Publications

2023

• Strain‐driven Switching Between Antiferromagnetic States in Frustrated Antiferromagnet UO2 Probed by Exchange Bias Effect
  
  • Tereshina-Chitrova Evgenia
  • Pourovskii Leonid
  • Khmelevskyi Sergii
  • Horak Lukas
  • Bao Zhaohui
  • Mackova Anna
  • Malinsky Petr
  • Gouder Thomas
  • Caciuffo Roberto
  Advanced Functional Materials, Wiley, 2023, 34 (13), pp.2311895. Frustrated antiferromagnets offer a captivating platform to study the intricate relationship of magnetic interactions, geometric constraints, and emergent phenomena. By controlling spin orientations, these materials can be tailored for applications in spintronics and quantum information processing. The research focuses on the interplay of magnetic and exchange anisotropy effects in artificial heterostructures based on a canonical frustrated antiferromagnet, UO 2 . The potential to manipulate the spin directions in this material and switch between distinct antiferromagnetic (AFM) states is investigated using substrate‐induced strain. The phenomenon is probed using exchange bias effects in stoichiometric UO 2 /Fe 3 O 4 bilayers. By employing many‐body first‐principles calculations magnetic configurations in the UO 2 layers are identified. Even a minor tetragonal distortion triggers a transition between AFM states of different symmetries, driven by a robust alteration of single‐ion anisotropy due to the distortion. Consequently, this change influences the arrangement of magnetic moments at the UO 2 /Fe 3 O 4 interface, affecting the magnitude of exchange bias. The findings showcase how epitaxial strain can effectively manipulate the AFM states in frustrated antiferromagnets by controlling single‐site anisotropy. (10.1002/adfm.202311895)
  DOI : 10.1002/adfm.202311895
• Flat-space Partial Waves From Conformal OPE Densities
  
  • van Rees Balt C
  • Zhao Xiang
  , 2023. We consider the behavior of the OPE density $c(\Delta,\ell)$ for conformal four-point functions in the flat-space limit where all scaling dimensions become large. We find evidence that the density reduces to the partial waves $f_\ell(s)$ of the corresponding scattering amplitude. The Euclidean inversion formula then reduces to the partial wave projection and the Lorentzian inversion formula to the Froissart-Gribov formula. The flat-space limit of the OPE density can however also diverge, and we delineate the domain in the complex $s$ plane where this happens. Finally we argue that the conformal dispersion relation reduces to an ordinary single-variable dispersion relation for scattering amplitudes.
• Combining Electrical Resistivity, Induced Polarization, and Self-Potential for a Better Detection of Ore Bodies
  
  • Su Zhaoyang
  • Revil André
  • Ghorbani Ahmad
  • Zhang Xin
  • Zhao Xiang
  • Richard Jessy
  Minerals, MDPI, 2023, 14. Electrical resistivity (ER), induced polarization (IP), and self-potential (SP) are three geophysical methods that have been broadly used in the realm of mineral exploration. These geophysical methods provide complementary information, each exhibiting a distinct sensitivity to various types of mineral deposits. Considering the relationship among these three methods, we propose an integrated approach that merges their respective information to offer an improved localization technique for ore bodies. First, we invert the electrical conductivity distribution through electrical resistance tomography (ERT). Then, we use the inverted conductivity distribution to invert the IP and SP data in terms of chargeability and source current density distributions. Then, we normalize the inverted chargeability and source current density distributions and we combine them to obtain an ore body index (ORI) χ used to delineate the potential locations of ore deposits. We design this index to be sensitive to the presence of ore bodies, which are reflected by either strong and localized source current density (SP) and/or strong chargeability values (IP). The proposed method is first validated using a synthetic model with two distinct anomalies characterized by different properties. The results show the limitation of individual inversion, as each method exclusively detects one of these anomalies. The combined approach allows a better characterization of the target. Then, the approach is applied to a sandbox experiment in which two metallic bodies are buried in water-saturated sand used as the background. Again, the proposed methodology is successfully applied to the detection of the metallic targets, improving their localization compared with individual methods. (10.3390/min14010012)
  DOI : 10.3390/min14010012
• Computer experiments and visualization in mathematics and physics A subjective short walk among some historical examples
  
  • Chazottes J.-R.
  • Monticelli Marc
  The Mathematical Intelligencer, Springer Verlag, 2023, 45, pp.320-334. In this short essay, we show how computer experiments, and especially visualization, allowed for the investigation and discovery of phenomena which would have passed unnoticed. We shall also highlight the importance of interactivity between the computer and the user. We do this by surveying several historical examples from mathematics and the physical sciences. Many pictures, and even hyperlinks to online interactive numerical experiments, are provided. Needless to say that we do not claim to be exhaustive, and that the chosen examples reflect our taste as well as our limited knowledge. Hyperlinks to several online interactive digital experiments are provided. (10.1007/s00283-023-10313-2)
  DOI : 10.1007/s00283-023-10313-2
• Fate of Quasiparticles at High Temperature in the Correlated Metal Sr 2 RuO 4
  
  • Hunter A.
  • Beck S.
  • Cappelli E.
  • Margot F.
  • Straub M.
  • Alexanian Y.
  • Gatti G.
  • Watson M. D.
  • Kim T. K.
  • Cacho C.
  • Plumb N. C.
  • Shi M.
  • Radović M.
  • Sokolov D. A.
  • Mackenzie A. P.
  • Zingl M.
  • Mravlje J.
  • Georges A.
  • Baumberger F.
  • Tamai A.
  Physical Review Letters, American Physical Society, 2023, 131 (23), pp.236502. (10.1103/PhysRevLett.131.236502)
  DOI : 10.1103/PhysRevLett.131.236502
• Aspects of Carrollian physics
  
  • Rivera Betancour David
  , 2023. The purpose of this thesis is to study aspects of Carrollian dynamics and its application to gravity with zero cosmological constant. Carrollian systems arise as the vanishing speed of light limit of Lorentzian theories. Here, general Carrollian and Galilean dynamical equations valid for arbitrary curved and time dependent Carrollian/Newton-Cartan geometries are constructed, with focus on fluid mechanics. In both cases the presence of a U(1) current is considered. The latter is done in two approaches: Carrollian/Galilean and Weyl invariance of the action, and by taking a large/small-c limit of the relativistic energy-momentum tensor and the U(1) current. In both cases, the dynamic is given by the conservation of a set of momenta that arise either as the variation of the action with respect to the different pieces of the Carrollian/Newton-Cartan geometry or appear at different orders in the c-expansion of the energy-momentum tensor. Although these two approaches agree, the limiting procedure is shown to be richer due to the possibility of capturing more general situations with extra degrees of freedom. In fact, it is this freedom that allows us to find under which conditions hydrodynamic-frame invariance is preserved when taking the large/small-c limit. We show that, although in the standard Galilean limit hydrodynamic-frame invariance is lost, it is recovered by adding two extra degrees of freedom in the large-c expansion of the heat current and U(1) currents. In the Carrollian case, hydrodynamic-frame invariance survives when the behavior of the energy-momentum tensor is guided by holographic Carrollian fluid results. We also present the analysis of the currents generated by Carrollian/Galilean isometries. In the Carrollian/Galilean instances, these currents are not guaranteed to be conserved and additional conditions must be imposed.The presented derivation for Carrollian dynamics is not valid only for fluids. The investigation of the scalar field on a general Carrollian spacetime is also presented, as well as the analysis of three dimensional Carrollian gravitational Chern-Simons extensions. In this analysis one finds electric and magnetic dynamics that are encoded at different orders in powers of the speed of light of the relativistic action. We furthermore unravel two more Carrollian Chern-Simons actions, dubbed paramagnetic and paraelectric, respectively.In relation to a possible flat version of the gauge/gravity duality, we also study some aspects of Ricci-flat dynamics from a Carrollian perspective. We show that Ricci-flat spacetimes can be expressed in a gauge covariant with respect to the null boundary. This gauge is an extension of the Newman-Unti gauge which is valid for asymptotically anti-de Sitter and flat spacetimes. The flat instance is reached as the vanishing cosmological constant limit of the anti-de Sitter case, which corresponds to a Carrollian limit at the boundary. Therefore, the resulting Ricci-flat solution space is reconstructed in terms of an infinite set of boundary Carrollian data. These are composed by the Carrollian conformal geometry, a finite set of momenta of the theory hosted at the boundary, and an infinite number of arbitrary tensors, obtained by expanding the original energy-momentum tensor in Laurent series, which obey Carrollian flux balance equations. We take advantage of the latter to define gravitational charges by using Carrollian boundary techniques and restricting the spacetime to the algebraically special Petrov type. With this construction we recover the mass and angular momentum multipolar expansion for the Kerr-Taub-NUT family. We also learn how the hidden Ehlers Möbius group acts on the boundary data at null infinity. We find that, for stationary spacetimes, this group is manifested as a local transformation for the Carrollian geometry and the boundary Carrollian observables. We reproduce the mass/nut rotation as the energy/Cotton rotation for the Kerr-Taub-NUT solution. (10.70675/f9e5dee8z7c4dz474bz983dz800a1d343006)
  DOI : 10.70675/f9e5dee8z7c4dz474bz983dz800a1d343006
• The phonon thermal Hall angle in black phosphorus
  
  • Li Xiaokang
  • Machida Yo
  • Subedi Alaska
  • Zhu Zengwei
  • Li Liang
  • Behnia Kamran
  Nature Communications, Nature Publishing Group, 2023, 14 (1), pp.1027. Abstract The origin of phonon thermal Hall Effect (THE) observed in a variety of insulators is yet to be identified. Here, we report on the observation of a thermal Hall conductivity in a non-magnetic elemental insulator, with an amplitude exceeding what has been previously observed. In black phosphorus (BP), the longitudinal ( κ i i ), and the transverse, κ i j , thermal conductivities peak at the same temperature and at this peak temperature, the κ i j / κ j j / B is ≈ 10 −4 −10 −3 T −1 . Both these features are shared by other insulators displaying THE, despite an absolute amplitude spreading over three orders of magnitude. The absence of correlation between the thermal Hall angle and the phonon mean-free-path imposes a severe constraint for theoretical scenarios of THE. We show that in BP a longitudinal and a transverse acoustic phonon mode anti-cross, facilitating wave-like transport across modes. The anisotropic charge distribution surrounding atomic bonds can pave the way for coupling between phonons and the magnetic field. (10.1038/s41467-023-36750-3)
  DOI : 10.1038/s41467-023-36750-3
• One-Half Topological Number in Entangled Quantum Physics
  
  • Hur Karyn Le
  Physical Review B, American Physical Society, 2023, 108 (23), pp.235144. A topological phase can be engineered in quantum physics from the Bloch sphere of a spin-1/2 showing a hedgehog structure as a result of a radial magnetic field. We elaborate on a relation between the formation of an entangled wavefunction at one pole, in a two-spins model, and an interesting pair of one-half topological numbers. Similar to Cooper pairs in superconductors, an Einstein-Podolsky-Rosen pair or Bell state at one pole produces a half-flux quantum, which here refers to the halved flux of the Berry curvature on the surface. These 1/2 numbers also refer to the presence of a free Majorana fermion at a pole on each sphere. The topological responses can be measured when driving from north to south and from a circularly polarized field at the poles revealing the quantized or half-quantized nature of the protected transverse currents. We show applications of entangled wavefunctions in band structures, introducing a local marker in momentum space, to characterize the topological response of two-dimensional semimetals in bilayer geometries. (10.1103/PhysRevB.108.235144)
  DOI : 10.1103/PhysRevB.108.235144
• Pion and photon beam initiated backward charmonium or lepton pair production
  
  • Pire Bernard
  • Semenov-Tian-Shansky Kirill
  • Shaikhutdinova Alisa
  • Szymanowski Lech
  AAPPS Bulletin, Association of Asia Pacific Physical Societies, 2023, 33 (1), pp.26. Hard exclusive reactions initiated by pion or photon beams within the near-backward kinematical regime specified by the small Mandelstam variable -u can be studied to access pion-to-nucleon and photon-to-nucleon Transition Distribution Amplitudes (TDAs). Checking the validity of collinear factorized description of pion and photon induced reactions in terms of TDAs allows to test the universality of TDAs between the space-like and time-like regimes that is the indispensable feature of the QCD collinear factorization approach. In this short review we consider the exclusive pion- and photo-production off nucleon of a highly virtual lepton pair (or heavy quarkonium) in the near-backward region. We first employ a simplistic cross channel nucleon exchange model of pion-to-nucleon TDAs to estimate the magnitude of the corresponding cross sections for the kinematical conditions of J-PARC. We then illustrate the flexibility of our approach by building a two parameter model for the photon-to-nucleon TDAs based on preliminary results for near threshold J/psi photoproduction at JLab and provide our estimates for near-backward J/psi photoproduction and Timelike Compton Scattering cross sections for the kinematical conditions of JLab and of future EIC and EicC. (10.1007/s43673-023-00094-3)
  DOI : 10.1007/s43673-023-00094-3
• Coexistence of $s$-wave superconductivity and phase separation in the half-filled extended Hubbard model with attractive interactions
  
  • Linnér E.
  • Dutreix C.
  • Biermann S.
  • Stepanov E. A.
  Physical Review B, American Physical Society, 2023, 108 (20), pp.205156. Understanding competing instabilities in systems with correlated fermions remains one of the holy grails of modern condensed matter physics. Among the fermionic lattice models used to this effect, the extended Hubbard model occupies a prime place due to the potential relevance of its repulsive and attractive versions for both electronic materials and artificial systems. Using the recently introduced multi-channel fluctuating field approach, we address the interplay of charge density wave, $s$-wave superconductivity, and phase separation fluctuations in the attractive extended Hubbard model. Despite of the fact that this model has been intensively studied for decades, our novel approach has allowed us to identify a novel phase that is characterised by the coexistence of $s$-wave superconductivity and phase separation. Our findings resonate with previous observations of interplaying phase separation and superconducting phases in electronic systems, most importantly in high-temperature superconductors. (10.1103/PhysRevB.108.205156)
  DOI : 10.1103/PhysRevB.108.205156
• New higher-spin curvatures in flat space
  
  • Boulanger Nicolas
  • Campoleoni Andrea
  • Pekar Simon
  Physical Review D, American Physical Society, 2023, 108 (10), pp.L101904. (10.1103/PhysRevD.108.L101904)
  DOI : 10.1103/PhysRevD.108.L101904
• Symmetry-broken perturbation theory to large orders in antiferromagnetic phases
  
  • Garioud Renaud
  • Šimkovic Fedor
  • Rossi Riccardo
  • Spada Gabriele
  • Schäfer Thomas
  • Werner Félix
  • Ferrero Michel
  , 2023.
• TMD Handbook
  
  • Boussarie Renaud
  • Burkardt Matthias
  • Constantinou Martha
  • Detmold William
  • Ebert Markus
  • Engelhardt Michael
  • Fleming Sean
  • Gamberg Leonard
  • Ji Xiangdong
  • Kang Zhong-Bo
  • Lee Christopher
  • Liu Keh-Fei
  • Liuti Simonetta
  • Mehen Thomas
  • Metz Andreas
  • Negele John
  • Pitonyak Daniel
  • Prokudin Alexei
  • Qiu Jian-Wei
  • Rajan Abha
  • Schlegel Marc
  • Shanahan Phiala
  • Schweitzer Peter
  • Stewart Iain W.
  • Tarasov Andrey
  • Venugopalan Raju
  • Vitev Ivan
  • Yuan Feng
  • Zhao Yong
  , 2023. This handbook provides a comprehensive review of transverse-momentum-dependent parton distribution functions and fragmentation functions, commonly referred to as transverse momentum distributions (TMDs). TMDs describe the distribution of partons inside the proton and other hadrons with respect to both their longitudinal and transverse momenta. They provide unique insight into the internal momentum and spin structure of hadrons, and are a key ingredient in the description of many collider physics cross sections. Understanding TMDs requires a combination of theoretical techniques from quantum field theory, nonperturbative calculations using lattice QCD, and phenomenological analysis of experimental data. The handbook covers a wide range of topics, from theoretical foundations to experimental analyses, as well as recent developments and future directions. It is intended to provide an essential reference for researchers and graduate students interested in understanding the structure of hadrons and the dynamics of partons in high energy collisions. (10.48550/arXiv.2304.03302)
  DOI : 10.48550/arXiv.2304.03302
• Emergent electronic landscapes in a novel valence-ordered nickelate with tri-component nickel coordination
  
  • Raji Aravind
  • Dong Zhengang
  • Porée Victor
  • Subedi Alaska
  • Li Xiaoyan
  • Mundet Bernat
  • Varbaro Lucia
  • Domínguez Claribel
  • Hadjimichael Marios
  • Feng Bohan
  • Nicolaou Alessandro
  • Rueff Jean-Pascal
  • Li Danfeng
  • Gloter Alexandre
  , 2023. The metal-hydride-based topochemical reduction process has produced novel thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and non-trivial ground states. Here, by such an oxygen (de-) intercalation method we synthesis a novel samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm$_{9}$Ni$_{9}$O$_{22}$ as revealed by four-dimensional scanning transmission electron microscopy, emerges from the intricate planes of {303}$_{\text{pc}}$ ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab-initio calculations show the coexistence of square-planar, pyramidal and octahedral Ni sites with mono-, bi- and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low-temperature. This new nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents a unique platform where mixed Ni valence can give rise to exotic phenomena. (10.48550/arXiv.2308.02855)
  DOI : 10.48550/arXiv.2308.02855
• Role of strange quarks in the D -term and cosmological constant term of the proton
  
  • Won Ho-Yeon
  • Kim Hyun-Chul
  • Kim June-Young
  Physical Review D, American Physical Society, 2023, 108 (9), pp.094018. (10.1103/PhysRevD.108.094018)
  DOI : 10.1103/PhysRevD.108.094018
• Reconstruction of classical skyrmions from Anderson towers: quantum Darwinism in action
  
  • Sotnikov O. M.
  • Stepanov E. A.
  • Katsnelson M. I.
  • Mila F.
  • Mazurenko V. V.
  Physical Review X, American Physical Society, 2023, 13 (4), pp.041027. The development of the quantum skyrmion concept is aimed at expanding the scope of the fundamental research and practical applications for classical topologically-protected magnetic textures, and potentially paves the way for creating new quantum technologies. Undoubtedly, this calls for establishing a connection between a classical skyrmion and its quantum counterpart: a skyrmion wave function is an intrinsically more complex object than a non-collinear configuration of classical spins representing the classical skyrmion. Up to date, such a quantum-classical relation was only established on the level of different physical observables, but not for classical and quantum states per se. In this work, we show that the classical skyrmion spin order can be reconstructed using only the low-energy part of the spectrum of the corresponding quantum spin Hamiltonian. This can be done by means of a flexible symmetry-free numerical realization of Anderson's idea of the towers of states (TOS) that allows one to study known, as well as unknown, classical spin configurations with a proper choice of the loss function. We show that the existence of the TOS in the spectrum of the quantum systems does not guarantee a priori that the classical skyrmion magnetization profile can be obtained as an outcome of the actual measurement. This procedure should be complemented by a proper decoherence mechanism due to the interaction with the environment. The later selects a specific combination of the TOS eigenfunctions before the measurement and, thus, ensures the transition from a highly-entangled quantum skyrmionic state to a classical non-collinear magnetic order that is measured in real experiments. The results obtained in the context of skyrmions allow us to take a fresh look at the problem of quantum antiferromagnetism. (10.1103/PhysRevX.13.041027)
  DOI : 10.1103/PhysRevX.13.041027
• On energy and particle production in cosmology: the particular case of the gravitino
  
  • Casagrande Gabriele
  • Dudas Emilian
  • Peloso Marco
  , 2023. It is well-known that the number of particles produced in cosmology, commonly defined in the literature from the Fock space of the instantaneous hamiltonian of the canonically normalized fields, is ambiguous. On the other hand, the energy computed from the energy-momentum tensor should be physical. We compare the corresponding Fock spaces and relate them through a Bogolyubov transformation. We find that for particles of spin $0$, $1$ and $3/2$ the two Fock spaces are different, whereas they are the same for spin $1/2$ fermions. For spin $0$ and $1$, for particles of high momenta the two Fock spaces align, as intuitively expected. For the spin $3/2$, one finds two puzzles. The first one is that the two corresponding Fock spaces do not match even in the limit of high momenta. The second is that whereas we provide evidence for the equivalence theorem between longitudinal gravitinos and the goldstino in terms of an exact matching between the lagrangians and the instantaneous hamiltonians for the canonically normalized fields, the energy operator computed from the Rarita--Schwinger action does not seem to be captured in a simple way by the goldstino action. Our results suggest a re-analysis of non-thermal gravitino production in cosmology.
• Revisiting the Landau criterion : A hydrodynamic and holographic approach to superfluid instabilities
  
  • Sottovia Filippo
  , 2023. Superfluidity is a state of matter whose main feature is the absence of viscous flow and heat transport. It originates from the spontaneous breaking of a U(1) continuous global symmetry below a critical temperature Tc. The gradient of the Goldstone field associated to the symmetry breaking determines the superfluid velocity. At finite temperature a superfluid and a normal fluid component coexist; both of them contribute to charge flow. Landau predicted that the superfluid becomes unstable at large superfluid velocities. He determined the critical velocity at low temperature by assuming boost invariance and that the normal fluid is composed of free bosonic quasiparticles with Bose-Einstein statistics.Hydrodynamics, on the other hand, gives a valid description of interacting systems at length- and timescales longer than the typical scales at which local thermal equilibrium is reached. In this thesis we employ a hydrodynamical approach in order to investigate the instabilities of superfluids at finite superflow. In particular, we find that a hydrodynamic collective mode crosses to the upper half complex frequency instabilities plane, which signals a dynamical instability. At the same time, however, this instability is also thermodynamic, as its onset is controlled by one of the second derivatives of the free energy changing sign.We study the onset of this instability in two main setups : the “probe limit”, where the fluctuations of the temperature and the normal fluid’s velocity are frozen, and a complete approach, which includes them.In both cases we test our findings with the help of gauge-gravity duality (holography). Up to our numerical precision, we find agreement between the hydrodynamic modes of the boundary theory and the quasinormal modes of the gravity theory. We are therewith able to verify our criterion in a holographic setup. Most importantly, our criterion, which is formulated in a model-independent way, is applies to interacting systems irrespective of the strength of interactions, does not rely on boost invariance and does not assume any specific quantum statistics. As a final check, we also show that it yields the Landau critical velocity for Galilean superfluids with Bose-Einstein quasiparticles. (10.70675/333d90ebzdf0bz4547zbe2bz660835458b51)
  DOI : 10.70675/333d90ebzdf0bz4547zbe2bz660835458b51
• Orbital-selective metal skin induced by alkali-metal-dosing Mott-insulating Ca2RuO4
  
  • Horio Masafumi
  • Forte Filomena
  • Sutter Denys
  • Kim Minjae
  • Fatuzzo Claudia
  • Matt Christian
  • Moser Simon
  • Wada Tetsuya
  • Granata Veronica
  • Fittipaldi Rosalba
  • Sassa Yasmine
  • Gatti Gianmarco
  • Rønnow Henrik
  • Hoesch Moritz
  • Kim Timur
  • Jozwiak Chris
  • Bostwick Aaron
  • Rotenberg Eli
  • Matsuda Iwao
  • Georges Antoine
  • Sangiovanni Giorgio
  • Vecchione Antonio
  • Cuoco Mario
  • Chang Johan
  Communications Physics, Nature Research, 2023, 6 (1), pp.323. Abstract Doped Mott insulators are the starting point for interesting physics such as high temperature superconductivity and quantum spin liquids. For multi-band Mott insulators, orbital selective ground states have been envisioned. However, orbital selective metals and Mott insulators have been difficult to realize experimentally. Here we demonstrate by photoemission spectroscopy how Ca 2 RuO 4 , upon alkali-metal surface doping, develops a single-band metal skin. Our dynamical mean field theory calculations reveal that homogeneous electron doping of Ca 2 RuO 4 results in a multi-band metal. All together, our results provide evidence for an orbital-selective Mott insulator breakdown, which is unachievable via simple electron doping. Supported by a cluster model and cluster perturbation theory calculations, we demonstrate a type of skin metal-insulator transition induced by surface dopants that orbital-selectively hybridize with the bulk Mott state and in turn produce coherent in-gap states. (10.1038/s42005-023-01436-1)
  DOI : 10.1038/s42005-023-01436-1
• Electronic structure of the highly conductive perovskite oxide SrMoO$_3$
  
  • Cappelli E.
  • Hampel A.
  • Chikina A.
  • Guedes E. Bonini
  • Gatti G.
  • Hunter A.
  • Issing J.
  • Biskup N.
  • Varela M.
  • Dreyer Cyrus
  • Tamai A.
  • Georges A.
  • Bruno F.
  • Radovic M.
  • Baumberger F.
  , 2022. We use angle-resolved photoemission to map the Fermi surface and quasiparticle dispersion of bulk-like thin films of SrMoO$_3$ grown by pulsed laser deposition. The electronic self-energy deduced from our data reveals weak to moderate correlations in SrMoO$_3$, consistent with our observation of well-defined electronic states over the entire occupied band width. We further introduce spectral function calculations that combine dynamical mean-field theory with an unfolding procedure of density functional calculations and demonstrate good agreement of this approach with our experiments. (10.48550/arXiv.2203.05820)
  DOI : 10.48550/arXiv.2203.05820
• Reconciling scaling of the optical conductivity of cuprate superconductors with Planckian resistivity and specific heat
  
  • Michon B.
  • Berthod Carole
  • Rischau C.
  • Ataei A.
  • Chen L.
  • Komiya S.
  • Ono S.
  • Taillefer L.
  • van der Marel D.
  • Georges A.
  , 2022. Materials tuned to a quantum critical point display universal scaling properties as a function of temperature $T$ and frequency $ω$. A long-standing puzzle regarding cuprate superconductors has been the observed power-law dependence of optical conductivity with an exponent smaller than one, in contrast to $T$-linear dependence of the resistivity and $ω$-linear dependence of the optical scattering rate. Here, we present and analyze resistivity and optical conductivity of La$_{2-x}$Sr$_x$CuO$_4$ with $x=0.24$. We demonstrate $\hbarω/k_{\mathrm{B}} T$ scaling of the optical data over a wide range of frequency and temperature, $T$-linear resistivity, and optical effective mass proportional to $\sim \ln T$ corroborating previous specific heat experiments. We show that a $T,ω$-linear scaling Ansatz for the inelastic scattering rate leads to a unified theoretical description of the experimental data, including the power-law of the optical conductivity. This theoretical framework provides new opportunities for describing the unique properties of quantum critical matter. (10.48550/arXiv.2205.04030)
  DOI : 10.48550/arXiv.2205.04030
• Real space representation of topological system: twisted bilayer graphene as an example
  
  • Zang Jiawei
  • Wang Jie
  • Georges Antoine
  • Cano Jennifer
  • Millis Andrew
  , 2022. We construct a Wannier basis for twisted bilayer graphene that is projected only from the Bloch functions of the twisted bilayer flat bands. The $C_3$ and $C_{2} \mathcal{T}$ symmetries act locally on the Wannier functions while the Wannier function charge density is strongly peaked at the triangular sites and becomes fully sublattice-polarized in the chiral limit. The Wannier functions have a power-law tail, due to the topological obstruction, but most of the charge density is concentrated within one unit cell so that the on-site local Coulomb interaction is much larger than the further neighbor interactions and in general the Hamiltonian parameters may be accurately estimated from a modest number of Wannier functions. One exception is the momentum space components of the single-particle Hamiltonian, where because of the topological obstruction convergence is non-uniform across the Brillouin zone. We observe, however, that mixed position and momentum space representations may be used to avoid this difficulty in the context of quantum embedding methods. Our work provides a new route to study systems with topological obstructions and paves the way for the future investigation of correlated states in twisted bilayer graphene, including studies of non-integer fillings and temperature dependence. (10.48550/arXiv.2210.11573)
  DOI : 10.48550/arXiv.2210.11573
• Tunable Stripe Order and Weak Superconductivity in the Moiré Hubbard Model
  
  • Wietek Alexander
  • Wang Jie
  • Zang Jiawei
  • Cano Jennifer
  • Georges Antoine
  • Millis Andrew
  , 2022. The moiré Hubbard model describes correlations in certain homobilayer twisted transition metal dichalcogenides. Using exact diagonalization and density matrix renormalization group methods, we find magnetic Mott insulating and metallic phases, which, upon doping exhibit intertwined charge and spin ordering and, in some regimes, pair binding of holes. The phases are highly tunable via an interlayer potential difference. Remarkably, the hole binding energy is found to be highly tunable revealing an experimentally accessible regime where holes become attractive. In this attractive regime, we study the superconducting correlation function and point out the possibility of weak superconductivity. (10.48550/arXiv.2204.04229)
  DOI : 10.48550/arXiv.2204.04229
• Temperature Dependence of Spin and Charge Orders in the Doped Two-Dimensional Hubbard Model
  
  • Xiao Bo
  • He Yuan-Yao
  • Georges Antoine
  • Zhang Shiwei
  , 2022. Competing and intertwined orders including inhomogeneous patterns of spin and charge are observed in many correlated electron materials, such as high-temperature superconductors. Introducing a new development of the constrained-path auxiliary-field quantum Monte Carlo (AFQMC) method, we study the interplay between thermal and quantum fluctuations in the two-dimensional Hubbard model. We obtain an accurate and systematic characterization of the evolution of the spin and charge correlations as a function of temperature $T$ and how it connects to the ground state, at three representative doping levels $δ= 1/5$, $1/8$, and $1/10$. We find increasing short-range commensurate antiferromagnetic correlations as $T$ is lowered. As the correlation length grows sufficiently large, a modulated spin-density-wave (SDW) appears. At $δ= 1/5$, the SDW saturates and remains short-ranged as $T \rightarrow 0$. In contrast, at $δ= 1/8$ and $1/10$ this evolves into a ground-state stripe phase. We study the relation between spin and charge orders and find that formation of charge order appears to be driven by that of the spin order. We identify a finite-temperature phase transition below which charge ordering sets in and discuss the implications of our results for the nature of this transition. (10.48550/arXiv.2202.11741)
  DOI : 10.48550/arXiv.2202.11741
• Distinct spin and orbital dynamics in Sr$_{2}$RuO$_{4}$
  
  • Suzuki H.
  • Wang L.
  • Bertinshaw J.
  • Strand H.
  • Käser S.
  • Krautloher M.
  • Yang Z.
  • Wentzell N.
  • Parcollet O.
  • Jerzembeck F.
  • Kikugawa N.
  • Mackenzie A.
  • Georges A.
  • Hansmann P.
  • Gretarsson H.
  • Keimer B.
  , 2022. The unconventional superconductor Sr$_2$RuO$_4$ has long served as a benchmark for theories of correlated-electron materials. The determination of the superconducting pairing mechanism requires detailed experimental information on collective bosonic excitations as potential mediators of Cooper pairing. We have used Ru $L_3$-edge resonant inelastic x-ray scattering to obtain comprehensive maps of the electronic excitations of Sr$_2$RuO$_4$ over the entire Brillouin zone. We observe multiple branches of dispersive spin and orbital excitations associated with distinctly different energy scales. The spin and orbital dynamical response functions calculated within the dynamical mean-field theory are in excellent agreement with the experimental data. Our results highlight the Hund metal nature of Sr$_{2}$RuO$_{4}$ and provide key information for the understanding of its unconventional superconductivity. (10.48550/arXiv.2212.00245)
  DOI : 10.48550/arXiv.2212.00245