Seminars and Events
Theory and Simulation of Materials
ICMM Coordinators: Elsa Prada, María José Calderón, Ramón Aguado, Sigmund Kohler
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27 May 2021, 12:00 h. online
Electronic structure of the superconducting layered nickelates
Universidade de Santiago de Compostela, Spain
For years, various systems have been considered cuprate analogues and superconductivity has been pursued profusely in some of them. Finally, in 2020 superconductivity was found in infinite-layered nickelates, that share some important structural and electronic-structure features with superconducting cuprates.
Infinite-layer nickelates are nominally Ni+: d9 systems, but they are metallic and are largely self-doped (the actual filling of the d band is less than 9 electrons). Our calculations show that doping with Sr modifies the self-doping effect, leading to a situation more similar to that in the cuprates. E.g. Ni2+ dopants are non-magnetic, just like the Zhang-Rice singlets in the cuprates.
Although LaNiO2 or NdNiO2 are non-magnetic metals, an antiferromagnetic insulating phase exists in the phase diagram of the superconducting cuprates when the dimensionality of the system is reduced. This can be done in various ways, as our ab initio calculations show.
Infinite-layer nickelates are part of a more general family of layered nickelates with formula Rn+1NinO2n+2, that span different doping regimes of the Ni d band (increasing as n increases). The confinement of the NiO2 layers introduces some differences in terms of the peculiar occupation of the Ni dz2 orbitals and also changing the magnetic properties[4,5].
It has been found that Pr4Ni3O8 presents a large orbital polarization of the d manifold, resembling the electronic structure of the cuprates. Electron-doping this system would recover the electronic structure of the parent phase of the cuprates, showing a promising way to search for superconductivity within this series.
 Danfeng Li et al., Nature 572, 624 (2019).
 J. Krishna et al., Phys. Rev. B 102, 224506 (2020).
 V. Pardo, A.S. Botana, arxiv/2012.02711 (2020).
 V. Pardo, W.E. Pickett, Phys. Rev. Lett. 105, 266402 (2010).
 A.S. Botana, V. Pardo et al., Phys. Rev. B 94, 081105 (2016).
 J. Zhang, A.S. Botana et al., Nature Physics 13, 864 (2017).
 A.S. Botana, V. Pardo, M.R. Norman, Phys. Rev. Materials 1, 021801(R) (2017).
13 May 2021, 12:00 h. online
Majorana bound states in topological insulators without a vortex
University of Basel, Switzerland
In my talk, I will discuss topological phases in a three-dimensional topological insulator (TI) wire with a non-uniform chemical potential induced by gating across the cross-section . This inhomogeneity in chemical potential lifts the degeneracy between two one-dimensional surface state subbands. A magnetic field applied along the wire, due to orbital effects, breaks time-reversal symmetry and lifts the Kramers degeneracy at zero-momentum. If placed in proximity to an s-wave superconductor, the system can be brought into a topological phase at relatively weak magnetic fields. Majorana bound states (MBSs), localized at the ends of the TI wire, emerge and are present for an exceptionally large region of parameter space in realistic systems. Unlike in previous proposals, these MBSs occur without the requirement of a vortex in the superconducting pairing potential, which represents a significant simplification for experiments. Our results open a pathway to the realisation of MBSs in present day TI wire devices. In the second part of my talk, I will switch from non-interacting systems, in which one neglects effects of strong electron-electron interactions, to interacting systems and, thus, to exotic fractional phases. In particular, I will focus on second-order TIs [2-5] and discuss two-dimensional fractional second-order topological superconductors, hosting zero-energy parafermion corner states.
 H. F. Legg, D. Loss, and J. Klinovaja, arXiv:2103.13412
 Y. Volpez, D. Loss, and J. Klinovaja, Phys. Rev. Lett. 122,126402 (2019).
 C.-H. Hsu, P. Stano, J. Klinovaja, and D. Loss, Phys. Rev. Lett. 121,196801 (2018).
 K. Laubscher, D. Loss, and J. Klinovaja, Phys. Rev. Research 1, 032017(R) (2019).
 K. Laubscher, D. Loss, and J. Klinovaja, Phys. Rev. Research 2, 013330 (2020).
06 May 2021, 12:00 h. online
Catching the quantum butterfly: Thermalization and information scrambling on a 53-qubit quantum processor
Google Research, Santa Barbara
Over the past decade, rapid progress in quantum engineering has allowed full fabrication and control of superconducting quantum processors with over 50 qubits and full planar connectivity, as exemplified by Google’s Sycamore processor. Using random circuits carefully designed to randomly explore the immense Hilbert space with 10^16 dimensions, a bit-string sampling task that proves difficult for classical computation has been recently demonstrated on Sycamore. In this presentation, I will first detail the formulation and execution of the original “quantum supremacy” experiment. I will then describe current works being done in our group, with a particular emphasis on the efforts to discover useful quantum computing applications in the Noisy Intermediate-Scale Quantum (NISQ) era. As an example, I will present preliminary experimental results on using Sycamore to study the physics of information scrambling and thermalization in quantum circuits that are challenging to analyze or simulate classically.
29 April 2021, 12:00 h. online
Topological Quantum Error Correction: From Theoretical Concepts to Experiments
RWTH Aachen University and Forschungszentrum Jülich, Germany
To date, the construction of a fault-tolerant quantum computer remains a fundamental scientific and technological challenge, due the influence of unavoidable noise which affects the fragile quantum states. In our talk, we first introduce basic concepts of quantum error correction and topological quantum codes, which allow one to protect quantum information during storage and processing. We then present strategies to detect and fight various sources of errors, including the loss of qubits , and present new intriguing theoretical connections between quantum error correction and classical statistical mechanics models and percolation theory . In the final part, I will discuss recent theory work from our group and experimental progress towards fault-tolerant quantum error correction with trapped ions [3,4].
 R. Stricker, et al., Nature 585, 207 (2020)
 D. Vodola, et al., Phys. Rev. Lett. 121, 060501 (2018)
 P. Parrado-Rodríguez et al., arXiv:2012.11366 (2020)
 A. Bermudez et al., Physical Review X 7, 041061 (2017)
22 April 2021, 12:00 h. online
Molecule-based magnetic heterostructures for spintronics and quantum computing
Universidad Autónoma de Madrid
Magnetic molecules are versatile building blocks for quantum computing and molecular spintronics. The molecular spin can encode quantum information in Qbits or even perform logic operations as Qgates with unmatched reproducibility and scalability. In spintronics, that same molecular spin could generate spin currents in molecular-based spin filters, switches or spin valves among other applications. However, the positioning of individual molecules into nanoscale devices, together with the typically insulating character of most molecule-based magnetic materials, challenges their implementation in nanoelectronics.
Here I will show how we overcome these limitations by creating heterostructures that combine magnetic molecules with one-dimensional single-walled carbon nanotubes (SWCNT). SWCNTs can act as electrical and mechanical backbones that protect and sense the molecular spin. Besides, they can be used as vessels to deterministically place the molecules in devices. In particular, I will show the encapsulation of spin cross-over (SCO) molecules within the SWCNT and their placement between nanoscale electrodes . The host SWCNT conductance is modified by the spin state of the guest encapsulated molecules. In turn, the confinement experienced by the molecules is translated into the appearance of a large thermal hysteresis in the SCO molecules. Besides, I will show the mechanical bond of magnetic porphyrin macrocycles embracing SWCNTs . I will show how we place these hybrids in superconducting resonators and how, in the future, this configuration may allow to reach a strong spin-photon coupling at the single molecule level.
 Leuenberger and Loss, Nature 410, 789 (2001), E. Moreno-Pineda et al., Chem Soc. Rev. 47, 501(2018)
 Pal et al., Nat. Commun. 10, 1 (2019)
 Villalva et al., Nat. Commun. 2021. DOI: 10.1038/s41467-021-21791-3
 de Juan-Fernández et al., Chem Sci 9, 6779 (2018)
15 April 2021, 12:00 h. online
Andreev bound states: detection and the road towards quantum technologies
Andreev bound states are the basis of the microscopic picture of the Josephson effect, describing the coupling of two superconductors via a weak link by the formation of localized, single quasiparticle states decoupled from the continuum above the superconducting gap. With the recent improvement of superconductor-semiconductor nanoscale hybrid devices, the readout and manipulation of quasiparticles occupying the Andreev levels became experimentally feasible.In this talk, I will discuss the experimental settings for the measurement of individual Andreev levels in this system. Then, I will provide an overview of the quantum manipulation of the quasiparticle charge and spin, and finally showcase our ongoing efforts towards scalable quantum technologies with Andreev levels.
08 April 2021, 12:00 h. online
Epitaxial growth of 2D materials: Insights from scanning tunneling microscopy
The success of graphene has spurred tremendous expectations and research efforts in the development of novel 2D materials with outstanding properties. A large variety of electronic structures, including band gaps and collective ground states, can be found in the direct analogs of graphene such as its insulating counterpart the hexagonal boron nitride (h-BN) or monolayers of transition-metal dichalcogenides, e.g. MoTe2.
In this talk, I will explore the epitaxial growth of 2D materials and hybrid structures on carefully chosen substrates. The quality of the resulting layers, their physical properties and their interaction with substrates will be examined by high-resolution scanning probe microscopy and first-principle calculations.
25 March 2021, 12:00 h. online
Scaling silicon-based quantum computers using CMOS technology
Quantum Motion Technologies
The spins of isolated electrons in silicon are one of the most promising solid-state systems on which to implement quantum information processing. With the recent demonstrations of long coherence times, high-fidelity spin readout, and one- and two-qubit gates, the basic requirements to build a quantum computer have been fulfilled. Now, scaling the technology to a number of qubits sufficiently large to perform computationally relevant calculations is one of the major objectives and several proposals for large scale integration have been put forward.
Recently, important developments in the field of nanodevice engineering have shown that qubits can be manufactured in a similar fashion to field-effect transistors (FET), creating an opportunity to leverage the scaling capabilities of the semiconductor industry to address the challenge. Quantum computing with silicon transistors fully profits from the most established industrial technology to fabricate large scale integrated circuits while facilitating the integration with conventional electronics for fast data processing of the binary outputs of the quantum processor.
In this talk, I will present a series of results on CMOS transistors at milikelvin temperatures that show this technology could provide a platform on to which implement electron-spin qubits. I will specially concentrate on our efforts to develop a qubit specific measurement technique that is accurate and scalable while being compatible with the industrial fabrication processes. With that, I will show the first measurements of an electron spin in a silicon industry-fabricated device and finally, I will present results on how digital and quantum devices can be combined with this technique to time-multiplex the readout of several qubits.
11 March 2021, 12:00 h. online
Quantum thermoelectrics: using interference, entanglement and non-equilibriumness
Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de
Electrons in a conductor react not only to voltage but also to temperature gradients. In their motion, they carry electric charge as well as energy. This makes it possible to think of devices that are absorb excess heat from their environment and convert it into useful power. This can be done in three terminal devices: Two terminals support the charge current with the third one serving as the heat source, enabling the separation of charge and heat flows. Mesoscopic (nanoscale) systems are good candidates for this, because of their high degree of tunability and rich variety of different effects that allow for the mechanism of heat to power conversion: Coulomb interactions , resonant tunneling , entanglement , quantum interference, or the absence of thermalization . I will review recent proposals and experimental implementations of three terminal energy harvesters.
 R. Sánchez, M. Büttiker, Phys. Rev. B 87, 075312 (2011); H.
Thierschmann et al., Nature Nanotech. 10, 854 (2015); B. Roche et al., Nature Comm. 6, 6738 (2015).
 A. N. Jordan, B. Sothmann, R. Sánchez and M. Büttiker, Phys. Rev. B 87, 075312 (2013); G. Jaliel et al., Phys. Rev. Let. 123, 117701 (2019).
 R. Sánchez,P. Burset, and A. L. Yeyati, Phys. Rev. B 98, 241414 (2018).
 R. Sánchez, J. Splettstoesser and R. S. Whitney, Phys. Rev. Lett. 123, 216801 (2019)
04 March 2021, 12:00 h. online
Colloidal design: building molecules and materials at the micro- and nanometer scale
Institute of Physics University of Amsterdam, Netherlands
Recent breakthroughs in the synthesis and design of colloidal building blocks allow the assembly of complex structures with unprecedented control over their architecture. In particular, patchy particles exhibiting highly directional interactions enable the assembly of “colloidal molecules”, and “colloidal graphene”, analogues of the atomic compounds at the colloidal scale. Such assembly control promises fascinating applications in the design of new functional materials at micrometer and nanometer length scales. In this talk, I will show how the combination of patchy particles and solvent-mediated interactions enables new control in the directional bonding that can be explored to build colloidal molecules and investigate their assembly kinetics and reactions. Using tetramer particles, we assemble colloidal analogs of well-known sp3-hybridized carbon compounds such as (cyclo)butane, butyne, cyclopentane, and cyclohexane, and investigate their transition states. Adsorbed at an attractive substrate, these particles assemble into two-dimensional materials such as graphene. This control applied to the nanoscale can assemble quantum dots, leading to new electronic states useful for active films in optoelectronic devices. These results demonstrate the opportunities for applications and exciting new science that can be explored with these novel colloidal architectures.
25 February 2021, 12:00 h. online
Evolution in uses of Antimony: from ancient Egyptian queen make-up to new 2D-materials
Departmento de Química Inorgánica, Institute for Advanced Research in Chemical Sciences (IAdChem), and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid.
In the search for 2D semiconductor materials, our research group and others have recently pointed out that antimonene, i.e. single or few-layer antimony, can be an interesting option. This seminar will aim to provide a perspective revision of our results on antimonene. I will describe several preparation methods developed in our group to prepare this material, going from top-down procedures, such as micromechanical exfoliation, which allows the isolation of monoatomic layers, or liquid phase exfoliation, to bottom-up synthesis that provides a way to produce large quantities of high-quality few-layer antimonene. Some selected physical and chemical properties found for the isolated antimonene materials will be discussed. Finally, I will show the first results obtained in the exfoliation of 3D covalent crystals, alpha-germanium, which opens new avenues for the 2D-materials.
18 February 2021, 12:00 h. online
Unconventional superconductivity in two-dimensional Van der Waals materials
Miguel M. Ugeda
Donostia International Physics Center (DIPC), Paseo Manuel de Lardizábal 4, 20018 San Sebastián, Spain.
Van der Waals materials provide an ideal platform to explore superconductivity in the presence of strong electronic correlations, which are detrimental of the conventional phonon-mediated Cooper pairing in the BCS-Eliashberg theory and, simultaneously, promote magnetic fluctuations. Despite recent progress in understanding superconductivity in layered materials, the glue pairing mechanism remains largely unexplored in the single-layer limit, where electron-electron interactions are dramatically enhanced. In this talk, I will present experimental evidence of unconventional Cooper pairing mediated by magnetic excitations in monolayers of Se-based transition metal superconductors (NbSe2 and TaSe2), two model strongly correlated 2D materials.
2D TMD materials will reduce the enormous complexity associated with the investigation of unconventional superconductivity, and will rapidly allow us to expand our current limited knowledge of non-phononic Cooper pairing. They offer unprecedented simplicity for modelling as compared to the most studied bulky unconventional superconductors, i.e., cuprates, Fe-pnictides and heavy-fermion compounds. In two dimensions, TMD superconductors are even simpler to model than twisted bilayer graphene, where superconductivity is intrinsically linked to specific magic angles. From the experimental point of view, our work opens the tantalizing possibility to explore unconventional superconductivity in simple, scalable and widely accessible 2D materials.
11 February 2021, 12:00 h. online
From Andreev to Majorana bound states in hybrid superconductor semiconductor nanowires
Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC)
Electronic excitations above the ground state must overcome an energy gap in superconductors with spatially-homogeneous s-wave pairing. In contrast, inhomogeneous superconductors such as those with magnetic impurities or weak links, or heterojunctions containing normal metals or quantum dots, can host subgap electronic excitations that are generically known as Andreev bound states (ABSs). With the advent of topological superconductivity, a new kind of ABS with exotic qualities, known as Majorana bound state (MBS), has been discovered. In this talk, I will focus on hybrid superconductor-semiconductor nanowires as one of the most flexible and promising experimental platforms to study ABSs and MBSs. I'll discuss how the combined effect of spin-orbit coupling and Zeeman field in these wires triggers the transition from ABSs into MBSs. I'll show theoretical progress beyond minimal models in understanding experiments, including the possibility of a type of robust zero mode that may emerge without a band-topological transition, called quasi-MBS or non-topological MBS in the field. Finally, I'll discuss the role of spatial non-locality, a special property of MBS wavefunctions that, together with non-Abelian braiding, is the key to realizing topological quantum computation.
This work has been recently published as a review in: Nature Reviews Physics 2, 575 (2020)
04 February 2021, 12:00 h. online
Quantum simulations of condensed matter systems with quantum technologies
Universidad de Sevilla
In this talk I will review recent research on quantum simulations of condensed matter systems with quantum controllable platforms, including trapped ions and superconducting circuits. Among others, we will describe efficient quantum algorithms for simulating fermionic systems in two and three spatial dimensions, as well as proposals for the quantum simulation of the quantum Rabi and Dicke models with analog and digital-analog quantum simulators.
28 January 2021, 12:00 h. online
Toroids, Active Nematics and Topological Defects
Department of Condensed Matter Physics, University of Barcelona
ICREA - Institució Catalana de Recerca i Estudis Avançats, Barcelona
We will discuss recent results with active nematics confined to either toroidal space. We will first review how we make and stabilize non-spherical droplets and describe how curvature affects defect arrangement on tori. We will show that despite the intrinsic activity and out-of-equilibrium character of the system, there are still remnants of the expected curvature-induced defect unbinding predicted for nematics in their ground state. Activity, however, augments the behavior leading to unexpected defect distributions. We will then focus on defect orientation and show that on flat space, there is short-range orientational correlations without long-range orientational order.
21 January 2021, 12:00 h. online
Iron-based superconductors – Materials, magnetism and tuning methods
Iron-based superconductors represent a second class of high-temperature superconductors after the copper oxides. As they enter their second decade, the large variety of their chemical structures, modes of magnetic ordering and the methods available to tune their properties are all the more appreciated. As with many unconventional superconductors, their magnetic order needs to be suppressed for superconductivity to appear. I will present an overview of the magnetism in several of these iron-based materials and the methods to suppress, or tune, it. Magnetism is particularly strongly coupled to the crystal lattice, which makes structural tuning parameters interesting. I will also discuss some of the other surprising effects that arise from this close interaction of structure, magnetism and superconductivity. Comparing diverse materials and their properties helps us understand some of the common principles within this class of unconventional superconductors.
14 January 2021, 12:00 h. online
Straintronics with 2D materials
Strain engineering is an interesting strategy to tune a material’s electronic properties by subjecting its lattice to a mechanical deformation. Conventional straining approaches, used for 3D materials (including epitaxial growth on a substrate with a lattice parameter mis-match, the use of a dielectric capping layer or heavy ions implantation) are typically limited to strains lower than 2% in most cases due to the low maximum strains sustained by brittle bulk semiconducting materials. Bulk silicon, for example, can be strained only up to 1.5% before breaking. Moreover, these straining approaches induce static deformations of the semiconductor materials and therefore they are not suitable for tunable functional devices.2D materials can be literally stretched, folded, bent or even pierced.  This outstanding stretchability (and the possibility of using dynamically varying strain) of 2D materials promises to revolutionize the field of strain engineering and could lead to "straintronic" devices – devices with electronic and optical properties that are engineered through the introduction of mechanical deformations.In this talk I will discuss our recent efforts to study strain engineering in 2D materials and to exploit it to fabricate strain tunable functional optoelectronic devices. [2-7].This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 755655, ERC-StG 2017 project 2D-TOPSENSE). ACG acknowledge funding from the EU Graphene Flagship funding (Grant Graphene Core 2, 785219). Roldán et al. Journal of Physics: Condensed Matter (2015) 27 (31), 313201 A. Castellanos-Gomez, et al. Nano letters (2013) 13 (11), 5361-5366 J. Quereda, et al. Nano letters (2016) 16 (5), 2931-2937 J.O. Island, et al. Nanoscale (2016) 8 (5), 2589-2593 R. Schmidt et al. 2D Materials (2016) 3 (2), 021011 R. Frisenda, et al. npj 2D Materials and Applications (2017) 1 (1), 10 P. Gant, et al. Materials Today (2019)