arXiv subjects

M. Michiardi

Publications and source records attributed to M. Michiardi.

At least 19 recordsLinked to original sources

arXiv2025

Control of intervalley scattering in Bi$_2$Te$_3$ via temperature-dependent band renormalization

The control of out-of-equilibrium electron dynamics in topological insulators is essential to unlock their potential in next-generation quantum technologies. However, the role of temperature on the renormalization of the electronic band structure and, consequently, on electron scattering processes is still elusive. Here, using high-resolution time- and angle-resolved photoemission spectroscopy (TR-ARPES), we show that even a modest ($\sim$15 meV) renormalization of the conduction band of Bi$_2$Te$_3$ can critically affect bulk and surface electron scattering processes. Supported by a kinetic Monte Carlo toy-model, we show that temperature-induced changes in the bulk band structure modulate the intervalley electron-phonon scattering rate, reshaping the out-of-equilibrium response. This work establishes temperature as an effective control knob for engineering scattering pathways in topological insulators.

cond-mat.str-el↗

arXiv2025

Enhanced coherence and layer-selective charge order in a trilayer cuprate superconductor

Trilayer cuprates hold the record for the highest superconducting critical temperatures ($T_{\text{c}}$), yet the underlying mechanism remains elusive. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES), we uncover a striking interplay between charge order, superconducting gap magnitude, and quasiparticle coherence in Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223). This constitutes ARPES-based evidence of charge order on the inner CuO$_2$ plane, as confirmed via resonant x-ray scattering (RXS); in addition, the same inner plane hosts a superconducting gap significantly larger than that of the overdoped outer planes, firmly establishing it as underdoped. Unexpectedly, despite its underdoped nature, the inner plane also exhibits an exceptional degree of quasiparticle coherence; suppressing charge-order fluctuations further enhances this, making it comparable to that of the overdoped outer planes at elevated electronic temperatures. These findings, supported by complementary three-layer single-band Hubbard calculations, reveal a unique interlayer mechanism in which both pairing strength and phase coherence are optimized when interfacing planes with distinct hole concentrations, providing new microscopic insight into the record $T_{\text{c}}$ of Bi2223.

cond-mat.str-el↗

arXiv2025

Selective electron-phonon coupling strength from nonequilibrium optical spectroscopy: The case of MgB$_2$

The coupling between quasiparticles and bosonic excitations rules the energy transfer pathways in condensed matter systems. The possibility of inferring the strength of specific coupling channels from their characteristic time scales measured in nonequilibrium experiments is still an open question. Here, we investigate MgB$_2$, in which conventional superconductivity at temperatures as high as 39 K is mediated by the strong coupling between the conduction electrons and the E$_{2g}$ phonon mode. By means of broadband time-resolved optical spectroscopy, we show that this selective electron-phonon coupling dictates the nonequilibrium optical response of MgB$_2$, at early times ($<$100 fs) after photoexcitation. Furthermore, based on an effective temperature model analysis, we estimate its contribution to the total electron-boson coupling function extracted from complementary equilibrium spectroscopy approaches, namely optical reflectivity and ARPES. The coupling strength with the E$_{2g}$ phonon modes is thus estimated to be $\lambda \simeq$ 0.56, which is approximately half of the total coupling constant, in agreement with ab-initio calculations from the literature. As a benchmark, broadband time-resolved optical spectroscopy is performed also on the isostructural and non-superconducting compound AlB$_2$, showing that the nonequilibrium optical response relaxes on a slower time scale due to the lack of strongly-coupled phonon modes. Our findings demonstrate the possibility to resolve and quantify selective electron-phonon coupling from nonequilibrium optical spectroscopy.

cond-mat.supr-con↗

arXiv2025

Universal electronic structure of multi-layered nickelates via oxygen-centered planar orbitals

Superconductivity has been demonstrated in the family of multi-layered nickelates La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$. Key questions remain open regarding the low-energy electronic states that support superconductivity in these compounds. Here we take advantage of the natural polymorphism between bilayer (2222) and alternating monolayer-trilayer (1313) stacking sequences that arises in bulk La$_3$Ni$_2$O$_7$ crystals, and by employing angle-resolved photoemission spectroscopy (ARPES) we identify a universal low-energy electronic structure in this family of materials. We observe the fingerprint of a doping-dependent spin-density wave (SDW) instability -- strong and coherent enough to reconstruct the Fermi surface, both by gapping out regions of the low-energy electronic structure as well as translating the $\beta$ pocket by a vector $Q_{t\beta}$ consistent with the results of previous neutron and x-ray scattering experiments. Using an effective tight-binding model, we simulate the spectral weight distribution observed in our ARPES dichroism experiments and establish that the low-energy electronic phenomenology is dominated by oxygen-centered planar orbitals, which evolve from the $d_{3x^2-r^2}$ and $d_{3y^2-r^2}$ symmetry characteristic of 3-spin polarons (3SP) to the familiar $d_{x^2-y^2}$ Zhang-Rice singlets (ZRS) that support high-temperature superconductivity in cuprates. By inclusion of magnetic moments on plaquettes of oxygen orbitals in our model, we show that ZRS-like states mediate the SDW. Combined with the observation that oxygen annealing is required to induce superconductivity in both thin films and bulk La$_3$Ni$_2$O$_7$, this demonstrates that the ZRS population dictates whether the ground state favors density-wave order or superconductivity -- with hole doping suppressing the former and stabilizing the latter, as in the cuprates.

cond-mat.supr-con↗

arXiv2024

Electronic switching of topology in LaSbTe

In the past two decades, various classes of topological materials have been discovered, yet the deliberate control of topology in a single material remains largely unexplored. Here, we demonstrate full experimental control over the topological nodal loop in the square-net material LaSb$_x$Te$_{2-x}$ by chemical substitution and electron doping. Using angle-resolved photoemission spectroscopy (ARPES), we show that changing the antimony concentration x from 0.9 to 1.0 in the bulk opens a gap larger than 400~meV in the nodal loop. Symmetry analysis establishes that this effect originates from the breaking of \textit{n} glide symmetry in the square-net layer. Remarkably, the same topological phase transition can also be driven reversibly on the surface of LaSb$_x$Te$_{2-x}$ by \textit{in situ} chemical gating via potassium deposition, enabling on-demand switching of topology. The control parameter for both the bulk and surface transition is the electron concentration, providing a pathway towards applications based on switching topology by electrostatic gating.

cond-mat.mtrl-sci↗

arXiv2023

Kink in cuprates: the role of the low-energy density of states

The 40-70 meV band-structure renormalization (so-called kink) in high-temperature cuprate superconductors - which has been mainly interpreted in terms of electron-boson coupling - is observed to be strongly suppressed both above the superconducting transition temperature and under optical excitation. We employ equilibrium and time- and angle-resolved photoemission spectroscopy, in combination with Migdal-Eliashberg simulations, to investigate the suppression of the near-nodal kink in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We show that the $\sim$30$\%$ decrease of the kink strength across the superconducting-to-normal-state phase transition can be entirely accounted for by the filling of the superconducting gap, without additional consideration of temperature-dependent electron-boson coupling. Our findings demonstrate that consideration of changes in the density of states is essential to quantitatively account for the band structure renormalization effects in cuprates.

cond-mat.supr-con↗

arXiv2022

Symmetry-enforced Fermi degeneracy in topological semimetal RhSb$_3$

Predictions of a topological electronic structure in the skutterudite TPn$_3$ family (T=transition metal, Pn=pnictogen) are investigated via magnetoresistance, quantum oscillations and angle-resolved photoemission experiments of RhSb$_3$, an unfilled skutterudite semimetal with low carrier density. Electronic band structure calculations and symmetry analysis of RhSb$_3$ indicate this material to be a zero-gap semimetal protected by symmetry with inverted valence/conduction bands that touch at the $Γ$ point close to the Fermi level. Transport experiments reveal an unsaturated linear magnetoresistance that approaches a factor of 200 at 60~T magnetic fields, and quantum oscillations observable up to 150~K that are consistent with a large Fermi velocity ($\sim 1.3\times 10^6$ m/s), high carrier mobility ($\sim 14$ m$^2$/Vs), and the existence of a small three dimensional hole pocket. A very small, sample-dependent effective mass falls to values as low as $0.018(2)$ of the bare electron mass and scales with Fermi wavevector. This, together with a non-zero Berry's phase and location of the Fermi level in the linear region of the valence band, suggests RhSb$_3$ as representative of a new class of toplogical semimeals with symmetry-enforced Fermi degeneracy at the high symmetry points.

cond-mat.mtrl-sci↗

arXiv2022

Optical manipulation of Rashba-split 2-Dimensional Electron Gas

In spintronic devices, the two main approaches to actively control the electrons' spin degree of freedom involve either static magnetic or electric fields. An alternative avenue relies on the application of optical fields to generate spin currents, which promises to bolster spin-device performance allowing for significantly faster and more efficient spin logic. To date, research has mainly focused on the optical injection of spin currents through the photogalvanic effect, and little is known about the direct optical control of the intrinsic spin splitting. Here, to explore the all-optical manipulation of a material's spin properties, we consider the Rashba effect at a semiconductor interface. The Rashba effect has long been a staple in the field of spintronics owing to its superior tunability, which allows the observation of fully spin-dependent phenomena, such as the spin-Hall effect, spin-charge conversion, and spin-torque in semiconductor devices. In this work, by means of time and angle-resolved photoemission spectroscopy (TR-ARPES), we demonstrate that an ultrafast optical excitation can be used to manipulate the Rashba-induced spin splitting of a two-dimensional electron gas (2DEG) engineered at the surface of the topological insulator Bi$_{2}$Se$_{3}$. We establish that light-induced photovoltage and charge carrier redistribution -- which in concert modulate the spin-orbit coupling strength on a sub-picosecond timescale -- can offer an unprecedented platform for achieving all optically-driven THz spin logic devices.

cond-mat.mes-hall↗

arXiv2021

Extremely large magnetoresistance from electron-hole compensation in the nodal loop semimetal ZrP$_2$

Several early transition metal dipnictides have been found to host topological semimetal states and exhibit large magnetoresistance. In this study, we use angle-resolved photoemission spectroscopy (ARPES) and magneto-transport to study the electronic properties of a new transition metal dipnictide ZrP$_2$. We find that ZrP$_2$ exhibits an extremely large and unsaturated magnetoresistance of up to 40,000 % at 2 K, which originates from an almost perfect electron-hole compensation. Our band structure calculations further show that ZrP$_2$ hosts a topological nodal loop in proximity to the Fermi level. Based on the ARPES measurements, we confirm the results of our calculations and determine the surface band structure. Our study establishes ZrP$_2$ as a new platform to investigate near-perfect electron-hole compensation and its interplay with topological band structures.

cond-mat.mtrl-sci↗

arXiv2021

Dynamic electron correlations with charge order wavelength along all directions in the copper oxide plane

In strongly correlated systems the strength of Coulomb interactions between electrons, relative to their kinetic energy, plays a central role in determining their emergent quantum mechanical phases. We perform resonant x-ray scattering on Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, a prototypical cuprate superconductor, to probe electronic correlations within the CuO$_2$ plane. We discover a dynamic quasi-circular pattern in the $x$-$y$ scattering plane with a radius that matches the wave vector magnitude of the well-known static charge order. Along with doping- and temperature-dependent measurements, our experiments reveal a picture of charge order competing with superconductivity where short-range domains along $x$ and $y$ can dynamically rotate into any other in-plane direction. This quasi-circular spectrum, a hallmark of Brazovskii-type fluctuations, has immediate consequences to our understanding of rotational and translational symmetry breaking in the cuprates. We discuss how the combination of short- and long-range Coulomb interactions results in an effective non-monotonic potential that may determine the quasi-circular pattern.

cond-mat.supr-con↗

arXiv2020

Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates

High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. Here, we combine equilibrium and time-resolved ARPES to track the temperature dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and double-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-Liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates.

cond-mat.supr-con↗

arXiv2019

Room temperature strain-induced Landau levels in graphene on a wafer-scale platform

Graphene is a powerful playground for studying a plethora of quantum phenomena. One of the remarkable properties of graphene arises when it is strained in particular geometries and the electrons behave as if they were under the influence of a magnetic field. Previously, these strain-induced pseudomagnetic fields have been explored on the nano- and micrometer-scale using scanning probe and transport measurements. Heteroepitaxial strain, in contrast, is a wafer-scale engineering method. Here, we show that pseudomagnetic fields can be generated in graphene through wafer-scale epitaxial growth. Shallow triangular nanoprisms in the SiC substrate generate strain-induced uniform fields of 41 T. This enables the observation of strain-induced Landau levels at room temperature, as detected by angle-resolved photoemission spectroscopy, and confirmed by model calculations and scanning tunneling microscopy measurements. Our work demonstrates the feasibility of exploiting strain-induced quantum phases in two-dimensional Dirac materials on a wafer-scale platform, opening the field to new applications.

cond-mat.mtrl-sci↗

arXiv2019

Emergence of pseudogap from short-range spin-correlations in electron doped cuprates

Electron interactions are pivotal for defining the electronic structure of quantum materials. In particular, the strong electron Coulomb repulsion is considered the keystone for describing the emergence of exotic and/or ordered phases of quantum matter as disparate as high-temperature superconductivity and charge- or magnetic-order. However, a comprehensive understanding of fundamental electronic properties of quantum materials is often complicated by the appearance of an enigmatic partial suppression of low-energy electronic states, known as the pseudogap. Here we take advantage of ultrafast angle-resolved photoemission spectroscopy to unveil the temperature evolution of the low-energy density of states in the electron-doped cuprate Nd$_{\text{2-x}}$Ce$_{\text{x}}$CuO$_{\text{4}}$, an emblematic system where the pseudogap intertwines with magnetic degrees of freedom. By photoexciting the electronic system across the pseudogap onset temperature T*, we report the direct relation between the momentum-resolved pseudogap spectral features and the spin-correlation length with an unprecedented sensitivity. This transient approach, corroborated by mean field model calculations, allows us to establish the pseudogap in electron-doped cuprates as a precursor to the incipient antiferromagnetic order even when long-range antiferromagnetic correlations are not established, as in the case of optimal doping.

cond-mat.str-el↗

arXiv2019

Cavity-enhanced high harmonic generation for XUV time-resolved ARPES

With its direct correspondence to electronic structure, angle-resolved photoemission spectroscopy (ARPES) is a ubiquitous tool for the study of solids. When extended to the temporal domain, time-resolved (TR)-ARPES offers the potential to move beyond equilibrium properties, exploring both the unoccupied electronic structure as well as its dynamical response under ultrafast perturbation. Historically, ultrafast extreme ultraviolet (XUV) sources employing high-order harmonic generation (HHG) have required compromises that make it challenging to achieve a high energy resolution - which is highly desirable for many TR-ARPES studies - while producing high photon energies and a high photon flux. We address this challenge by performing HHG inside a femtosecond enhancement cavity (fsEC), realizing a practical source for TR-ARPES that achieves a flux of over 10$^{11}$ photons/s delivered to the sample, operates over a range of 8-40 eV with a repetition rate of 60 MHz. This source enables TR-ARPES studies with a temporal and energy resolution of 190 fs and 22 meV, respectively. To characterize the system, we perform ARPES measurements of polycrystalline Au and MoTe$_2$, as well as TR-ARPES studies on graphite.

physics.optics↗

arXiv2018

Role of matrix elements in the time-resolved photoemission signal

Time- and angle-resolved photoemission spectroscopy accesses the ultrafast evolution of quasiparticles and many-body interactions in solid-state systems. However, the momentum- and energy-resolved transient photoemission intensity may not be unambiguously related to the intrinsic relaxation dynamics of photoexcited electrons. In fact, interpretation of the time-dependent photoemission signal can be affected by the transient evolution of both the one-electron removal spectral function as well as the photoemission dipole matrix elements. Here we investigate the topological insulator Bi$_{1.1}$Sb$_{0.9}$Te$_2$S to demonstrate, by means of a careful probe-polarization study, the transient contribution of matrix elements to the time-resolved photoemission signal.

cond-mat.mtrl-sci↗

arXiv2018

Influence of Spin Orbit Coupling in the Iron-Based Superconductors

We report on the influence of spin-orbit coupling (SOC) in the Fe-based superconductors (FeSCs) via application of circularly-polarized spin and angle-resolved photoemission spectroscopy. We combine this technique in representative members of both the Fe-pnictides and Fe-chalcogenides with ab initio density functional theory and tight-binding calculations to establish an ubiquitous modification of the electronic structure in these materials imbued by SOC. The influence of SOC is found to be concentrated on the hole pockets where the superconducting gap is generally found to be largest. This result contests descriptions of superconductivity in these materials in terms of pure spin-singlet eigenstates, raising questions regarding the possible pairing mechanisms and role of SOC therein.

cond-mat.supr-con↗

arXiv2018

Influence of an anomalous temperature-dependence of the phase coherence length on the conductivity of magnetic topological insulators

Magnetotransport constitutes a useful probe to understand the interplay between electronic band topology and magnetism in spintronics devices based on topological materials. A recent theory of Lu and Shen [Phys. Rev. Lett. 112, 146601 (2014)] on magnetically doped topological insulators predicts that quantum corrections $Δκ$ to the temperature-dependence of the conductivity can change sign during the Curie transition. This phenomenon has been attributed to a suppression of the Berry phase of the topological surface states at the Fermi level, caused by a magnetic energy gap. Here, we demonstrate experimentally that $Δκ$ can reverse its sign even when the Berry phase at the Fermi level remains unchanged, provided that the inelastic scattering length decreases with temperature below the Curie transition.

cond-mat.mtrl-sci↗

arXiv2018

Collapse of superconductivity in cuprates via ultrafast quenching of phase coherence

The possibility of driving phase transitions in low-density condensates through the loss of phase coherence alone has far-reaching implications for the study of quantum phases of matter. This has inspired the development of tools to control and explore the collective properties of condensate phases via phase fluctuations. Electrically-gated oxide interfaces, ultracold Fermi atoms, and cuprate superconductors, which are characterized by an intrinsically small phase-stiffness, are paradigmatic examples where these tools are having a dramatic impact. Here we use light pulses shorter than the internal thermalization time to drive and probe the phase fragility of the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ cuprate superconductor, completely melting the superconducting condensate without affecting the pairing strength. The resulting ultrafast dynamics of phase fluctuations and charge excitations are captured and disentangled by time-resolved photoemission spectroscopy. This work demonstrates the dominant role of phase coherence in the superconductor-to-normal state phase transition and offers a benchmark for non-equilibrium spectroscopic investigations of the cuprate phase diagram.

cond-mat.supr-con↗