# Matériaux et Phénomènes Quantiques

### Présentation

Le laboratoire Matériaux et Phénomènes Quantiques (MPQ) est une unité mixte de recherche (UMR 7162) du CNRS et de l’Université Paris Diderot, installée sur le campus de Paris Rive Gauche. Elle est composée d’environ 120 personnes au total dont 51 permanent.e.s.

Le laboratoire est spécialisé dans l’étude des matériaux quantiques de frontière et dans le développement de dispositifs quantiques innovants. Ces activités reposent sur un large spectre de compétences théoriques et expérimentales alliant la physique des matériaux, le transport et l’optique, et des plateformes technologiques de salle blanche, de spectroscopie et de microscopie électronique haute résolution.

Les activités de recherche du laboratoire MPQ se déclinent selon les thèmes suivants :

- nouveaux matériaux à l’échelle nano : nanoparticules, nanocristaux, nanotubes fonctionnalisés, matériaux multiferroïques, etc.
- nouveaux états de la matière : fluides quantiques de lumière, couplage ultra-fort en cavité, supraconducteurs non-conventionnels, systèmes fortement corrélés, phases topologiques, etc.
- systèmes nano-optiques innovants : optomécanique, nanophotonique non-linéaire, nanoplasmonique, etc.
- ingénierie quantique et information quantique : composants optoélectroniques quantiques, circuits photoniques quantiques, ions piégés, matériaux et composants hybrides organique/inorganique, ingénierie des surfaces/interfaces.

Les projets actuels du laboratoire incluent le développement de nouvelles sondes pour l’étude des matériaux quantiques, comme la spectroscopie Raman résolue en temps, la microscopie AFM opto-mécanique et la microscopie tunnel sous excitation optique. Réciproquement, les matériaux de frontière sont mis à profit pour la réalisation de nouvelles fonctionnalités dans des senseurs optomécaniques, des circuits photoniques non-linéaires et quantiques, ou encore dans des expériences de transport mésoscopique en cavité optique.

### [hal-01944732] Demonstration of an Effective Ultrastrong Coupling between Two Oscillators

Date: 26 Mar 2019 - 14:36

Desc: When the coupling rate between two quantum systems becomes as large as their characteristic frequencies, it induces dramatic effects on their dynamics and even on the nature of their ground state. The case of a qubit coupled to a harmonic oscillator in this ultrastrong coupling regime has been investigated theoretically and experimentally. Here, we explore the case of two harmonic oscillators in the ultrastrong coupling regime. Probing the properties of their ground state remains out of reach in natural implementations. Therefore, we have realized an analog quantum simulation of this coupled system by dual frequency pumping a nonlinear superconducting circuit. The pump amplitudes directly tune the effective coupling rate. We observe spectroscopic signature of a mode hybridization that is characteristic of the ultrastrong coupling. We experimentally demonstrate a key property of the ground state of this simulated ultrastrong coupling between modes by observing simultaneous single- and two-mode squeezing of the radiated field below vacuum fluctuations.

### [hal-01987541] Temperature-, Light-, and Soft X-ray-Induced Spin Crossover in a Single Layer of Fe-II-Pyrazolylborate Molecules in Direct Contact with Gold (vol 122, pg 727, 2018)

Date: 26 Mar 2019 - 14:29

Desc: [...]

### [hal-01987542] Temperature-, Light-, and Soft X-ray-Induced Spin Crossover in a Single Layer of Fe-II-Pyrazolylborate Molecules in Direct Contact with Gold

Date: 26 Mar 2019 - 14:29

Desc: Understanding the properties of spin-crossover molecules in direct contact with metals is crucial for their future integration in electronic and spintronic devices. By X-ray absorption spectroscopy, we investigate the properties of Fe-II((3,5-(CH3)(2)Pz)(3)BH)(2) molecules in the form of monolayer islands on a metallic substrate, namely, Au(111). We demonstrate that the spin crossover transition can be thermally induced from the high spin state to a mixed spin state phase containing one-third of high-spin state and two-thirds of low-spin-state molecules in agreement with previous work by scanning tunneling microscopy. In addition, at 4.4 K, the spin crossover from the low spin state to the high spin state can also be induced by X-ray and by light excitations.

### [hal-01438633] Interface roughness transport in terahertz quantum cascade detectors

Date: 26 Mar 2019 - 14:24

Desc: Infrared detectors based on a quantum cascade have been proposed to suppress the dark current which is a limiting factor in quantum well infrared photodetectors. Those detectors have been mainly designed for the midinfrared wavelength. Operating in the terahertz range involves a complete change of regime of transport since the photon energy is lower than the optical phonon energy. Thanks to a two dimensional model of transport, we have identified interface roughness as the key interaction in such a structure. Interface parameters, evaluated by scanning transmission electron microscopy, are used to study their influence on the resistance of the device. In order to overcome the limiting factor of quantum well infrared photodetectors QWIPs, a high dark current, photo-voltaic detectors inspired from quantum cascade laser QCL and named quantum cascade detectors QCD have been designed. 1–6 They consist in several periods composed of different quantum wells. As in QWIPs, this device is unipo-lar and the infrared absorption occurs between two subbands. The separation in energy between the subbands corresponds to the detection wavelength. Unlike the QWIP, where the excited electron is extracted from the top subband by an external electric field, in QCDs, it escapes by resonant tun-neling to the adjacent quantum well. Then, the electron cascades from subband to subband down to the next period. Such detectors were initially designed in 3–5 and 8 – 12 m wavelength ranges. In this case, the involved energy differences between subbands and the operating temperatures are high enough for efficient longitudinal optical LO phonon emission and absorption. A previous study has determined LO phonons as the dominant scattering process in the transport without illumination. 6 However, this argument fails in terahertz THz range as lower energies are involved. In this range, the dominant scattering process has still to be identified. In this letter, first, we highlight the dominant interaction in a THz QCD, interface roughness IR scattering. Then, we use scanning transmission electron microscopy STEM measurements to extract the key parameters of IR: the magnitude of the roughness and the mean distance between defects. Only few studies have been carried out to design a THz QCD, since only one other device has been presented. 7 The QCD under study is a GaAs– Al 0.27 Ga 0.73 As heterostruc-ture. It consists in forty periods of five coupled quantum wells. The layer sequence in Å ´ , starting from the first barrier, is as follows: 75/ 52 / 30/ 48 / 50/ 44 / 60/ 50 / 68/ 54. Barriers are in bold. The first two wells of a period are doped with Si donors 3 10 17 cm −3 in the central third of the well. The photonic transitions E 5-E 1 and E 4-E 1 are expected to lead to a maximum absorption at 70 m. This device, designed, and grown by the University Paris Diderot and the III–V Laboratory , will be presented and characterized in a future publication. In the THz range, the dominant interaction is a priori difficult to identify. For this purpose, we used a hopping transport code 8 between two dimensional states based on the Fermi golden rule. Wave functions were evaluated using a two band kp model. All the following scattering processes are taken into account in our simulation: interaction between electron and LO phonon LO, acoustical phonon AC, alloy disorder AL, interface roughness IR, ionized impurities II, and electrons EE. We have evaluated the different relaxation rates in the cascade for each of the six processes. Calculations have been made for an electron hopping from the bottom of the initial subband K i =0, at a temperature of 10 K. The interaction parameters are given in Ref. 8. Table I presents the intracascade scattering rates 1 / ij , and between two different cascades 1 / i j. The numbering system of the levels is the same as in Ref. 6. Interface roughness is identified as the main interaction, the corresponding ratse 1 / ij are at least one order of magnitude above the others. As LO phonon interaction in 3 – 12 m QCD, IR can be considered as the interaction which has to be taken into account when evaluating the transport properties in THz QCDs. Since these scattering rates have been appraised, the net scattering rates G ij between two levels can be deduced. The G ij are taking into account both population of each level and scattering rates. G ij are given by: G ij = 0 f FD E i 1 − f FD E f 1 / ij E i − E f dE i with f FD the Fermi Dirac distribution. We assume an electronic population at the equilibrium , since the intrasubband relaxation is faster than the in-tersubband one. 9,10 The G ij are used for the calculation of R 0 A where R 0 is the resistance per period of the pixel and A the area of the pixel, via the following relation 6 R 0 A = k b T / e 2 iC jC G ij , where C designates one cascade and C the following one. Figure 1 represents R 0 A as a func-a Electronic

### [hal-01438579] Modeling of dark current in midinfrared quantum well infrared photodetectors

Date: 26 Mar 2019 - 14:24

Desc: We present a model for the description of dark IV curves in midinfrared quantum well infrared photodetectors at low temperatures, in a regime where dark current is dominated by interwell tunneling. The model separates the IV curve into a low-field and a high-field region allowing us to identify the effects ascribed to miniband transport and carrier localization, respectively. At low fields the system is thought as a superlattice and described by means of a high-density correction of the Esaki-Tsu model. This approach allows us to simulate current saturation phenomena that occur at low temperatures at intermediate fields. On the other hand, high-field transport effects are described in the localized Wannier-Stark basis in order to account for tunneling and field-assisted thermionic emission effects. We then compare simulations with our measurements of the IV curves of mid-IR quantum well infrared photodetectors finding good quantitative agreement between theory and experiment.

### Autres contacts

Université Paris Diderot - Paris 7

U.F.R. Physique

Bâtiment Condorcet

10, rue Alice Domon et Léonie Duquet

75205 PARIS CEDEX 13