Publications

2020

• Chirality-induced tunneling asymmetry at a semiconductor interface
  
  • Rozhansky I.
  • To D. Quang
  • Jaffrès H.
  • Drouhin H.-J.
  Physical Review B, American Physical Society, 2020, 102 (4). (10.1103/PhysRevB.102.045428)
  DOI : 10.1103/PhysRevB.102.045428
• h / e oscillations in interlayer transport of delafossites
  
  • Putzke Carsten
  • Bachmann Maja
  • Mcguinness Philippa
  • Zhakina Elina
  • Sunko Veronika
  • Konczykowski Marcin
  • Oka Takashi
  • Moessner Roderich
  • Stern Ady
  • König Markus
  • Khim Seunghyun
  • Mackenzie Andrew
  • Moll Philip J.W.
  Science, American Association for the Advancement of Science (AAAS), 2020, 368 (6496), pp.1234-1238. Microstructures can be carefully designed to reveal the quantum phase of the wave-like nature of electrons in a metal. Here, we report phase-coherent oscillations of out-of-plane magnetoresistance in the layered delafossites PdCoO2 and PtCoO2 The oscillation period is equivalent to that determined by the magnetic flux quantum, h/e, threading an area defined by the atomic interlayer separation and the sample width, where h is Planck's constant and e is the charge of an electron. The phase of the electron wave function appears robust over length scales exceeding 10 micrometers and persisting up to temperatures of T > 50 kelvin. We show that the experimental signal stems from a periodic field modulation of the out-of-plane hopping. These results demonstrate extraordinary single-particle quantum coherence lengths in delafossites. (10.1126/science.aay8413)
  DOI : 10.1126/science.aay8413
• A linearly polarized electromagnetic wave as a swarm of photons half of which have spin −1 and half of which have spin +1
  
  • Coddens Gerrit
  , 2020. We illustrate our solution for the particle-wave duality and the paradox of the cat of Schr\"odinger on the example of the photons which occur in a linearly polarized electromagnetic wave. The +1 and -1 spin states correspond to the left and right circular polarizations, as pointed out by Raman and Bhagavantam \cite{Raman}. This example provides also insight into the solutions of some other puzzles, e.g. how electromagnetic waves can propagate in vacuum, the supposed collapse of the wave function, the quantization of the electromagnetic wave, the reason for the Born rule and why we can solve quantum-mechanical wave equations by adopting boundary conditions that correspond to a coherent source even if the source is incoherent.
• Scattering description of Andreev molecules
  
  • Pillet Jean-Damien
  • Benzoni Vincent
  • Griesmar Joël
  • Smirr Jean-Loup
  • Girit Çağlar
  SciPost Physics Core, SciPost Foundation, 2020, 2 (2). An Andreev molecule is a system of closely spaced superconducting weak links accommodating overlapping Andreev Bound States. Recent theoretical proposals have considered one-dimensional Andreev molecules with a single conduction channel. Here we apply the scattering formalism and extend the analysis to multiple conduction channels, a situation encountered in epitaxial superconductor/semiconductor weak links. We obtain the multi-channel bound state energy spectrum and quantify the contribution of the microscopic non-local processes leading to the formation of Andreev molecules. (10.21468/SciPostPhysCore.2.2.009)
  DOI : 10.21468/SciPostPhysCore.2.2.009
• Interface science in JMS
  
  • Hardouin Duparc Olivier B M
  • Lartigue-Korinek Sylvie
  Journal of Materials Science, Springer Verlag, 2020, 55, pp.16861 - 16863. (10.1007/s10853-020-04947-2)
  DOI : 10.1007/s10853-020-04947-2
• Dynamic Structure Factor and Dielectric Function of Valence Electrons in Lithium Hydride: An Inelastic X‐Ray Scattering Study at Finite Momentum Transfer
  
  • Paredes-Mellone Oscar A.
  • Koskelo Jaakko
  • Ceppi Sergio
  • Stutz Guillermo E.
  physica status solidi (b), Wiley, 2020, 257 (6), pp.1900780. (10.1002/pssb.201900780)
  DOI : 10.1002/pssb.201900780
• Probing polyelectrolyte adsorption in charged nanochannels by streaming potential measurements
  
  • Christoulaki Anastasia
  • Lairez Didier
  • Dubois Emmanuelle
  • Jouault Nicolas
  ACS Macro Letters, Washington, D.C : American Chemical Society, 2020, 9 (6), pp.794-798. It remains a great experimental challenge to obtain quantitative information on the polyelectrolyte (PE) behavior confined in charged nanoporous materials. Here, we propose an original approach using transverse streaming potential measurements (TSPMs), an efficient technique providing information on the electrical surface properties of nanoporous materials through the ζ-potential determination. We conduct TSPMs within the thin double-layer approximation on a model system composed of individual nanochannels, a nanoporous anodic aluminum oxide (AAO) membrane, filled with a well-known PE, sodium polystyrenesulfonate (NaPSS). We demonstrate that TSPMs can provide the AAO ζ-potential under different experimental conditions and monitor the PE penetration in AAO with positive or negative surface charge. On the positive surface, the PE irreversibly adsorbs, while it does not when the surface is negatively charged, indicating the electrostatic nature of the PE adsorption. In the context of experimental limitations to investigate PE behavior on concave surfaces, this study shows that the TSPM is suitable to extract quantitative information and can be exploited to gain an understanding of the PE adsorption and desorption in a confined medium. (10.1021/acsmacrolett.0c00172)
  DOI : 10.1021/acsmacrolett.0c00172
• Luminescence of non-bridging oxygen hole centers as a marker of particle irradiation of $\alpha$-quartz
  
  • Skuja Linards
  • Ollier N.
  • Kajihara K.
  Radiation Measurements, Elsevier, 2020, 135, pp.106373. The origin of the "red" emission bands in the 600 nm-700 nm region, observed in quartz crystals used for luminescence dating and environmental dosimetry, is still controversial. Their reported spectral and lifetime characteristics are often similar to those of oxygen dangling bonds ("non-bridging oxygen hole centers, NBOHCs") in glassy silicon dioxide. The presence of these "surface radical type" centers in quartz crystal requires sites with highly disordered local structure forming nano-voids characteristic to the structure of glassy SiO 2. Such sites are introduced in the tracks of nuclear particles (α-irradiation, neutrons, ions). In case of electrons they are created only at large doses (>5 GGy), approaching amorphization threshold. This study reports a comparison of NBOHC photoluminescence in synthetic quartz and silica glass irradiated by neutrons or 2.5 MeV electrons, and suggests that the red NBOHC photoluminescence band in quartz may serve as a selective marker of an exposure to particle irradiation. It can be distinguished from other red-region luminescence bands by lifetimes in 5-25 μs range, characteristic vibrational structures in the low-temperature spectra and presence of resonance excitation band at ~620 nm. (10.1016/j.radmeas.2020.106373)
  DOI : 10.1016/j.radmeas.2020.106373
• Electron irradiation effects on superconductivity in PdTe$_2$ : an application of a generalized Anderson theorem
  
  • Timmons E. I.
  • Teknowijoyo S.
  • Kończykowski M.
  • Cavani O.
  • Tanatar M. A.
  • Ghimire Sunil
  • Cho Kyuil
  • Lee Yongbin
  • Ke Liqin
  • Jo Na Hyun
  • Bud'Ko S. L.
  • Canfield P. C.
  • Orth Peter P.
  • Scheurer Mathias S.
  • Prozorov R.
  Physical Review Research, American Physical Society, 2020, 2 (2), pp.023140. Low-temperature (∼20 K) electron irradiation with 2.5 MeV relativistic electrons was used to study the effect of controlled nonmagnetic disorder on the normal and superconducting properties of the type-II Dirac semimetal PdTe2. We report measurements of longitudinal and Hall resistivity, thermal conductivity and London penetration depth using the tunnel-diode resonator technique for various irradiation doses. The normal-state electrical resistivity follows the Matthiessen rule with an increase of the residual resistivity at a rate of ∼0.77 μ$\Omega$ cm/(C/cm$^2$ ). London penetration depth and thermal conductivity results show that the superconducting state remains fully gapped. The superconducting transition temperature is suppressed at a nonzero rate that is about 16 times slower than described by the Abrikosov-Gor’kov dependence, applicable to magnetic impurity scattering in isotropic, single-band s-wave superconductors. To gain information about the gap structure and symmetry of the pairing state, we perform a detailed analysis of these experimental results based on insight from a generalized Anderson theorem for multiband superconductors. This imposes quantitative constraints on the gap anisotropies for each of the possible pairing candidate states. We conclude that the most likely pairing candidate is an unconventional A$^{+−}_{1g}$ state. While we cannot exclude the conventional A$^{++}_{1g}$ and the triplet A$_{1u}$, we demonstrate that these candidates require additional assumptions about the orbital structure of the disorder potential to be consistent with our experimental results, e.g., a ratio of inter- to intraband scattering for the singlet state significantly larger than 1. Due to the generality of our theoretical framework, we believe that it will also be useful for irradiation studies in other spin-orbit-coupled multiorbital systems (10.1103/PhysRevResearch.2.023140)
  DOI : 10.1103/PhysRevResearch.2.023140
• Local and mean-field approaches for modeling semiconductor spin-lasers
  
  • Drong Mariusz
  • Fördös Tibor
  • Jaffrès Henri
  • Peřina Jan
  • Postava Kamil
  • Pištora Jaromír
  • Drouhin Henri-Jean
  Journal of Optics, Institute of Physics (IOP), 2020, 22 (5), pp.055001. (10.1088/2040-8986/ab7d8a)
  DOI : 10.1088/2040-8986/ab7d8a
• Controlled introduction of defects to delafossite metals by electron irradiation
  
  • Sunko V.
  • Mcguinness P. H.
  • Chang C. S.
  • Zhakina E.
  • Khim S.
  • Dreyer C. E.
  • Konczykowski M.
  • Borrmann H.
  • Moll P. J. W.
  • König M.
  • Muller D. A.
  • Mackenzie A. P.
  Physical Review X, American Physical Society, 2020, 10 (2), pp.021018. The delafossite metals PdCoO 2 , PtCoO 2 , and PdCrO 2 are among the highest conductivity materials known, with low-temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure or are a consequence of highly unusual levels of crystalline perfection. We report the results of experiments in which high-energy electron irradiation was used to introduce point disorder to the Pd and Pt layers in which the conduction occurs. We obtain the cross section for formation of Frenkel pairs in absolute units, and cross-check our analysis with first-principles calculations of the relevant atomic displacement energies. We observe an increase of resistivity that is linear in defect density with a slope consistent with scattering in the unitary limit. Our results enable us to deduce that the as-grown crystals contain extremely low levels of in-plane defects of approximately 0.001%. This confirms that crystalline perfection is the most important factor in realizing the long mean free paths and highlights how unusual these delafossite metals are in comparison with the vast majority of other multicomponent oxides and alloys. We discuss the implications of our findings for future materials research. (10.1103/PhysRevX.10.021018)
  DOI : 10.1103/PhysRevX.10.021018
• Anharmonic coupling, thermal transport and acoustic wave attenuation in cubic semiconductors and bismuth.
  
  • Markov Maxime
  • Sjakste Jelena
  • Vast Nathalie
  • Perrin Bernard
  • Paulatto Lorenzo
  Journal of Physics: Conference Series, IOP Science, 2020, 1461, pp.012165. In this work we present our recent results of the ab initio calculations of anharmonic coupling in cubic semiconductors and bismuth. Our results allow us to explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy in the subterahertz domain, which shows a plateau between 0.6 and 1 THz at low temperatures. The plateau is explained by the competition between different phonon-phonon scattering processes such as Herring's mechanism, which dominates at low frequencies, saturates, and disappears at higher frequencies. We found an excellent agreement between measurements performed by some of us, and new ab initio calculations of third-order anharmonic processes. We predict that the same phenomenon should occur in other cubic semiconductors. In the case of bismuth, we discuss the occurrence of the hydrodynamic heat transport regime at low temperatures, in consistency with the experimental observations. Bismuth is one of the rare materials in which second sound has been experimentally observed. Our calculations predict the occurrence of the Poiseuille phonon flow in Bi between 1.5 K and 3.5 K for sample size of 3.86 mm and 9.06 mm, in consistency with the experimental observations. We will also discuss a Gedanken experiment allowing to assess the occurrence of the hydrodynamic regime in any bulk material. (10.1088/1742-6596/1461/1/012165)
  DOI : 10.1088/1742-6596/1461/1/012165
• Correlation satellites in optical and loss spectra
  
  • Cudazzo Pierluigi
  • Reining Lucia
  Physical Review Research, American Physical Society, 2020, 2 (1). Coupling of excitations leads to intriguing effects on the spectra of materials. We propose a cumulant formulation for neutral electronic excitations which opens the way to describe effects such as double plasmon satellites or exciton-exciton coupling. Our approach starts from the GW + Bethe-Salpeter approximation to many-body perturbation theory which is based on a quasiparticle picture, and it adds coupling of excitations through a consistent inclusion of dynamically screened interactions. This requires one to consider scattering contributions that are usually neglected. The result is formulated in a way that highlights essential physics, that can be implemented as a postprocessing tool in first-principles codes, and that suggests which kind of materials and measurements should exhibit strong effects. This is illustrated using a model. (10.1103/PhysRevResearch.2.012032)
  DOI : 10.1103/PhysRevResearch.2.012032
• Electron energy loss spectroscopy of thin slabs with supercell calculations
  
  • Giorgetti Christine
  • Iagupov Ilia
  • Véniard Valérie
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2020. Electron energy loss spectroscopy in the low loss regime is widely used to access to the screening of the Coulomb potential as a function of the momentum transfer. This screening is strongly reduced for low dimensional materials and this spectroscopy is a technique of choice to study the resulting quantum connement. Time-dependent density functional theory within an ab initio formalism, is particularly suited to simulate angular resolved electron energy loss spectra, taking benet from the reciprocal space description. For an isolated object, the standard procedure based on the supercell approach dramatically fails for the out-of-plane optical response of the surface and we have proposed a new scheme called Selected-G, 1 leading to a slab potential. In this paper, we show that the standard procedure also aects the in-plane components of the EEL spectra. Applying the Selected-G procedure, we show that the full expression of the slab potential is crucial to describe slabs of nite thickness. We compare our formalism to other cuto procedures, and show that if they provide spectra with the correct spectral weight, allowing the good description of plasmon dispersion, the amplitude of the peaks depends on the choice of the supercell. Our results, which provide spectra independent of vacuum, will have a strong impact on the calculation of properties such as quasiparticle corrections. (10.1103/PhysRevB.101.035431)
  DOI : 10.1103/PhysRevB.101.035431
• Effect of quenched disorder on the quantum spin liquid state of the triangular-lattice antiferromagnet 1$T$-TaS$_2$
  
  • Murayama H.
  • Sato Y.
  • Taniguchi T.
  • Kurihara R.
  • Xing X. Z.
  • Huang W.
  • Kasahara S.
  • Kasahara Y.
  • Kimchi I.
  • Yoshida M.
  • Iwasa Y.
  • Mizukami Y.
  • Shibauchi T.
  • Konczykowski M.
  • Matsuda Y.
  Physical Review Research, American Physical Society, 2020, 2, pp.013099. A quantum spin liquid (QSL) is an exotic state of matter characterized by quantum entanglement and the absence of any broken symmetry. A longstanding open problem, which is key for fundamental understanding the mysterious QSL states, is how the quantum fluctuations respond to randomness due to quenched disorder. Transition metal dichalcogenide 1T-TaS2 is a candidate material that hosts a QSL ground state with spin-1/2 on the two-dimensional perfect triangular lattice. Here, we performed systematic studies of low-temperature heat capacity and thermal conductivity on pure, Se-substituted, and electron-irradiated crystals of 1T-TaS2, where the substitution of S by Se induces weak disorder and electron irradiation induces strong quenched disorder. In pure 1T-TaS2, the linear temperature term of the heat capacity γ T and the finite residual linear term of the thermal conductivity in the zero-temperature limit κ0/T ≡ κ/T (T → 0) are clearly resolved, consistent with the presence of gapless spinons with a Fermi surface. Moreover, while the strong magnetic field slightly enhances κ0/T , it strongly suppresses γ . These unusual contrasting responses to magnetic field imply the coexistence of two types of gapless excitations with itinerant and localized characters. Introduction of additional weak random exchange disorder in 1T-Ta(S1−xSex )2 leads to vanishing of κ0/T , indicating that the itinerant gapless excitations are sensitive to the disorder. On the other hand, in both pure and Se-substituted systems, the magnetic contribution of the heat capacity obeys a universal scaling relation, which is consistent with a theory that assumes the presence of localized orphan spins forming random singlets. These results appear to capture an essential feature of the QSL state of 1T-TaS2; localized orphan spins induced by disorder form random valence bonds and are surrounded by a QSL phase with spinon Fermi surface. Electron irradiation in pure 1T-TaS2 largely enhances γ and changes the scaling function dramatically, suggesting a possible new state of spin liquid. (10.1103/PhysRevResearch.2.013099)
  DOI : 10.1103/PhysRevResearch.2.013099
• Ab initio description of the electron-phonon coupling and calculations of charge transport properties in bulk semiconductors and 2D materials
  
  • Sjakste Jelena
  , 2020.
• Insights into one-body density matrices using deep learning
  
  • Wetherell Jack
  • Costamagna Andrea
  • Gatti Matteo
  • Reining Lucia
  Faraday Discussions, Royal Society of Chemistry, 2020. (10.1039/D0FD00061B)
  DOI : 10.1039/D0FD00061B
• Electron irradiation induced aging effects on radiative recombination properties of quadruple cation organic-inorganic perovskite layers
  
  • Aversa Pierfrancesco
  • Öz Senol
  • Jung Eunhwan
  • Plantevin Olivier
  • Cavani Olivier
  • Ollier Nadège
  • Bourée Jean-Eric
  • Geffroy Bernard
  • Miyasaka Tsutomu
  • Mathur Sanjay
  • Corbel Catherine
  Emergent Materials, Springer, 2020, 3, pp.133-160. Understanding the role of defects in hybrid organic inorganic perovskites (HOIPs) is critically important to engineer the stability and performance of photovoltaic devices based on HOIPs. Recent reports on multi-cation compositions of general formula (A$^1$,A$^2$,A$^3$,A$^4$)Pb(X$^1$,X$^2$,X$^3$)$_3$, where the A sites can be occupied by a distribution of 2–4 metallic/organic cations and X sites with halide anions have shown stabilization effects against the well-known methyl ammonium lead triiodide (CH$_3$NH$_3$PbI$_3$), although the underlying mechanism is not fully elucidated. Herein, polycrystalline layers of 4APb(IBr)$_3$ perovskite, where A is occupied by a combination of Cs$^+$ (cesium ion), GA$^+$ (guanidinium), MA$^+$ (methylammonium), and FA$^+$(formamidinium) ions were synthesized. To gain insight on the role of intrinsic defects, electron irradiation was used for introducing point defects in a controlled way in the quadruple-cation HOIPs. Our results show that the engineered defects in perovskites strongly influenced the absorption, photoluminescence, and time-resolved photoluminescence of these materials, probably due to introduction of additional energy levels that modify electronic and light emitting properties of the material. Furthermore, the irradiation-induced defects were found to strongly affect the aging behavior of HOIPs and modify their radiative recombination properties. (10.1007/s42247-020-00096-z)
  DOI : 10.1007/s42247-020-00096-z
• Design of auxiliary systems for spectroscopy
  
  • Vanzini Marco
  • Sottile Francesco
  • Reshetnyak Igor
  • Ciuchi Sergio
  • Reining Lucia
  • Gatti Matteo
  Faraday Discussions, Royal Society of Chemistry, 2020. The Kohn-Sham system is the prototypical example of an auxiliary system that targets, in principle exactly, an observable like the electronic density without the need to calculate the complicated many-body wavefunction. Although the Kohn-Sham system does not describe excited-state properties directly, it represents a very successful strategy guideline also for many spectroscopy applications. Here we propose a generalization of the Kohn-Sham idea. In many situations one is interested only in limited answers to specific questions, whereas in state-of-the-art approaches a lot of information is generally calculated that is not needed for the interpretation of experimental spectra. For example, while the target is a spectrum S(ω) like the optical absorption of a solid, within time-dependent density-functional theory (TDDFT) one calculates the whole response function χ(r, r , ω). Analogously , within many-body perturbation theory (MBPT) one calculates the whole one-particle Green's function G(r, r , ω), while only the total spectral function A(ω) is needed for angle-integrated pho-toemission spectra. In this contribution, we advocate the possibility to design auxiliary systems with effective potentials or kernels that target only the specific spectral properties of interest and are simpler than the self-energy of MBPT or the exchange-correlation kernel of TDDFT. In particular, we discuss the fundamentals and prototypical applications of simplified effective kernels for optical absorption and spectral potentials for photoemission, and we discuss how to express these potentials or kernels as functionals of the density. (10.1039/D0FD00067A)
  DOI : 10.1039/D0FD00067A
• Synthesis of boron carbide from its elements at high pressures and high temperatures
  
  • Chakraborti A.
  • Vast N.
  • Le Godec Y.
  Solid State Sciences, Elsevier, 2020, 104, pp.106265. The formation of boron carbide under high pressures and from elemental reactants has been studied. To this end, stoichiometric mixtures of commercial crystalline beta rhombohedral boron and amorphous glassy carbon have been subjected to temperatures ranging from 1473 K to 2473 K at pressures of 2 GPa and 5 GPa. Similar syntheses have been repeated for crystalline boron and graphite. As the synthesis pressure increases, the temperature of formation of boron carbide is found to increase as well. Moreover, the reaction temperature is shown to be affected by the choice of the reactants. (10.1016/j.solidstatesciences.2020.106265)
  DOI : 10.1016/j.solidstatesciences.2020.106265
• Non-Gaussian tail in the force distribution: a hallmark of correlated disorder in the host media of elastic objects
  
  • Sánchez Jazmín Aragón
  • Rumi Gonzalo
  • Maldonado Raúl Cortés
  • Bolecek Néstor René Cejas
  • Puig Joaquín
  • Pedrazzini Pablo
  • Nieva Gladys
  • Dolz Moira
  • Konczykowski Marcin
  • van Der Beek Cornelis
  • Kolton Alejandro
  • Fasano Yanina
  Scientific Reports, Nature Publishing Group, 2020, 10 (1), pp.19452. Inferring the nature of disorder in the media where elastic objects are nucleated is of crucial importance for many applications but remains a challenging basic-science problem. Here we propose a method to discern whether weak-point or strong-correlated disorder dominates based on characterizing the distribution of the interaction forces between objects mapped in large fields-of-view. We illustrate our proposal with the case-study system of vortex structures nucleated in type-II superconductors with different pinning landscapes. Interaction force distributions are computed from individual vortex positions imaged in thousands-vortices fields-of-view in a two-orders-of-magnitude-wide vortex-density range. Vortex structures nucleated in point-disordered media present Gaussian distributions of the interaction force components. In contrast, if the media have dilute and randomly-distributed correlated disorder, these distributions present non-Gaussian algebraically-decaying tails for large force magnitudes. We propose that detecting this deviation from the Gaussian behavior is a fingerprint of strong disorder, in our case originated from a dilute distribution of correlated pinning centers. (10.1038/s41598-020-76529-w)
  DOI : 10.1038/s41598-020-76529-w
• Strategies to build functionals of the density, or functionals of Green’s functions: what can we learn?
  
  • Aouina Ayoub
  • Gatti Matteo
  • Reining Lucia
  Faraday Discussions, Royal Society of Chemistry, 2020. (10.1039/D0FD00068J)
  DOI : 10.1039/D0FD00068J
• Spectroscopy of buried states in black phosphorus with surface doping
  
  • Chen Zhesheng
  • Dong Jingwei
  • Giorgetti Christine
  • Papalazarou Evangelos
  • Marsi Marino
  • Zhang Zailan
  • Tian Bingbing
  • Ma Qingwei
  • Cheng Yingchun
  • Rueff Jean-Pascal
  • Taleb-Ibrahimi Amina
  • Perfetti Luca
  2D Materials, IOP Publishing, 2020, 7 (3), pp.035027. Electrostatic gating or alkali metal evaporation can be successfully employed to tune the interface of layered black phosphorus (BP) from a semiconductor to a 2D Dirac semimetal. Although Angle Resolved Photoelectron Spectroscopy (ARPES) experiments have captured the collapse of the band gap in the inversion layer, a quantitative estimation of band structure evolution has been hindered by the short escape depth and matrix elements of the probed photoelectrons. Here, we precisely monitor the evolution of electronic states using time-resolved ARPES at a photon energy of 6.3 eV. The probing depth of laser-based ARPES is long enough to observe the buried electronic states originating from the valence band maximum. Our data shows that the band gap has a maximal value of 0.32 eV in the pristine sample, and that it shrinks down monotonically by increasing the carrier concentration in the topmost layer. Most interestingly, the band velocity of the valence band increases by a factor of two along the armchair direction, surpassing the value reported in graphene on silicon carbide (SiC). This control of band structure via external gating will be of interest with regard to the design of optoelectronic devices. (10.1088/2053-1583/ab8ec1)
  DOI : 10.1088/2053-1583/ab8ec1
• Symmetry of order parameters in multiband superconductors LaRu$_4$As$_{12}$ and PrOs$_4$Sb$_{12}$ probed by local magnetization measurements
  
  • Juraszek J.
  • Wawryk R.
  • Henkie Z.
  • Konczykowski M.
  • Cichorek T.
  Physical Review Letters, American Physical Society, 2020, 124, pp.027001. The temperature dependencies of the lower critical field Hc1ðTÞ of several filled-skutterudite superconductors were investigated by local magnetization measurements. While LaOs4As12 and PrRu4As12 exhibit the Hc1ðTÞ dependencies consistent with the single-band BCS prediction, for LaRu4As12 (the superconducting temperature Tc ¼ 10.4 K) with a similar three-dimensional Fermi surface, we observe a sudden increase in Hc1ðTÞ deep in a superconducting state below about 0.32Tc. Remarkably, a rapid rise of Hc1ðTÞ at approximately the same reduced temperature 0.27Tc is also found for the heavy-fermion compound PrOs4Sb12 (Tc ≃ 1.78 K), in fair accord with the earlier macroscopic study. We attribute the unusual Hc1ðTÞ dependencies of LaRu4As12 and PrOs4Sb12 to a kink structure in their superfluid densities due to different contributions from two nearly decoupled bands. Whereas LaRu4As12 is established as a two-band isotropic s-wave superconductor, nonsaturating behavior of Hc1ðTÞis observed for PrOs4Sb12, indicative of an anisotropic structure of a smaller gap. For this superconductor with broken time-reversal symmetry, our findings suggest a superconducting state with multiple symmetries of the order parameters. (10.1103/PhysRevLett.124.027001)
  DOI : 10.1103/PhysRevLett.124.027001