arXiv subjects

Christopher W. Nicholson

Publications and source records attributed to Christopher W. Nicholson.

13 recordsLinked to original sources

arXiv2025

Conduction Band Structure and Ultrafast Dynamics of Ferroelectric $\alpha$-GeTe(111)

$\alpha$-GeTe(111) is a non-centrosymmetric ferroelectric (FE) material for which a significative lattice distortion combined with a strong spin-orbit interaction gives rise to giant Rashba split states in the bulk and at the surface, which have been intensively probed in the occupied valence states using static angle-resolved photoemission spectroscopy (ARPES). Nevertheless, its unoccupied conduction band structure remains unexplored, in particular the experimental determination of its electronic band gap across momentum space. Using time-resolved ARPES based on high-repetition rate and extreme ultraviolet femtosecond (fs) laser, we unveil the band structure of $\alpha$-GeTe(111) in the full Brillouin zone, both in the valence and conduction states, as well as the exploration of its out-of-equilibrium dynamics. Our work confirms the semiconducting nature of $\alpha$-GeTe(111) with a 0.85 eV indirect band gap, which provides an upper limit for comparison to density functional theory calculations. We finally reveal the dominant scattering mechanisms of photoexcited carriers during the out-of-equilibrium dynamics under fs light pulses.

cond-mat.mtrl-sci↗

arXiv2024

Composite antiferromagnetic and orbital order with altermagnetic properties at a cuprate/manganite interface

Heterostructures from complex oxides allow one to combine various electronic and magnetic orders as to induce new quantum states. A prominent example is the coupling between superconducting and magnetic orders in multilayers from high-Tc cuprates and manganites. A key role is played here by the interfacial CuO2 layer whose distinct properties remain to be fully understood. Here, we study with resonant inelastic X-ray scattering (RIXS) the magnon excitations of this interfacial CuO2 layer. In particular, we show that the underlying antiferromagnetic exchange interaction at the interface is strongly suppressed to J ~ 70 meV, as compared to J ~ 130 meV for the CuO2 layers away from the interface. Moreover, we observe an anomalous momentum dependence of the intensity of the interfacial magnon mode and show that it suggests that the antiferromagnetic order is accompanied by a particular kind of orbital order that yields a so-called altermagnetic state. Such a two-dimensional altermagnet has recently been predicted to enable new spintronic applications and superconducting proximity effects.

cond-mat.str-el↗

arXiv2023

Observation of the metallic mosaic phase in 1$T$-TaS$_2$ at equilibrium

The transition-metal dichalcogenide tantalum disulphide (1$T$-TaS$_2$) hosts a commensurate charge density wave (CCDW) at temperatures below 165~K where it also becomes insulating. The low temperature CCDW phase can be driven into a metastable "mosaic" phase by means of either laser or voltage pulses which shows a large density of CDW domain walls as well as a closing of the electronic band gap. The exact origins of this pulse-induced metallic mosaic are not yet fully understood. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we observe the occurrence of such a metallic mosaic phase on the surface of TaS$_2$ without prior pulse excitation over continuous areas larger than $100 \times 100$~nm$^2$ and macroscopic areas on the millimetre scale. We attribute the appearance of the mosaic phase to the presence of surface defects which cause the formation of the characteristic dense domain wall network. Based on our STM measurements, we further argue how the appearance of the metallic behaviour in the mosaic phase could be explained by local stacking differences of the top layer. Thus, we provide a potential avenue to explain the origin of the pulse induced mosaic phase.

cond-mat.str-el↗

arXiv2022

Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric $α$-GeTe(111)

Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, $α$-GeTe(111) is a non-centrosymmetric ferroelectric (FE) semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond (fs) light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of $α$-GeTe(111) using time and angle-resolved photoemission spectroscopy (tr-ARPES). Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage (SPV), namely an increase corresponding to a 13 % enhancement of the lattice distortion. This opens the route for the control of the FE polarization in $α$-GeTe(111) and FE semiconducting materials in quantum heterostructures.

cond-mat.mtrl-sci↗

arXiv2022

Modified interlayer stacking and insulator to correlated-metal transition driven by uniaxial strain in 1$T$-TaS$_{2}$

Interlayer coupling is strongly implicated in the complex electronic properties of 1$T$-TaS$_2$ , but the interplay between this and electronic correlations remains unresolved. Here, we employ angle-resolved photoemission spectroscopy (ARPES) to reveal the effect of uniaxial strain engineering on the electronic structure and interlayer coupling in 1$T$-TaS$_2$ . The normally insulating ground state is transformed into a correlated-metal phase under strain, as evidenced by the emergence of a narrow band at the Fermi level. Temperature dependent ARPES measurements reveal that the metallic behaviour only develops below the commensurate charge density wave (CCDW) transition, where interlayer dimerization produces a band-insulator in unstrained samples. Electronic structure calculations demonstrate that the correlated metallic behaviour is stabilized by a previously predicted but unobserved bulk stacking structure with a modified interlayer coupling of the Ta d$_{z^{2}}$ electrons. Our combined approach lays bare the role of correlations and interlayer coupling in 1$T$-TaS$_2$ , providing critical input for understanding superconductivity under pressure and the metastable hidden phase induced using non-equilibrium protocols in this platform material for correlated physics.

cond-mat.str-el↗

arXiv2022

Mott versus hybridization gap in the low-temperature phase of $1T$-TaS$_2$

We compute the correlated electronic structure of stacked $1T$-TaS$_2$ bilayers using the $GW$ + EDMFT method. Depending on the surface termination, the semi-infinite uncorrelated system is either band-insulating or exhibits a metallic surface state. For realistic values of the onsite and intersite interactions, a Mott gap opens in the surface state, but this gap is smaller than the gap originating from the bilayer structure. Our results are consistent with recent scanning tunneling spectroscopy measurements for different terminating layers, and with our own photoemission measurements, which indicate the coexistence of spatial regions with different gaps in the electronic spectrum.

cond-mat.str-el↗

arXiv2022

Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi$_2$B$_2$C

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. Strong coupling of phonons to electrons near the Fermi surface, which reduces the phonon lifetimes and broadens the phonon peaks in scattering experiments, is often associated with Fermi surface nesting. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi$_2$B$_2$C. This new scenario likely applies to a wide range of compounds.

cond-mat.supr-con↗

arXiv2021

Direct measurement of key exciton properties: energy, dynamics and spatial distribution of the wave function

Excitons, Coulomb-bound electron-hole pairs, are the fundamental excitations governing the optoelectronic properties of semiconductors. While optical signatures of excitons have been studied extensively, experimental access to the excitonic wave function itself has been elusive. Using multidimensional photoemission spectroscopy, we present a momentum-, energy- and time-resolved perspective on excitons in the layered semiconductor WSe$_2$. By tuning the excitation wavelength, we determine the energy-momentum signature of bright exciton formation and its difference from conventional single-particle excited states. The multidimensional data allows to retrieve fundamental exciton properties like the binding energy and the exciton-lattice coupling and to reconstruct the real-space excitonic distribution function via Fourier transform. All quantities are in excellent agreement with microscopic calculations. Our approach provides a full characterization of the exciton properties and is applicable to bright and dark excitons in semiconducting materials, heterostructures and devices.

cond-mat.mtrl-sci↗

arXiv2020

Novel Cu-based dz2 order at a YBa2Cu3O7/ manganite interface

The interplay of nearly degenerate orders in quantum materials can lead to a myriad of emergent phases. A prominent case is that of the high-Tc cuprates for which the relationship between superconductivity and a short-ranged, incommensurate charge density wave in the CuO2 planes involving the dx2-y2 orbitals (Cu-CDW) is a subject of great current interest. Strong modifications of the strength and coherence of this Cu-CDW have been achieved by applying large magnetic fields, uniaxial pressure, or via the interfacial coupling in cuprate/manganite multilayers. However, such modifications do not alter the dominant orbital character. Here we investigate cuprate/manganite multilayers with resonant inelastic X-ray scattering (RIXS) and show that a new kind of Cu-based density wave order can be induced that has not been previously observed in the cuprates. This order has an unusually small wave vector of Q = 0.096 reciprocal lattice units (r.l.u.), a large correlation length of about 40 nm, and a predominant dz2 orbital character, instead of the typical dx2-y2 character. Its appearance is determined by the hole doping of the manganite which is a key parameter controlling the interfacial charge transfer and orbital reconstruction. We anticipate that the observation of a previously unknown type of density wave order at the YBCO interface will allow for fresh perspectives on the enigmatic relation between superconductivity and charge order (CO) in the cuprates.

cond-mat.str-el↗

arXiv2020

Role of a higher dimensional interaction in stabilizing charge density waves in quasi-1D NbSe$_3$ revealed by angle-resolved photoemission spectroscopy

We revisit charge density wave (CDW) behavior in the archetypal quasi-one-dimensional (quasi-1D) material NbSe$_3$ by high-resolution angle-resolved photoemission spectroscopy measurements utilizing a microfocused laser with a photon energy of 6.3 eV. We present a detailed view of the electronic structure of this complex multiband system and unambiguously resolve CDW gaps at the Fermi level ($E_F$). By employing a tight-binding model, we argue that these gaps are the result of interband coupling between electronic states that reside predominantly on distinct 1D chains within the material. Two such localized states are found to couple to an electronic state that extends across multiple 1D chains, highlighting the importance of a higher-dimensional interaction in stabilizing the CDW ordering in this material. In addition, the temperature evolution of intrachain gaps caused by the CDW periodicities far below $E_F$ deviate from the behavior expected for a Peierls-type mechanism driven by nesting; the upper and lower bands of the renormalized CDW dispersions maintain a fixed peak-to-peak distance while the gaps are gradually removed at higher temperatures. This points toward a gradual loss of long-range phase coherence as the dominant effect in reducing the CDW order parameter, which may correspond to the loss of coherence between the coupled chains. Furthermore, one of the gaps is observed above the bulk and surface CDW transition temperatures, implying the persistence of short-range incoherent CDW order. The influence of such higher-dimensional interactions likely plays an important role in a range of low-dimensional systems.

cond-mat.str-el↗

arXiv2018

Beyond the molecular movie: dynamics of bands and bonds during a photo-induced phase transition

Ultrafast non-equilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photo-induced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a non-equilibrium population of excited electronic states. Utilizing femtosecond photoemission we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photo-holes in shaping the potential energy landscape, and enables a combined momentum and real space description of PIPTs, including the ultrafast formation of chemical bonds.

cond-mat.str-el↗

arXiv2017

Dimensional Crossover in a Charge Density Wave Material Probed by Angle-Resolved Photoemission Spectroscopy

High-resolution angle-resolved photoemission spectroscopy (ARPES) data reveal evidence of a crossover from one-dimensional (1D) to three-dimensional (3D) behavior in the prototypical charge density wave (CDW) material NbSe3. In the low-temperature 3D regime, gaps in the electronic structure are observed due to two incommensurate CDWs, in agreement with x-ray diffraction and electronic-structure calculations. At higher temperatures we observe a spectral weight depletion that approaches the power-law behavior expected in 1D. From the warping of the quasi-1D Fermi surface at low temperatures, we extract the energy scale of the dimensional crossover. This is corroborated by a detailed analysis of the density of states, which reveals a change in dimensional behavior dependent on binding energy. Our results offer an important insight into the dimensionality of excitations in quasi-1D materials.

cond-mat.str-el↗

arXiv2017

Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2

Manipulation of spin and valley degrees of freedom is a key step towards realizing novel quantum technologies, for which atomically thin transition metal dichalcogenides (TMDCs) have been established as promising candidates. In monolayer TMDCs, the lack of inversion symmetry gives rise to a spin-valley correlation of the band structure allowing for valley-selective electronic excitation with circularly polarized light. Here we show that, even in centrosymmetric samples of 2H-WSe2, circularly polarized light can generate spin-, valley- and layer-polarized excited states in the conduction band. Employing time- and angle-resolved photoemission spectroscopy (trARPES) with spin-selective excitation, the dynamics of valley and layer pseudospins of the excited carriers are investigated. Complementary time-dependent density functional theory (TDDFT) calculations of the excited state populations reveal a strong circular dichroism of the spin-, valley- and layer-polarizations and a pronounced 2D character of the excited states in the K valleys. We observe scattering of carriers towards the global minimum of the conduction band on a sub-100 femtosecond timescale to states with three-dimensional character facilitating inter-layer charge transfer. Our results establish the optical control of coupled spin-, valley- and layer-polarized states in centrosymmetric materials and suggest the suitability of TMDC multilayer materials for valleytronic and spintronic device concepts.

cond-mat.mes-hall↗