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Seminarios y Eventos

Seminars and Events

Teoría y Simulación de Materiales: Seminarios Alternativos

Coordinadores: lvaro Gmez Len, Sigmund Kohler

19 de marzo de 2020, 12:00 h. Sala de Seminarios, 182


Low-dimensional magnetic heterostructures for quantum computing and spintronics

Enrique Burzur

Molecular spins have been proposed as Qbits due to their chemical tunability, scalability and reproducibility. The control of these spin Qbits can be done in quantum circuits, like coplanar superconducting resonators, where the spins are coupled to the
photon generated in a central transmission line. The strong spin-photon coupling, necessary to achieve a coherent control of the qubit, is however limited to large assembles of spins. Strategies to increase the coupling and thereof reach the single molecular spin, have tried to reduce the width of the transmission line to constrictions where the molecules are deposited. We propose to downscale further and reach the single-spin sensitivity limit by directly replacing the superconducting constriction with low-dimensional magnetic heterostructures. We will show the different chemical strategies to achieve the magnetic heterostructures, their integration in nanoscale devices and their additional applications in spintronics.

27 de febrero de 2020, 12:00 h. Sala de Seminarios, 182

Artificial intelligence for condensed matter physics

Jorge Bravo Abad
Dpto. de Fsica Terica de la Materia Condensada

This seminar aims at stimulating a discussion on the potential of artificial intelligence as a novel tool in condensed matter physics research. To do that, I will first provide an overview of artificial intelligence from the standpoint of a condensed matter physicist. Then, I will provide illustrative examples of how artificial intelligence enables solving problems that otherwise would be very challenging (if not directly impossible) to tackle, particularly focusing on our own work on the topic.

06 de febrero de 2020, 12:00 h. Sala de Seminarios, 182

New anomaly induced transport in Weyl matter

Mara A. H. Vozmediano
Instituto de Ciencia de Materiales de Madrid

After the synthesis of graphene (described at low energies as massless Dirac fermions in 2+1 dimensions) in 2005, Weyl semimetals were synthesized in 2015. Although they can be seen as 3D graphene, a series of new phenomena arise from the fundamental differences between chiral fermions in two and three dimensions. Chiral imbalance in 3D implies a set of anomaly related transport phenomena first discussed in the context of high energy collisions (quark-gluon plasma). Examples are the chiral magnetic effect: generation of an electric current parallel to en applied magnetic field, or the axial magnetic effect: generation of an energy current parallel to an axial magnetic field [1]. In this talk we will see some of the implications of these phenomena to the novel Dirac materials. I will explain the origin of the anomaly--induced response functions and review the experimental evidences found so far. Finally I will describe novel response functions associated to the scale anomaly in Dirac and Weyl semimetals [2]. I will try to be pedagogical.

[1] D. E. Kharzeev, arXiv:1312.3348.
[2] M. N. Chernodub, A. Cortijo, and M. A. H. Vozmediano, Phys. Rev. Lett. 120, 206601 (2018);
M. N. Chernodub, M. A. H. Vozmediano, Phys. Rev. Res. 1, 032002(R) (2019);
V. Arjona, M. N. Chernodub, M. A. H. Vozmediano, Phys. Rev. B 99, 235123 (2019);
M. Chernodub and M. A. H. Vozmediano, Phys. Rev. Res. 1, 032040(R) (2019).

31 de enero de 2020, 12:00 h. Saln de Actos

Probing the Quantum Mechanics of Many-Body Chaos

Klaus Richter
Regensburg University

The dynamics and spread of quantum information in complex many-body systems is presently attracting a lot of attention across various fields, ranging from cold atom physics via condensed quantum matter to high energy physics and quantum gravity. This includes questions of how a quantum system thermalizes and phenomena like many-body interference and localization, more generally non-classicality in many-particle quantum physics. Here concepts that are based on echoes, i.e. "rewinding" time, provide a powerful way to monitor complex quantum dynamics and its stability. Central to these developments are so-called out-of-time-order correlators (OTOCs) as sensitive probes for chaos and the temporal growth of complexity in interacting systems. We will address such phenomena for quantum critical and quantum chaotic systems using semiclassical path integral techniques based on interfering Feynman paths, thereby bridging the classical and quantum many-body world. These methods enable us to compute echoes and OTOCs including entanglement and correlation effects. Moreover, on the numerical side we devise a semiclassical method for Bose-Hubbard systems far-out-of equilibrium that allows us to calculate many-body quantum interference on time scales far beyond the famous Ehrenfest/scrambling time.

23 de enero de 2020, 12:00 h. Sala de Seminarios, 182

Exploring the metal-insulator transition in vanadium oxide thin films

Mariela Menghini

One of the consequences of strong electron-electron interactions in transition metal oxides is the transition from the Mott insulating phase to a metal, and therefore several orders of magnitude change in resistance, as external parameters are varied. The metal-insulator transition (MIT) in these materials has been a long-standing topic of research both from a theoretical and experimental point of view. In particular, two of the phases from the family of vanadium oxides are of great interest, namely vanadium sesquioxide (V_2 O_3) and vanadium dioxide (VO_2). The first one has a very rich phase diagram and is considered a paradigmatic example of a pure metal-Mott insulator phase transition. On the other hand, VO_2 with a phase transition close to room temperature, has gained lot of attention due to the possible applications of MIT materials in devices. In this talk, I will discuss how strain, doping and defects influence the metal-insulator phase transition in V_2 O_3 thin films. Finally, I will also present electrical and optical properties of VO_2 films and their integration in silicon photonic devices.

12 de diciembre de 2019, 12:00 h. Sala de Seminarios, 182

Coupling Atom-Like Spins in Semiconductors: Toward Scalable Quantum Computing

Frederico Martins
University of New South Wales

In a world where the amount of data to process is steadily increasing, the quantum nature of matter offers new possibilities to develop concepts, which may overcome nowadays technologies. Implications are expected in research areas that can range from quantum computation, cryptography, and quantum simulation.

To be useful, a qubit (the elementary quantum unit of information) needs to be both isolated from its environment and precisely controllable, which places strict requirements on its physical realization. In particular, spins in solids are one of the most promising realizations due to their potential for scalability and miniaturization. Furthermore, in these systems, quantum control has been established and electron spin coherence times now exceed several seconds. Even so, a critical challenge in these systems consists of developing a robust two-qubit gate that can be scaled up to a larger network.

In this seminar, I will overview the first attempts to do two-qubit operations with spin qubits and I will introduce a new mechanism for long-range interaction. Making use of independent readout of two electron spins, we demonstrate coherent exchange interaction mediated by a multielectron quantum dot. This result provides a possible route to the realization of multi-qubit quantum circuits based on single spins.

21 de noviembre de 2019, 12:00 h. Sala de Seminarios, 182

Coherent population trapping by dark states formation in carbon nanotubes

Milena Grifoni
Regensburg University

Illumination of atoms by resonant lasers can pump electrons into a coherent superposition of hyperfine levels which can no longer absorb the light. Such superposition is known as dark state, because fluorescent light emission is then suppressed. Here we report an all-electric analogue of this destructive interference effect in a carbon nanotube quantum dot. The dark states are a coherent superposition of valley (angular momentum) states which are decoupled from either the drain or the source leads. Their emergence is visible in asymmetric current-voltage characteristics, with missing current steps and current suppression which depend on the polarity of the applied source-drain bias. Our results demonstrate coherent-population trapping by all-electric means in an artificial atom.

17 de octubre de 2019, 12:00 h. Sala de Seminarios, 182

Unconventional quantum optics in topological waveguide QED

Miguel Bello

Abstract: In the race to find the next big technological application of quantum systems, quantum emitters coupled to photonic crystals seems a promising platform with many potential applications. Specifically, they can be used to simulate spin Hamiltonians whose interactions can be tailored by tuning the properties of the photonic crystals or those of the emitters. In this talk I will review a recent work [1] where we explore the physics emerging when quantum emitters interact with a topological 1D photonic crystal, namely a photonic analogue of the SSH model. We will see that the topology of the crystal reflects itself in the effective dipole-dipole interactions produced between the emitters and also the single-photon scattering properties.

[1] M. Bello, G. Platero, J. I. Cirac, and A. Gonzlez-Tudela,
"Unconventional quantum optics in topological waveguide QED". Sci.
Adv. 5, eaaw0297 (2019).

10 de octubre de 2019, 12:00 h. Sala de Seminarios, 182

Drift instabilities in graphene as a source of THz radiation

Hugo Teras
Instituto Superior Tcnico, University of Lisbon

Electrically injected terahertz (THz) radiation sources are extremely appealing given their versatility and miniaturization potential, opening the venue for integrated-circuit THz technology. In this work, we show that coherent THz frequency combs in the range 0.5 THz<ω/2π<10 THz can be generated making use of graphene plasmonics. Our setup consists of a graphene field-effect transistor with asymmetric boundary conditions, with the radiation originating from a plasmonic instability that can be controlled by direct current injection. To continue the debate, we discuss some of the controversy still present around the various hydrodynamical models for graphene and present a strategy to mitigate it based on microscopic wave kinetics.

09 de octubre de 2019, 12:00 h. Sala de Seminarios, 182

Long-range chirality dimer order in the spin S = 1/2 Heisenberg chain with alternating Dzyaloshinskii-Moriya interaction

George I. Japaridze
Ilia State University

The ground state phase diagram of a spin S=1/2 XXZ Heisenberg chain with spatially modulated Dzyaloshinskii-Moriya (DM) interaction parametrized by D0, D1 is studied using the continuum-limit bosonization approach and extensive density matrix renormalization group computations. It is shown that at finite values of the parameters, depending on the value of an effective anisotropy parameter, the ground state phase diagram of the model contains four phases: the ferromagnetic phase, the gapless Luttinger-liquid (LL) phase,the gapped composite (C1) phase characterized by coexistence of the long-range-ordered (LRO) dimerization pattern with the LRO alternating spin current (chirality) pattern, and the gapped composite (C2) phase characterized in addition to the coexisting spin dimerization and alternating chirality patterns, by the presence of LRO antiferromagnetic order. The transition from the LL to the C1 phase belongs to the Berezinskii-Kosterlitz-Thouless universality class, while the transition from C1 to C2 phase is of the Ising type. Magnetic phase diagram of a spin S = 1=2 antiferromagnetic two-leg ladder with period two lattice units modulated DM interaction along the legs will is also discussed.

26 de septiembre de 2019, 12:00 h. Sala de Seminarios, 182

Electron-Phonon Interaction in Nanoelectronic Circuits:
toward the control of single phonons

Stefan Ludwig
Paul Drude Institute for Solid-State Electronics, Berlin, Germany

Electron-phonon interaction strongly influences the dynamics of mesoscopic quantum circuits; in nanostructures it can be further enhanced by size effects. This leads to detector backaction [1] and dephasing of solid state qubits [2]. Can we take a more optimistic perspective and aim at a full control of phonons? Are coherent electron-phonon-hybrid nanosystems interesting for quantum technology applications? In this talk I will present a series of experiments performed in semiconductor based mesoscopic samples ranging from phonon-mediated energy transfer in a two-dimensional electron-system to coherent electron-phonon interaction in lateral coupled quantum dot systems [1-4]. Finally, I will provide an outlook on how we plan to achieve strong coupling between confined electrons and surface phonons stored in cavities.

[1] Phonon-mediated vs. Coulombic Back-Action in Quantum Dot circuits; D. Harbusch et al.: Phys. Rev. Lett. 104, 196801 (2010).
[2] Characterization of Qubit Dephasing by Landau-Zener-Stckelberg-Majorana Interferometry; F. Forster et al.: Phys. Rev. Lett. 112, 116803 (2014).
[3] Phonon-mediated non-equilibrium interaction between nanoscale devices; G.J. Schinner et al.: Phys. Rev. Lett. 102, 186801 (2009).
[4] Quantum interference and phonon-mediated back-action in lateral quantum-dot circuits; G. Granger et al.: Nature Phys. 8, 522527 (2012).

19 de septiembre de 2019, 12:00 h. Sala de Seminarios, 182

Transport phenomena in hybrid superconductor devices

Robert HusseinEve
University of Konstanz

Cooper-pairs are a source of correlated electrons. Their nonlocal breaking can be used in entanglement generation, thermoelectric transport, and spin-current-control. On the basis of a double-quantum-dot-superconductor three-terminal system, we demonstrate that Cooper-pair splitting induces a thermo-current in the superconducting lead without any transfer of charge between the two normal metal leads [1]. Conversely, we show that a nonlocal heat exchange between the normal leads is mediated by nonlocal Andreev reflection. Coupling each quantum dot to an electromagnetic or mechanical resonator, realizes a device which can simultaneously cool down both resonators into their ground state [2]. We show that the process of cross-Andreev reflection can additionally be employed to coherently transfer single photons, and therewith heat, between both the oscillators. Furthermore, we investigate the Josephson current through a suspended carbon nanotube quantum dot, which acts likewise as a mechanical resonator. We discuss that a finite resonator coupling can induce phase transitions between groundstates of different total spin. A footprint also manifests in the critical current. Finally, we study spin-dependent quasiparticle and Cooper-pair transport through an island interfaced with two superconductors and two ferromagnets [3]. We demonstrate that unequal spin-mixing conductances for the two interfaces with the ferromagnets result in equal-spin triplet correlations on the island, detectable via a net charge current between the two magnets. The setup enables the generation, control, and detection of typically elusive equal-spin triplet Cooper-pairs.
[1] R. Hussein et al, PRB 99, 075429 (2019).
[2] M. Mantovani et al, arXiv:1907.04308 (2019).
[3] A. Rezaei et al, arXiv:1908.09610 (2019)

12 de septiembre de 2019, 12:00 h. Sala de Seminarios, 182

Quantum sensing with NV centers. Limits of spectral resolution

Javier Prior
Departamento de Fsica Aplicada, Universidad Politcnica de Cartagena

In Quantum Sensing (QS) exploiting the quantum properties of entanglement and coherence, quantities such as charge, magnetic and electric fields, pressure, and temperature can be measured with a sensitivity which significantly exceeds any classical methodologies. New concepts of QS currently being developed could provide groundbreaking measurement techniques. It is believed that the field of QS will transform science and technology in the next decade. Among the most prominent devices, NV centers in diamonds represent one of the most promising platforms for sensing or metrology and they are proposed as a new generation for Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI).

The limits of frequency resolution in nano-NMR experiments have been discussed extensively in recent years. It is believed that there is a crucial difference between the ability to resolve a few frequencies and the precision of estimating a single one. Whereas the efficiency of single frequency estimation gradually increases with the square root of the number of measurements, the ability to resolve two frequencies is limited by the specific timescale of the signal and cannot be compensated for by extra measurements. In this talk, we show theoretically and demonstrate experimentally that the relationship between these quantities is more subtle and both are only limited by the Cramr-Rao bound of a single frequency estimation [1].

[1] Amit Rotem, Tuvia Gefen, Santiago Oviedo-Casado, Javier Prior, Simon Schmitt, Yoram Burak, Liam McGuiness, Fedor Jelezko, and Alex Retzker Phys. Rev. Lett. 122, 060503 (2019).


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