductor. These results establish a spectroscopic foundation for modeling and engineering emergent quantum phases in this moiré platform.

cond-mat.mes-hall↗

arXiv2025

Line shapes in time- and angle-resolved photoemission spectroscopy explored by machine learning

Time- and angle-resolved photoemission spectroscopy is a powerful technique for investigating the dynamics of excited carriers in quantum materials. Typically, data analysis proceeds via the inspection of time distribution curves (TDCs), which represent the time-dependent photoemission intensity in a region of interest -- often chosen somewhat arbitrarily -- in energy-momentum space. Here, we employ $k$-means, an unsupervised machine learning technique, to systematically investigate trends in TDC line shape for quasi-free-standing monolayer graphene and for a simple analytical model. Our analysis reveals how finite energy and time resolution can affect the TDC line shape. We discuss how this can be taken into account in a quantitative analysis, and under what conditions the time-dependent photoemission intensity after laser excitation can be approximated by a simple exponential decay.

cond-mat.str-el↗

arXiv2024

Van der Waals engineering of ultrafast carrier dynamics in magnetic heterostructures

Heterostructures composed of the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ and its non-magnetic counterpart Bi$_2$Te$_3$ host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$ following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy, and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi$_2$Te$_4$ and Bi$_2$Te$_3$ surface terminations of MnBi$_4$Te$_7$, revealing an instantaneous occupation of states associated with the Bi$_2$Te$_3$ surface layer followed by carrier extraction into the adjacent MnBi$_2$Te$_4$ layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi$_2$Te$_4$ septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi$_2$Te$_4$-based van der Waals compounds.

cond-mat.mtrl-sci↗

arXiv2024

Direct view of gate-tunable miniband dispersion in graphene superlattices near the magic twist angle

Superlattices from twisted graphene mono- and bi-layer systems give rise to on-demand many-body states such as Mott insulators and unconventional superconductors. These phenomena are ascribed to a combination of flat bands and strong Coulomb interactions. However, a comprehensive understanding is lacking because the low-energy band structure strongly changes when the electron filling is varied. Here, we gain direct access to the filling-dependent low energy bands of twisted bilayer graphene (TBG) and twisted double bilayer graphene (TDBG) by applying micro-focused angle-resolved photoemission spectroscopy to in situ gated devices. Our findings for the two systems are in stark contrast: The doping dependent dispersion for TBG can be described in a simple model, combining a filling-dependent rigid band shift with a many-body related bandwidth change. In TDBG, on the other hand, we find a complex behaviour of the low-energy bands, combining non-monotonous bandwidth changes and tuneable gap openings. Our work establishes the extent of electric field tunability of the low energy electronic states in twisted graphene superlattices and can serve to underpin the theoretical understanding of the resulting phenomena.

cond-mat.mes-hall↗

arXiv2024

Ultrafast carrier dynamics throughout the three-dimensional Brillouin zone of the Weyl semimetal PtBi$_2$

Using time- and angle-resolved photoemission spectroscopy, we examine the unoccupied electronic structure and electron dynamics of the type-I Weyl semimetal PtBi$_2$. Using the ability to change the probe photon energy over a wide range, we identify the predicted Weyl points in the unoccupied three-dimensional band structure and we discuss the effect of $k_\perp$ broadening in the normally unoccupied states. We characterise the electron dynamics close to the Weyl point and in other parts of three-dimensional Brillouin zone using $k$-means, an unsupervised machine learning technique. This reveals distinct differences -- in particular, dynamics that are faster in the parts of the Brillouin zone that host most of the bulk Fermi surface than in parts close to the Weyl points.

cond-mat.str-el↗

arXiv2024

Revealing flat bands and hybridization gaps in a twisted bilayer graphene device with microARPES

Controlling the electronic structure of two-dimensional materials using the combination of twist angle and electrostatic doping is an effective means to induce emergent phenomena. In bilayer graphene with an interlayer twist angle near the magic angle, the electronic dispersion is strongly modified by a manifold of hybridizing moiré Dirac cones leading to flat band segments with strong electronic correlations. Numerous technical challenges arising from spatial inhomogeneity of interlayer interactions, twist angle and device functionality have so far limited momentum-resolved electronic structure measurements of these systems to static conditions. Here, we present a detailed characterization of the electronic structure exhibiting miniband dispersions for twisted bilayer graphene, near the magic angle, integrated in a functional device architecture using micro-focused angle-resolved photoemission spectroscopy. The optimum conditions for visualizing the miniband dispersion are determined by exploiting the spatial resolution and photon energy tunability of the light source and applied to extract a hybridization gap size of $(0.14 \pm 0.03)$~eV and flat band segments extending across a moiré mini Brillouin zone. \textit{In situ} electrostatic gating of the sample enables significant electron-doping, causing the conduction band states to shift below the Fermi energy. Our work emphasizes key challenges in probing the electronic structure of magic angle bilayer graphene devices and outlines conditions for exploring the doping-dependent evolution of the dispersion that underpins the ability to control many-body interactions in the material.

cond-mat.mes-hall↗

arXiv2023

Access to the full 3D Brillouin zone with time resolution, using a new tool for pump-probe ARPES

Here we report the first time- and angle-resolved photoemission spectroscopy (TR-ARPES) with the new Fermiologics "FeSuMa" analyzer. The new experimental setup has been commissioned at the Artemis laboratory of the UK Central Laser Facility. We explain here some of the advantages of the FeSuMa for TR-ARPES and discuss how its capabilities relate to those of hemispherical analyzers and momentum microscopes. We have integrated the FeSuMa into an optimized pump-probe beamline that permits photon-energy- (i.e., kz-) dependent scanning, using probe energies generated from high harmonics in a gas jet. The advantages of using the FeSuMa in this situation include the possibility of taking advantage of its "fisheye" mode of operation.

cond-mat.mtrl-sci↗

arXiv2022

Programming moiré patterns in 2D materials by bending

Moiré superlattices in twisted two-dimensional materials have generated tremendous excitement as a platform for achieving quantum properties on demand. However, the moiré pattern is highly sensitive to the interlayer atomic registry, and current assembly techniques suffer from imprecise control of the average twist angle, spatial inhomogeneity in the local twist angle, and distortions due to random strain. Here, we demonstrate a new way to manipulate the moiré patterns in hetero- and homo-bilayers through in-plane bending of monolayer ribbons, using the tip of an atomic force microscope. This technique achieves continuous variation of twist angles with improved twist-angle homogeneity and reduced random strain, resulting in moiré patterns with highly tunable wavelength and ultra-low disorder. Our results pave the way for detailed studies of ultra-low disorder moiré systems and the realization of precise strain-engineered devices.

cond-mat.mes-hall↗

arXiv2021

Bulk band structure of Sb$_2$Te$_3$ determined by angle-resolved photoemission spectroscopy

The bulk band structure of the topological insulator \sbte~ is investigated by angle-resolved photoemission spectroscopy. Of particular interest is the dispersion of the uppermost valence band with respect to the topological surface state Dirac point. The valence band maximum has been calculated to be either near the Brillouin zone centre or in a low-symmetry position in the $\barΓ-\bar{M}$ azimuthal direction. In order to observe the full energy range of the valence band, the strongly p-doped crystals are counter-doped by surface alkali adsorption. The data show that that the absolute valence band maximum is likely to be found at the bulk $Γ$ point and predictions of a low-symmetry position with an energy higher than the surface Dirac point can be ruled out.

cond-mat.mtrl-sci↗

arXiv2021

Fermi surface tomography

Fermi surfaces, three-dimensional (3D) abstract interfaces that define the occupied energies of electrons in a solid, are important for characterizing and predicting the thermal, electrical, magnetic, and optical properties of crystalline metals and semiconductors [1]. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta (kF) from energy and angular distribution of photoelectrons dislodged by monochromatic light [2]. Existing electron analyzers are able to determine a number of kF-vectors simultaneously, but current technical limitations prohibit a direct high-resolution 3D Fermi surface mapping. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces and restricting a detailed comparison with the widely available nowadays calculated 3D Fermi surfaces. Here we show that using a simpler instrumentation, based on the Fourier electron optics combined with a retardation field of the detector, it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface recorded in the full Brillouin zone along the kz-direction as well as other experimental results featuring multiple advantages of our technique. In combination with various light sources, including synchrotron radiation, our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences.

cond-mat.mtrl-sci↗

arXiv2020

Hot carrier-assisted switching of the electron-phonon interaction in 1$T$-VSe$_2$

We apply an intense infrared laser pulse in order to perturb the electronic and vibrational states in the three-dimensional charge density wave material 1$T$-VSe$_2$. Ultrafast snapshots of the light-induced hot carrier dynamics and non-equilibrium quasiparticle spectral function are collected using time- and angle-resolved photoemission spectroscopy. The hot carrier temperature and time-dependent electronic self-energy are extracted from the time-dependent spectral function, revealing that incoherent electron-phonon interactions heat the lattice above the charge density wave critical temperature on a timescale of $(200 \pm 40)$~fs. Density functional perturbation theory calculations establish that the presence of hot carriers alters the overall phonon dispersion and quenches efficient low-energy acoustic phonon scattering channels, which results in a new quasi-equilibrium state that is experimentally observed.

cond-mat.mtrl-sci↗

arXiv2020

Ultrafast triggering of insulator-metal transition in two-dimensional VSe$_2$

Assembling transition metal dichalcogenides (TMDCs) at the two-dimensional (2D) limit is a promising approach for tailoring emerging states of matter such as superconductivity or charge density waves (CDWs). Single-layer (SL) VSe$_2$ stands out in this regard because it exhibits a strongly enhanced CDW transition with a higher transition temperature compared to the bulk in addition to an insulating phase with an anisotropic gap at the Fermi level, causing a suppression of anticipated 2D ferromagnetism in the material. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these electronic phases in SL VSe$_2$ using ultrafast pump-probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap on a timescale of 480 fs, which we disentangle from the ensuing hot carrier dynamics. Our work thereby reveals that the phase transition in SL VSe$_2$ is driven by electron-lattice coupling and demonstrates the potential for controlling electronic phases in 2D materials with light.

cond-mat.mtrl-sci↗

arXiv2020

Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from nanoARPES

The possibility of triggering correlated phenomena by placing a singularity of the density of states near the Fermi energy remains an intriguing avenue towards engineering the properties of quantum materials. Twisted bilayer graphene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. While the twist angle-dependence of these properties has been explored, direct demonstration of electrostatic control of the superlattice bands over a wide energy range has, so far, been critically missing. This work examines a functional twisted bilayer graphene device using in-operando angle-resolved photoemission with a nano-focused light spot. A twist angle of 12.2$^{\circ}$ is selected such that the superlattice Brillouin zone is sufficiently large to enable identification of van Hove singularities and flat band segments in momentum space. The doping dependence of these features is extracted over an energy range of 0.4 eV, expanding the combinations of twist angle and doping where they can be placed at the Fermi energy and thereby induce new correlated electronic phases in twisted bilayer graphene.

cond-mat.mtrl-sci↗