Showing 1–50 of 109 results for author: Huber, D

Observation of ring states in a delicate topological insulator

Authors: Caroline Tornow, Julia Rupprecht, Pascal Engeler, Ute Drechsler, Kukka-Emilia Huhtinen, Chiara Devescovi, Sebastian D. Huber

Abstract: Topological insulators are typically characterized by particularly stable properties, such as global invariants, and can be identified by probing their robust surface states. A recently discovered novel form of band topology, delicate topology, challenges this paradigm: its defining property, multicellularity, can be removed by introducing a coupling to local orbitals anywhere in the spectrum, eve… ▽ More Topological insulators are typically characterized by particularly stable properties, such as global invariants, and can be identified by probing their robust surface states. A recently discovered novel form of band topology, delicate topology, challenges this paradigm: its defining property, multicellularity, can be removed by introducing a coupling to local orbitals anywhere in the spectrum, even far above the relevant band gap. This makes it hard to diagnose delicate topology with conventional probes that access only low-energy degrees of freedom. Here, we introduce strong local impurities as a spectroscopic probe of a delicate topological insulator which we realize in a phononic metamaterial. By tuning the impurity strength and performing orbital-resolved readout, we observe recently proposed indicators of topology: ring states, in-gap bound states whose frequencies remain pinned in the strong-impurity limit while their real-space profiles form a pronounced ring around the impurity site. We find that these ring states persist even when the multicellularity in our system is removed by a weakly hybridizing additional orbital. Our results establish impurity-induced ring states as probes of complex multiband physics, including delicate topological phases. △ Less

Submitted 17 April, 2026; originally announced April 2026.

Bulk-dissociated topological bands without spin-orbit coupling in hetero-dimensional superconducting metamaterials

Authors: Joseph J. Cuozzo, Sayed Ali Akbar Ghorashi, Dale Huber, Wei Pan, François Léonard

Abstract: Topological superconductors (TSCs) in superconducting hybrid heterostructures, which integrate superconducting and non-superconducting materials, have been intensely investigated with the hope of discovering exotic non-Abelian anyons for fault-tolerant quantum computing. In this effort, a challenge for hybrid superconducting systems is controlling hybridization, which is often a balance between en… ▽ More Topological superconductors (TSCs) in superconducting hybrid heterostructures, which integrate superconducting and non-superconducting materials, have been intensely investigated with the hope of discovering exotic non-Abelian anyons for fault-tolerant quantum computing. In this effort, a challenge for hybrid superconducting systems is controlling hybridization, which is often a balance between enhancing the superconducting proximity effect at the cost of suppressing desirable electronic properties such as strong spin-orbit interactions. Hence, discovering hybrid superconducting systems with topological properties controlled and enhanced by material geometry design without spin-orbit interactions would be intriguing to explore. In this work, we theoretically study a square superconducting network decorated with spin-polarized magnetic adatoms. We find that localized Yu-Shiba-Rusinov bound states at magnetic adatom sites collectively form a weak topological superconducting phase despite the absence of spin-orbit interactions. We then demonstrate that by tuning the Fermi energy of the network, the system can transition from a weak TSC phase to a bulk-dissociated TSC phase where the edge state bands separate from the bulk, giving rise to unexpected features such as nodal lines and co-existing bulk-dissociated edge and corner modes. Moreover, our findings highlight how hetero-dimensional superconducting metamaterials can serve as a useful template for controlling the coupling and dissociation between electronic degrees of freedom of different dimensionalities. △ Less

Submitted 13 April, 2026; v1 submitted 9 April, 2026; originally announced April 2026.

Comments: 28 pages, 6 figures, 1 table

Interplay of local and global quantum geometry in the stability of flat-band superfluids

Authors: Kukka-Emilia Huhtinen, Matteo Dürrnagel, Valerio Peri, Sebastian D. Huber

Abstract: Quantum geometry strongly impacts physical properties in flat-band systems. We consider its role in bosonic condensation and superfluidity on flat bands, and show that the superfluid weight has an important contribution proportional to the condensate quantum metric. Based on this result, we uncover conditions under which flat-band superfluidity is unlikely. For instance, we find that stable flat-b… ▽ More Quantum geometry strongly impacts physical properties in flat-band systems. We consider its role in bosonic condensation and superfluidity on flat bands, and show that the superfluid weight has an important contribution proportional to the condensate quantum metric. Based on this result, we uncover conditions under which flat-band superfluidity is unlikely. For instance, we find that stable flat-band superfluidity in a two-dimensional system requires at least three bands within Bogoliubov theory. Because the quantum geometry at the condensation momentum plays a disproportionately large role, a large integrated quantum metric is not sufficient for flat-band superfluidity, but how the quantum metric is distributed in the Brillouin zone is crucial. △ Less

Submitted 9 March, 2026; originally announced March 2026.

Comments: 13 pages, 2 figures

Superfluid stiffness of superconductors with delicate topology

Authors: Tijan Prijon, Sebastian D. Huber, Kukka-Emilia Huhtinen

Abstract: We consider superconductivity in two-dimensional delicate topological bands, where the total Chern number vanishes but the Brillouin zone can be divided into subregions with a quantized nontrivial Chern number. We formulate a lower bound on the geometric contribution to the superfluid weight in terms of the sum of the absolute values of these sub-Brillouin zone Chern numbers. We verify this bound… ▽ More We consider superconductivity in two-dimensional delicate topological bands, where the total Chern number vanishes but the Brillouin zone can be divided into subregions with a quantized nontrivial Chern number. We formulate a lower bound on the geometric contribution to the superfluid weight in terms of the sum of the absolute values of these sub-Brillouin zone Chern numbers. We verify this bound in Chern dartboard insulators, where the delicate topology is protected by mirror symmetry. In iso-orbital models, where the mirror representation is the same along all high-symmetry lines, the lower bound increases linearly with the number of mirror planes. This work points to delicate bands as promising candidates for particularly stable superconductivity, especially in narrow bands where the kinetic energy is suppressed due to lattice effects. △ Less

Submitted 11 February, 2026; v1 submitted 22 July, 2025; originally announced July 2025.

Comments: 7+3 pages, 5 figures

Journal ref: Phys. Rev. B 113, 064504 (2026)

Electrokinetic nanofluidic sensing of DNA nanostar condensate

Authors: Kuang-Hua Chou, Alexander Eden, David E. Huber, Sumita Pennathur, Deborah Kuchnir Fygenson

Abstract: We demonstrate electronic sensing of DNA nanostar (NS) condensate. Specifically, we use electrokinetic nanofluidics to observe and interpret how temperature-induced NS condensation affects nanochannel current. The increase in current upon filling a nanochannel with NS condensate indicates that its electrophoretic mobility is about half that of a single NS and its effective ionic strength is… ▽ More We demonstrate electronic sensing of DNA nanostar (NS) condensate. Specifically, we use electrokinetic nanofluidics to observe and interpret how temperature-induced NS condensation affects nanochannel current. The increase in current upon filling a nanochannel with NS condensate indicates that its electrophoretic mobility is about half that of a single NS and its effective ionic strength is $\sim35$% greater than that of 150mM NaCl in phosphate buffer. $ζ$-potential measurements before and after exposure to NS show that condensate binds the silica walls of a nanochannel more strongly than individual NS do under identical conditions. This binding increases electroosmotic flow, possibly enough to completely balance, or even exceed, the electrophoretic velocity of NS condensate. Although the current through a flat nanochannel is erratic in the presence of NS condensate, tilting the nanochannel to accumulate NS condensate at one entrance (and away from the other) results in a robust electronic signature of the NS phase transition at temperatures $T_c$ = $f$([NaCl]) that agree with those obtained by other methods. Electrokinetic nanofluidic detection and measurement of NS condensate thus provides a foundation for novel biosensing technologies based on liquid-liquid phase separation. △ Less

Submitted 14 May, 2025; v1 submitted 11 December, 2024; originally announced December 2024.

Time-resolved diamond magnetic microscopy of superparamagnetic iron-oxide nanoparticles

Authors: B. A. Richards, N. Ristoff, J. Smits, A. Jeronimo Perez, I. Fescenko, M. D. Aiello, F. Hubert, Y. Silani, N. Mosavian, M. Saleh Ziabari, A. Berzins, J. T. Damron, P. Kehayias, D. Egbebunmi, J. E. Shield, D. L. Huber, A. M. Mounce, M. P. Lilly, T. Karaulanov, A. Jarmola, A. Laraoui, V. M. Acosta

Abstract: Superparamagnetic iron-oxide nanoparticles (SPIONs) are promising probes for biomedical imaging, but the heterogeneity of their magnetic properties is difficult to characterize with existing methods. Here, we perform widefield imaging of the stray magnetic fields produced by hundreds of isolated ~30-nm SPIONs using a magnetic microscope based on nitrogen-vacancy centers in diamond. By analyzing th… ▽ More Superparamagnetic iron-oxide nanoparticles (SPIONs) are promising probes for biomedical imaging, but the heterogeneity of their magnetic properties is difficult to characterize with existing methods. Here, we perform widefield imaging of the stray magnetic fields produced by hundreds of isolated ~30-nm SPIONs using a magnetic microscope based on nitrogen-vacancy centers in diamond. By analyzing the SPION magnetic field patterns as a function of applied magnetic field, we observe substantial field-dependent transverse magnetization components that are typically obscured with ensemble characterization methods. We find negligible hysteresis in each of the three magnetization components for nearly all SPIONs in our sample. Most SPIONs exhibit a sharp Langevin saturation curve, enumerated by a characteristic polarizing applied field, B_c. The B_c distribution is highly asymmetric, with a standard deviation (1.4 mT) that is larger than the median (0.6 mT). Using time-resolved magnetic microscopy, we directly record SPION Néel relaxation, after switching off a 31 mT applied field, with a temporal resolution of ~60 ms that is limited by the ring-down time of the electromagnet coils. For small bias fields B_{hold}=1.5-3.5 mT, we observe a broad range of SPION Néel relaxation times--from milliseconds to seconds--that are consistent with an exponential dependence on B_{hold}. Our time-resolved diamond magnetic microscopy study reveals rich SPION sample heterogeneity and may be extended to other fundamental studies of nanomagnetism. △ Less

Submitted 3 February, 2025; v1 submitted 20 November, 2024; originally announced November 2024.

Comments: Main text: 9 pages, 5 figures. Entire manuscript including Appendices: 31 pages, 24 figures

Journal ref: ACS nano 19 (10), 10048-10058 (2025)

Design and characterization of all two-dimensional fragile topological bands

Authors: Samuel Bird, Chiara Devescovi, Pascal Engeler, Agnes Valenti, Doruk Efe Gökmen, Robin Worreby, Valerio Peri, Sebastian D. Huber

Abstract: Designing topological materials with specific topological indices is a complex inverse problem, traditionally tackled through manual, intuition-driven methods that are neither scalable nor efficient for exploring the vast space of possible material configurations. In this work, we develop an algorithm that leverages the covariance matrix adaptation evolution strategy to optimize the Fourier repres… ▽ More Designing topological materials with specific topological indices is a complex inverse problem, traditionally tackled through manual, intuition-driven methods that are neither scalable nor efficient for exploring the vast space of possible material configurations. In this work, we develop an algorithm that leverages the covariance matrix adaptation evolution strategy to optimize the Fourier representation of the periodic functions shaping the designer material's characteristics. This includes mass profiles or dielectric tensors for phononic and photonic crystals, respectively, as much as synthetic potentials applicable to electronic and ultra-cold atomic systems. We demonstrate our methodology with a detailed characterization of a class of topological bands known as "fragile topological", showcasing the algorithm's capability to address both topological characteristics and spectral quality. This automation not only streamlines the design process but also significantly expands the potential for identifying and constructing high quality designer topological materials across the wide range of platforms, and is readily extendable to other setups, including higher-dimensional and non-linear systems. △ Less

Submitted 14 October, 2024; originally announced October 2024.

Comments: 4 pages + 45 pages SI, 4 + 49 figures

Journal ref: PNAS Nexus, Volume 4, Issue 9, September 2025, pgaf285

Enhanced magnetic dichroism in darkfield UV photoemission electron microscopy

Authors: Maximilian Paleschke, David Huber, Friederike Wührl, Cheng-Tien Chiang, Frank O. Schumann, Jürgen Henk, Wolf Widdra

Abstract: Photoemission electron microscopy (PEEM) has evolved into an indispensable tool for structural and magnetic characterization of surfaces at the nanometer scale. In strong contrast to synchrotron-radiation-based X-ray PEEM as a leading method for element-specific magnetic properties via magnetic circular dichroism (MCD), laboratory ultraviolet (UV) PEEM has seen limited application with much smalle… ▽ More Photoemission electron microscopy (PEEM) has evolved into an indispensable tool for structural and magnetic characterization of surfaces at the nanometer scale. In strong contrast to synchrotron-radiation-based X-ray PEEM as a leading method for element-specific magnetic properties via magnetic circular dichroism (MCD), laboratory ultraviolet (UV) PEEM has seen limited application with much smaller dichroism effects for in-plane magnetization. Here we introduce darkfield PEEM as a novel approach to enhance MCD contrast in threshold photoemission, enabling efficient MCD imaging with significantly enhanced contrast by an order-of-magnitude for Fe(001). This advancement paves the way for MCD imaging on femtosecond timescales using modern lasers. The experimental results will be quantitatively benchmarked against advanced relativistic photoemission calculations. △ Less

Submitted 15 May, 2025; v1 submitted 7 September, 2024; originally announced September 2024.

Theory of Coulomb driven nematicity in a multi-valley two-dimensional electron gas

Authors: Vladimir Calvera, Agnes Valenti, Sebastian D. Huber, Erez Berg, Steven A. Kivelson

Abstract: The properties of a two-dimensional electron gas (2DEG) in a semiconductor host with two valleys related by an underlying $C_4$ rotational symmetry are studied using Hartree-Fock (HF) and various other many-body approaches. A familiar artifact of the HF approach is a degeneracy between the valley polarized - ``Ising nematic'' - and spin polarized - ferromagnetic - phases, which is inconsistent wit… ▽ More The properties of a two-dimensional electron gas (2DEG) in a semiconductor host with two valleys related by an underlying $C_4$ rotational symmetry are studied using Hartree-Fock (HF) and various other many-body approaches. A familiar artifact of the HF approach is a degeneracy between the valley polarized - ``Ising nematic'' - and spin polarized - ferromagnetic - phases, which is inconsistent with recent variational Monte Carlo (VMC) results. Correlation effects, computed either within the random phase approximation (RPA) or the T-matrix approximation, enhance the valley susceptibility relative to the spin susceptibility. Extrapolating the results to finite interaction strength, we find a direct first-order transition from a symmetry-unbroken state to a spin unpolarized Ising nematic fluid with full valley polarization, in qualitative agreement with VMC. The RPA results are also reminiscent of experiments on the corresponding 2DEG in AlAs heterostructures. △ Less

Submitted 12 August, 2024; v1 submitted 18 June, 2024; originally announced June 2024.

Comments: 6 + 25 pages, 12 figures

Journal ref: Phys. Rev. B 111, 155135 (2025)

Nematic metal in a multi-valley electron gas: Variational Monte Carlo analysis and application to AlAs

Authors: Agnes Valenti, Vladimir Calvera, Steven A. Kivelson, Erez Berg, Sebastian D. Huber

Abstract: The two-dimensional electron gas is of fundamental importance in quantum many-body physics. We study a minimal extension of this model with $C_4$ (as opposed to full rotational) symmetry and an electronic dispersion with two valleys with anisotropic effective masses. Using variational Monte Carlo simulations, we find a broad intermediate range of densities with a metallic valley-polarized, spin-un… ▽ More The two-dimensional electron gas is of fundamental importance in quantum many-body physics. We study a minimal extension of this model with $C_4$ (as opposed to full rotational) symmetry and an electronic dispersion with two valleys with anisotropic effective masses. Using variational Monte Carlo simulations, we find a broad intermediate range of densities with a metallic valley-polarized, spin-unpolarized ground-state. Our results are of direct relevance to the recently discovered ``nematic'' state in AlAs quantum wells. For the effective mass anisotropy relevant to this system, $m_x/m_y\approx 5.2$, we obtain a transition from an anisotropic metal to a valley-polarized metal at $r_s \approx 12$ (where $r_s$ is the dimensionless Wigner-Seitz radius). At still lower densities, we find a (possibly metastable) valley and spin-polarized state with a reduced electronic anisotropy. △ Less

Submitted 27 July, 2023; originally announced July 2023.

Comments: 5+20 pages

Journal ref: Phys. Rev. Lett. 132, 266501 (2024)

Hyperfine-interaction limits polarization entanglement of photons from semiconductor quantum dots

Authors: Christian Schimpf, Francesco Basso Basset, Maximilian Aigner, Wolfgang Atteneder, Laia Gínes, Gabriel Undeutsch, Marcus Reindl, Daniel Huber, Dorian Gangloff, Evgeny Chekhovich, Christian Schneider, Sven Höfling, Ana Predojević, Rinaldo Trotta, Armando Rastelli

Abstract: Excitons in quantum dots are excellent sources of polarization-entangled photon pairs, but a quantitative understanding of their interaction with the nuclear spin bath is still missing. Here we investigate the role of hyperfine energy shifts using experimentally accessible parameters and derive an upper limit to the achievable entanglement fidelity. Our results are consistent with all available li… ▽ More Excitons in quantum dots are excellent sources of polarization-entangled photon pairs, but a quantitative understanding of their interaction with the nuclear spin bath is still missing. Here we investigate the role of hyperfine energy shifts using experimentally accessible parameters and derive an upper limit to the achievable entanglement fidelity. Our results are consistent with all available literature, indicate that spin-noise is often the dominant process limiting the entanglement in InGaAs quantum dots, and suggest routes to alleviate its effect. △ Less

Submitted 27 April, 2023; v1 submitted 12 February, 2023; originally announced February 2023.

Compression theory for inhomogeneous systems

Authors: Doruk Efe Gökmen, Sounak Biswas, Sebastian D. Huber, Zohar Ringel, Felix Flicker, Maciej Koch-Janusz

Abstract: The physics of complex systems stands to greatly benefit from the qualitative changes in data availability and advances in data-driven computational methods. Many of these systems can be represented by interacting degrees of freedom on inhomogeneous graphs. However, the lack of translational invariance presents a fundamental challenge to theoretical tools, such as the renormalization group, which… ▽ More The physics of complex systems stands to greatly benefit from the qualitative changes in data availability and advances in data-driven computational methods. Many of these systems can be represented by interacting degrees of freedom on inhomogeneous graphs. However, the lack of translational invariance presents a fundamental challenge to theoretical tools, such as the renormalization group, which were so successful in characterizing the universal physical behaviour in critical phenomena. Here we show that compression theory allows the extraction of relevant degrees of freedom in arbitrary geometries, and the development of efficient numerical tools to build an effective theory from data. We demonstrate our method by applying it to a strongly correlated system on an Ammann-Beenker quasicrystal, where it discovers an exotic critical point with broken conformal symmetry. We also apply it to an antiferromagnetic system on non-bipartite random graphs, where any periodicity is absent. △ Less

Submitted 26 November, 2024; v1 submitted 27 January, 2023; originally announced January 2023.

Comments: Version accepted in Nature Communications

Journal ref: Nature Communications 15, 10214 (2024)

Phases, instabilities and excitations in a two-component lattice model with photon-mediated interactions

Authors: Leon Carl, Rodrigo Rosa-Medina, Sebastian D. Huber, Tilman Esslinger, Nishant Dogra, Tena Dubcek

Abstract: Engineering long-range interacting spin systems with ultra cold atoms offers the possibility to explore exotic magnetically ordered phases in strongly-correlated scenarios. Quantum gases in optical cavities provide a versatile experimental platform to further engineer photon-mediated interactions and access the underlying microscopic processes by probing the cavity field. Here, we study a two-comp… ▽ More Engineering long-range interacting spin systems with ultra cold atoms offers the possibility to explore exotic magnetically ordered phases in strongly-correlated scenarios. Quantum gases in optical cavities provide a versatile experimental platform to further engineer photon-mediated interactions and access the underlying microscopic processes by probing the cavity field. Here, we study a two-component spin Bose-Hubbard system with cavity-mediated interactions. We provide a comprehensive overview of its phase diagram and transitions in experimentally relevant regimes. The interplay of different energy scales yields a rich phase diagram with superfluid and insulating phases exhibiting density modulation or spin ordering. In particular, the combined effect of contact and global-range interactions gives rise to an antiferromagnetically ordered phase for arbitrarily small spin-dependent light-matter coupling, while long-range and inter-spin contact interactions introduce regions of instability and phase separation in the phase diagram. We further study the low energy excitations above the antiferrogmagnetic phase. Besides particle-hole branches, it hosts spin-exchange excitations with a tunable energy gap. The studied lattice model can be readily realized in cold-atom experiments with optical cavities. △ Less

Submitted 20 October, 2022; originally announced October 2022.

Comments: 8(+10) pages, 4(+7) figures

Untrained physically informed neural network for image reconstruction of magnetic field sources

Authors: A. E. E. Dubois, D. A. Broadway, A. Stark, M. A. Tschudin, A. J. Healey, S. D. Huber, J. -P. Tetienne, E. Greplova, P. Maletinsky

Abstract: Predicting measurement outcomes from an underlying structure often follows directly from fundamental physical principles. However, a fundamental challenge is posed when trying to solve the inverse problem of inferring the underlying source-configuration based on measurement data. A key difficulty arises from the fact that such reconstructions often involve ill-posed transformations and that they a… ▽ More Predicting measurement outcomes from an underlying structure often follows directly from fundamental physical principles. However, a fundamental challenge is posed when trying to solve the inverse problem of inferring the underlying source-configuration based on measurement data. A key difficulty arises from the fact that such reconstructions often involve ill-posed transformations and that they are prone to numerical artefacts. Here, we develop a numerically efficient method to tackle this inverse problem for the reconstruction of magnetisation maps from measured magnetic stray field images. Our method is based on neural networks with physically inferred loss functions to efficiently eliminate common numerical artefacts. We report on a significant improvement in reconstruction over traditional methods and we show that our approach is robust to different magnetisation directions, both in- and out-of-plane, and to variations of the magnetic field measurement axis orientation. While we showcase the performance of our method using magnetometry with Nitrogen Vacancy centre spins in diamond, our neural-network-based approach to solving inverse problems is agnostic to the measurement technique and thus is applicable beyond the specific use-case demonstrated in this work. △ Less

Submitted 27 July, 2022; originally announced July 2022.

Comments: 11 pages, 6 figures

Magnetic impurity in a one-dimensional few-fermion system

Authors: Lukas Rammelmüller, David Huber, Matija Čufar, Joachim Brand, Hans-Werner Hammer, Artem G. Volosniev

Abstract: We present a numerical analysis of spin-$\frac{1}{2}$ fermions in a one-dimensional harmonic potential in the presence of a magnetic point-like impurity at the center of the trap. The model represents a few-body analogue of a magnetic impurity in the vicinity of an $s$-wave superconductor. Already for a few particles we find a ground-state level crossing between sectors with different fermion pari… ▽ More We present a numerical analysis of spin-$\frac{1}{2}$ fermions in a one-dimensional harmonic potential in the presence of a magnetic point-like impurity at the center of the trap. The model represents a few-body analogue of a magnetic impurity in the vicinity of an $s$-wave superconductor. Already for a few particles we find a ground-state level crossing between sectors with different fermion parities. We interpret this crossing as a few-body precursor of a quantum phase transition, which occurs when the impurity `breaks' a Cooper pair. This picture is further corroborated by analyzing density-density correlations in momentum space. Finally, we discuss how the system may be realized with existing cold-atoms platforms. △ Less

Submitted 4 April, 2022; originally announced April 2022.

Comments: SciPost Submission

Journal ref: SciPost Phys. 14, 006 (2023)

A modular implementation of an effective interaction approach for harmonically trapped fermions in 1D

Authors: Lukas Rammelmüller, David Huber, Artem G. Volosniev

Abstract: We introduce a generic and accessible implementation of an exact diagonalization method for studying few-fermion models. Our aim is to provide a testbed for the newcomers to the field as well as a stepping stone for trying out novel optimizations and approximations. This userguide consists of a description of the algorithm, and several examples in varying orders of sophistication. In particular, w… ▽ More We introduce a generic and accessible implementation of an exact diagonalization method for studying few-fermion models. Our aim is to provide a testbed for the newcomers to the field as well as a stepping stone for trying out novel optimizations and approximations. This userguide consists of a description of the algorithm, and several examples in varying orders of sophistication. In particular, we exemplify our routine using an effective-interaction approach that fixes the low-energy physics. We benchmark this approach against the existing data, and show that it is able to deliver state-of-the-art numerical results at a significantly reduced computational cost. △ Less

Submitted 5 November, 2022; v1 submitted 9 February, 2022; originally announced February 2022.

Comments: 22 + 4 pages

Journal ref: SciPost Phys. Codebases 12 (2023)

Binary classification of spoken words with passive phononic metamaterials

Authors: Tena Dubček, Daniel Moreno-Garcia, Thomas Haag, Parisa Omidvar, Henrik R. Thomsen, Theodor S. Becker, Lars Gebraad, Christoph Bärlocher, Fredrik Andersson, Sebastian D. Huber, Dirk-Jan van Manen, Luis Guillermo Villanueva, Johan O. A. Robertsson, Marc Serra-Garcia

Abstract: Mitigating the energy requirements of artificial intelligence requires novel physical substrates for computation. Phononic metamaterials have a vanishingly low power dissipation and hence are a prime candidate for green, always-on computers. However, their use in machine learning applications has not been explored due to the complexity of their design process: Current phononic metamaterials are re… ▽ More Mitigating the energy requirements of artificial intelligence requires novel physical substrates for computation. Phononic metamaterials have a vanishingly low power dissipation and hence are a prime candidate for green, always-on computers. However, their use in machine learning applications has not been explored due to the complexity of their design process: Current phononic metamaterials are restricted to simple geometries (e.g. periodic, tapered), and hence do not possess sufficient expressivity to encode machine learning tasks. We design and fabricate a non-periodic phononic metamaterial, directly from data samples, that can distinguish between pairs of spoken words in the presence of a simple readout nonlinearity; hence demonstrating that phononic metamaterials are a viable avenue towards zero-power smart devices. △ Less

Submitted 7 July, 2023; v1 submitted 14 November, 2021; originally announced November 2021.

Comments: 13 pages, 11 figures

Superfluid weight bounds from symmetry and quantum geometry in flat bands

Authors: Jonah Herzog-Arbeitman, Valerio Peri, Frank Schindler, Sebastian D. Huber, B. Andrei Bernevig

Abstract: Flat-band superconductivity has theoretically demonstrated the importance of band topology to correlated phases. In two dimensions, the superfluid weight, which determines the critical temperature through the Berezinksii-Kosterlitz-Thouless criteria, is bounded by the Fubini-Study metric at zero temperature. We show this bound is nonzero within flat bands whose Wannier centers are obstructed from… ▽ More Flat-band superconductivity has theoretically demonstrated the importance of band topology to correlated phases. In two dimensions, the superfluid weight, which determines the critical temperature through the Berezinksii-Kosterlitz-Thouless criteria, is bounded by the Fubini-Study metric at zero temperature. We show this bound is nonzero within flat bands whose Wannier centers are obstructed from the atoms - even when they have identically zero Berry curvature. Next, we derive general lower bounds for the superfluid weight in terms of momentum space irreps in all 2D space groups, extending the reach of topological quantum chemistry to superconducting states. We find that the bounds can be naturally expressed using the formalism of real space invariants (RSIs) that highlight the separation between electronic and atomic degrees of freedom. Finally, using exact Monte Carlo simulations on a model with perfectly flat bands and strictly local obstructed Wannier functions, we find that an attractive Hubbard interaction results in superconductivity as predicted by the RSI bound beyond mean-field. Hence, a nonzero superfluid weight constitutes a nontrivial bulk property that distinguishes obstructed bands from trivial bands in the presence of interactions. △ Less

Submitted 26 February, 2022; v1 submitted 27 October, 2021; originally announced October 2021.

Comments: 5+31 pages, 3+2 figures, 2 tables

Journal ref: Phys. Rev. Lett. 128, 087002 (2022)

Symmetries and phase diagrams with real-space mutual information neural estimation

Authors: Doruk Efe Gökmen, Zohar Ringel, Sebastian D. Huber, Maciej Koch-Janusz

Abstract: Real-space mutual information (RSMI) was shown to be an important quantity, formally and from a numerical standpoint, in finding coarse-grained descriptions of physical systems. It very generally quantifies spatial correlations, and can give rise to constructive algorithms extracting relevant degrees of freedom. Efficient and reliable estimation or maximization of RSMI is, however, numerically cha… ▽ More Real-space mutual information (RSMI) was shown to be an important quantity, formally and from a numerical standpoint, in finding coarse-grained descriptions of physical systems. It very generally quantifies spatial correlations, and can give rise to constructive algorithms extracting relevant degrees of freedom. Efficient and reliable estimation or maximization of RSMI is, however, numerically challenging. A recent breakthrough in theoretical machine learning has been the introduction of variational lower bounds for mutual information, parametrized by neural networks. Here we describe in detail how these results can be combined with differentiable coarse-graining operations to develop a single unsupervised neural-network based algorithm, the RSMI-NE, efficiently extracting the relevant degrees of freedom in the form of the operators of effective field theories, directly from real-space configurations. We study the information contained in the statistical ensemble of constructed coarse-graining transformations, and its recovery from partial input data using a secondary machine learning analysis applied to this ensemble. In particular, we show how symmetries, also emergent, can be identified. We demonstrate the extraction of the phase diagram and the order parameters for equilibrium systems, and consider also an example of a non-equilibrium problem. △ Less

Submitted 18 October, 2021; v1 submitted 31 March, 2021; originally announced March 2021.

Comments: Accepted version. Added new discussion of symmetries in the ensemble of coarse-graining rules. 27 pages, 13 figures. arXiv admin note: substantial text overlap with arXiv:2101.11633

Journal ref: Phys. Rev. E 104, 064106 (2021)

Correlation-Enhanced Neural Networks as Interpretable Variational Quantum States

Authors: Agnes Valenti, Eliska Greplova, Netanel H. Lindner, Sebastian D. Huber

Abstract: Variational methods have proven to be excellent tools to approximate ground states of complex many body Hamiltonians. Generic tools like neural networks are extremely powerful, but their parameters are not necessarily physically motivated. Thus, an efficient parametrization of the wave-function can become challenging. In this letter we introduce a neural-network based variational ansatz that retai… ▽ More Variational methods have proven to be excellent tools to approximate ground states of complex many body Hamiltonians. Generic tools like neural networks are extremely powerful, but their parameters are not necessarily physically motivated. Thus, an efficient parametrization of the wave-function can become challenging. In this letter we introduce a neural-network based variational ansatz that retains the flexibility of these generic methods while allowing for a tunability with respect to the relevant correlations governing the physics of the system. We illustrate the success of this approach on topological, long-range correlated and frustrated models. Additionally, we introduce compatible variational optimization methods for exploration of low-lying excited states without symmetries that preserve the interpretability of the ansatz. △ Less

Submitted 8 March, 2021; originally announced March 2021.

Comments: 14 pages, 18 figures, code available at https://github.com/cmt-qo/cm-cRBM

Journal ref: Phys. Rev. R 4, L012010 (2022)

Scalable Hamiltonian learning for large-scale out-of-equilibrium quantum dynamics

Authors: Agnes Valenti, Guliuxin Jin, Julian Léonard, Sebastian D. Huber, Eliska Greplova

Abstract: Large-scale quantum devices provide insights beyond the reach of classical simulations. However, for a reliable and verifiable quantum simulation, the building blocks of the quantum device require exquisite benchmarking. This benchmarking of large scale dynamical quantum systems represents a major challenge due to lack of efficient tools for their simulation. Here, we present a scalable algorithm… ▽ More Large-scale quantum devices provide insights beyond the reach of classical simulations. However, for a reliable and verifiable quantum simulation, the building blocks of the quantum device require exquisite benchmarking. This benchmarking of large scale dynamical quantum systems represents a major challenge due to lack of efficient tools for their simulation. Here, we present a scalable algorithm based on neural networks for Hamiltonian tomography in out-of-equilibrium quantum systems. We illustrate our approach using a model for a forefront quantum simulation platform: ultracold atoms in optical lattices. Specifically, we show that our algorithm is able to reconstruct the Hamiltonian of an arbitrary size quasi-1D bosonic system using an accessible amount of experimental measurements. We are able to significantly increase the previously known parameter precision. △ Less

Submitted 1 March, 2021; originally announced March 2021.

Comments: 13 pages, 13 figures, code: https://gitlab.com/QMAI/papers/manybodydynlearning

Morphology of three-body quantum states from machine learning

Authors: David Huber, Oleksandr V. Marchukov, Hans-Werner Hammer, Artem G. Volosniev

Abstract: The relative motion of three impenetrable particles on a ring, in our case two identical fermions and one impurity, is isomorphic to a triangular quantum billiard. Depending on the ratio $κ$ of the impurity and fermion masses, the billiards can be integrable or non-integrable (also referred to in the main text as chaotic). To set the stage, we first investigate the energy level distributions of th… ▽ More The relative motion of three impenetrable particles on a ring, in our case two identical fermions and one impurity, is isomorphic to a triangular quantum billiard. Depending on the ratio $κ$ of the impurity and fermion masses, the billiards can be integrable or non-integrable (also referred to in the main text as chaotic). To set the stage, we first investigate the energy level distributions of the billiards as a function of $1/κ\in [0,1]$ and find no evidence of integrable cases beyond the limiting values $1/κ=1$ and $1/κ=0$. Then, we use machine learning tools to analyze properties of probability distributions of individual quantum states. We find that convolutional neural networks can correctly classify integrable and non-integrable states.The decisive features of the wave functions are the normalization and a large number of zero elements, corresponding to the existence of a nodal line. The network achieves typical accuracies of 97%, suggesting that machine learning tools can be used to analyze and classify the morphology of probability densities obtained in theory or experiment. △ Less

Submitted 2 August, 2021; v1 submitted 9 February, 2021; originally announced February 2021.

Comments: version accepted for publication in New Journal of Physics (Focus Issue on Machine Learning Across Physics)

Journal ref: New J. Phys. 23, 065009 (2021)

Statistical physics through the lens of real-space mutual information

Authors: Doruk Efe Gökmen, Zohar Ringel, Sebastian D. Huber, Maciej Koch-Janusz

Abstract: Identifying the relevant coarse-grained degrees of freedom in a complex physical system is a key stage in developing powerful effective theories in and out of equilibrium. The celebrated renormalization group provides a framework for this task, but its practical execution in unfamiliar systems is fraught with ad hoc choices, whereas machine learning approaches, though promising, often lack formal… ▽ More Identifying the relevant coarse-grained degrees of freedom in a complex physical system is a key stage in developing powerful effective theories in and out of equilibrium. The celebrated renormalization group provides a framework for this task, but its practical execution in unfamiliar systems is fraught with ad hoc choices, whereas machine learning approaches, though promising, often lack formal interpretability. Recently, the optimal coarse-graining in a statistical system was shown to exist, based on a universal, but computationally difficult information-theoretic variational principle. This limited its applicability to but the simplest systems; moreover, the relation to standard formalism of field theory was unclear. Here we present an algorithm employing state-of-art results in machine-learning-based estimation of information-theoretic quantities, overcoming these challenges. We use this advance to develop a new paradigm in identifying the most relevant field theory operators describing properties of the system, going beyond the existing approaches to real-space renormalization. We evidence its power on an interacting model, where the emergent degrees of freedom are qualitatively different from the microscopic building blocks of the theory. Our results push the boundary of formally interpretable applications of machine learning, conceptually paving the way towards automated theory building. △ Less

Submitted 19 October, 2021; v1 submitted 27 January, 2021; originally announced January 2021.

Comments: Version accepted for publication in Physical Review Letters. See also the companion manuscript arXiv:2103.16887 "Symmetries and phase diagrams with real-space mutual estimation neural estimation"

Journal ref: Phys. Rev. Lett. 127, 240603 (2021)

Fragile topology and flat-band superconductivity in the strong-coupling regime

Authors: Valerio Peri, Zhida Song, B. Andrei Bernevig, Sebastian D. Huber

Abstract: In flat bands, superconductivity can lead to surprising transport effects. The superfluid "mobility", in the form of the superfluid weight $D_s$, does not draw from the curvature of the band but has a purely band-geometric origin. In a mean-field description, a non-zero Chern number or fragile topology sets a lower bound for $D_s$, which, via the Berezinskii-Kosterlitz-Thouless mechanism, might ex… ▽ More In flat bands, superconductivity can lead to surprising transport effects. The superfluid "mobility", in the form of the superfluid weight $D_s$, does not draw from the curvature of the band but has a purely band-geometric origin. In a mean-field description, a non-zero Chern number or fragile topology sets a lower bound for $D_s$, which, via the Berezinskii-Kosterlitz-Thouless mechanism, might explain the relatively high superconducting transition temperature measured in magic-angle twisted bilayer graphene (MATBG). For fragile topology, relevant for the bilayer system, the fate of this bound for finite temperature and beyond the mean-field approximation remained, however, unclear. Here, we use numerically exact Monte Carlo simulations to study an attractive Hubbard model in flat bands with topological properties akin to those of MATBG. We find a superconducting phase transition with a critical temperature that scales linearly with the interaction strength. We then investigate the robustness of the superconducting state to the addition of trivial bands that may or may not trivialize the fragile topology. Our results substantiate the validity of the topological bound beyond the mean-field regime and further stress the importance of fragile topology for flat-band superconductivity. △ Less

Submitted 25 November, 2020; v1 submitted 5 August, 2020; originally announced August 2020.

Comments: 5 pages, 3 figures + supplemental material (14 pages, 4 figures)

Journal ref: Phys. Rev. Lett. 126, 027002 (2021)

Quantum cryptography with highly entangled photons from semiconductor quantum dots

Authors: Christian Schimpf, Marcus Reindl, Daniel Huber, Barbara Lehner, Saimon F. Covre Da Silva, Santanu Manna, Michal Vyvlecka, Philip Walther, Armando Rastelli

Abstract: State-of-the-art quantum key distribution systems are based on the BB84 protocol and single photons generated by lasers. These implementations suffer from range limitations and security loopholes, which require expensive adaptation. The use of polarization entangled photon pairs substantially alleviates the security threads while allowing for basically arbitrary transmission distances when embedde… ▽ More State-of-the-art quantum key distribution systems are based on the BB84 protocol and single photons generated by lasers. These implementations suffer from range limitations and security loopholes, which require expensive adaptation. The use of polarization entangled photon pairs substantially alleviates the security threads while allowing for basically arbitrary transmission distances when embedded in quantum repeater schemes. Semiconductor quantum dots are capable of emitting highly entangled photon pairs with ultra-low multi-pair emission probability even at maximum brightness. Here we report on the first implementation of the BBM92 protocol using a quantum dot source with an entanglement fidelity as high as 0.97(1). For a proof of principle, the key generation is performed between two buildings, connected by 350 metre long fiber, resulting in an average key rate of 135 bits/s and a qubit error rate of 0.019 over a time span of 13 hours, without resorting to time- or frequency-filtering techniques. Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By embedding them in state-of-the-art photonic structures, key generation rates in the Gbit/s range are at reach. △ Less

Submitted 24 July, 2020; originally announced July 2020.

Comments: 7 pages, 2 figures

Weyl orbits without an external magnetic field

Authors: Valerio Peri, Tena Dubček, Agnes Valenti, Roni Ilan, Sebastian D. Huber

Abstract: Weyl semimetals in a magnetic field give rise to interesting non-local electronic orbits: the ballistic transport through the bulk enabled by the chiral Landau levels is combined with a momentum-space sliding along the surface Fermi-arc driven by the Lorentz force. Bulk chiral Landau levels can also be induced by axial fields whose sign depends on the chirality of the Weyl point. However, the micr… ▽ More Weyl semimetals in a magnetic field give rise to interesting non-local electronic orbits: the ballistic transport through the bulk enabled by the chiral Landau levels is combined with a momentum-space sliding along the surface Fermi-arc driven by the Lorentz force. Bulk chiral Landau levels can also be induced by axial fields whose sign depends on the chirality of the Weyl point. However, the microscopic perturbations that give rise to them can be described in terms of gauge fields only in the low-energy sectors around the Weyl points. In addition, since pseudo-fields are intrinsic, there is no apparent reason for a Lorentz force that causes sliding along the Fermi-arcs. Therefore, the existence of non-local orbits driven exclusively by pseudo-fields is not obvious. Here, we show that for systems with at least four Weyl points in the bulk spectrum, non-local orbits can be induced by axial fields alone. We discuss the underlying mechanisms by a combination of analytical semi-classical theory, the microscopic numerical study of wave-packet dynamics, and a surface Green's function analysis. △ Less

Submitted 26 August, 2020; v1 submitted 8 April, 2020; originally announced April 2020.

Comments: 12 pages, 5 figures

Journal ref: Phys. Rev. B 101, 235117 (2020)

Acoustic spin-Chern insulator induced by synthetic spin-orbit coupling with spin conservation breaking

Authors: Weiyin Deng, Xueqin Huang, Jiuyang Lu, Valerio Peri, Feng Li, Sebastian D. Huber, Zhengyou Liu

Abstract: Topologically protected surface modes of classical waves hold the promise to enable a variety of applications ranging from robust transport of energy to reliable information processing networks. The integer quantum Hall effect has delivered on that promise in the electronic realm through high-precision metrology devices. However, both the route of implementing an analogue of the quantum Hall effec… ▽ More Topologically protected surface modes of classical waves hold the promise to enable a variety of applications ranging from robust transport of energy to reliable information processing networks. The integer quantum Hall effect has delivered on that promise in the electronic realm through high-precision metrology devices. However, both the route of implementing an analogue of the quantum Hall effect as well as the quantum spin Hall effect are obstructed for acoustics by the requirement of a magnetic field, or the presence of fermionic quantum statistics, respectively. Here, we use a two-dimensional acoustic crystal with two layers to mimic spin-orbit coupling, a crucial ingredient of topological insulators. In particular, our setup allows us to free ourselves of symmetry constraints as we rely on the concept of a non-vanishing "spin" Chern number. We experimentally characterize the emerging boundary states which we show to be gapless and helical. Moreover, in an H-shaped device we demonstrate how the transport path can be selected by tuning the geometry, enabling the construction of complex networks. △ Less

Submitted 16 October, 2019; originally announced October 2019.

Journal ref: Nature Communications 11, 3227 (2020)

Experimental characterization of fragile topology in an acoustic metamaterial

Authors: Valerio Peri, Zhi-Da Song, Marc Serra-Garcia, Pascal Engeler, Raquel Queiroz, Xueqin Huang, Weiyin Deng, Zhengyou Liu, B. Andrei Bernevig, Sebastian D. Huber

Abstract: Symmetries crucially underlie the classification of topological phases of matter. Most materials, both natural as well as architectured, possess crystalline symmetries. Recent theoretical works unveiled that these crystalline symmetries can stabilize fragile Bloch bands that challenge our very notion of topology: while answering to the most basic definition of topology, one can trivialize these ba… ▽ More Symmetries crucially underlie the classification of topological phases of matter. Most materials, both natural as well as architectured, possess crystalline symmetries. Recent theoretical works unveiled that these crystalline symmetries can stabilize fragile Bloch bands that challenge our very notion of topology: while answering to the most basic definition of topology, one can trivialize these bands through the addition of trivial Bloch bands. Here, we fully characterize the symmetry properties of the response of an acoustic metamaterial to establish the fragile nature of the low-lying Bloch bands. Additionally, we present a spectral signature in the form of spectral flow under twisted boundary conditions. △ Less

Submitted 25 November, 2020; v1 submitted 15 October, 2019; originally announced October 2019.

Comments: 4 pages, 3 figures + supplementary information (21 pages, 13 figures, 7 tables). Note the accompanying paper by Zhi-Da Song et al. "Real Space Invariants: Twisted Bulk-Boundary Correspondence of Fragile Topology"' posted on the same day on the arXiv

Journal ref: Science 367, 797 (2020)

Single-Particle-Picture Breakdown in laterally weakly confining GaAs Quantum Dots

Authors: Daniel Huber, Barbara Ursula Lehner, Diana Csontosová, Marcus Reindl, Simon Schuler, Saimon Filipe Covre da Silva, Petr Klenovský, Armando Rastelli

Abstract: We present a detailed investigation of different excitonic states weakly confined in single GaAs/AlGaAs quantum dots obtained by the Al droplet-etching method. For our analysis we make use of temperature-, polarization- and magnetic field-dependent $μ$-photoluminescence measurements, which allow us to identify different excited states of the quantum dot system. Besides that, we present a comprehen… ▽ More We present a detailed investigation of different excitonic states weakly confined in single GaAs/AlGaAs quantum dots obtained by the Al droplet-etching method. For our analysis we make use of temperature-, polarization- and magnetic field-dependent $μ$-photoluminescence measurements, which allow us to identify different excited states of the quantum dot system. Besides that, we present a comprehensive analysis of g-factors and diamagnetic coefficients of charged and neutral excitonic states in Voigt and Faraday configuration. Supported by theoretical calculations by the Configuration interaction method, we show that the widely used single-particle Zeeman Hamiltonian cannot be used to extract reliable values of the g-factors of the constituent particles from excitonic transition measurements. △ Less

Submitted 11 September, 2019; originally announced September 2019.

Journal ref: Phys. Rev. B 100, 235425 (2019)

In-medium bound states of two bosonic impurities in a one-dimensional Fermi gas

Authors: D. Huber, H. -W. Hammer, A. G. Volosniev

Abstract: We investigate the ground-state energy of a one-dimensional Fermi gas with two bosonic impurities. We consider spinless fermions with no fermion-fermion interactions. The fermion-impurity and impurity-impurity interactions are modelled with Dirac delta functions. First, we study the case where impurity and fermions have equal masses, and the impurity-impurity two-body interaction is identical to t… ▽ More We investigate the ground-state energy of a one-dimensional Fermi gas with two bosonic impurities. We consider spinless fermions with no fermion-fermion interactions. The fermion-impurity and impurity-impurity interactions are modelled with Dirac delta functions. First, we study the case where impurity and fermions have equal masses, and the impurity-impurity two-body interaction is identical to the fermion-impurity interaction, such that the system is solvable with the Bethe ansatz. For attractive interactions, we find that the energy of the impurity-impurity subsystem is below the energy of the bound state that exists without the Fermi gas. We interpret this as a manifestation of attractive boson-boson interactions induced by the fermionic medium, and refer to the impurity-impurity subsystem as an in-medium bound state. For repulsive interactions, we find no in-medium bound states. Second, we construct an effective model to describe these interactions, and compare its predictions to the exact solution. We use this effective model to study non-integrable systems with unequal masses and/or potentials. We discuss parameter regimes for which impurity-impurity attraction induced by the Fermi gas can lead to the formation of in-medium bound states made of bosons that repel each other in the absence of the Fermi gas. △ Less

Submitted 17 December, 2019; v1 submitted 7 August, 2019; originally announced August 2019.

Comments: version accepted for publication in Physical Review Research

Journal ref: Phys. Rev. Research 1, 033177 (2019)

Highly indistinguishable single photons from incoherently and coherently excited GaAs quantum dots

Authors: Marcus Reindl, Jonas H. Weber, Daniel Huber, Christian Schimpf, Saimon F. Covre da Silva, Simone L. Portalupi, Rinaldo Trotta, Peter Michler, Armando Rastelli

Abstract: Semiconductor quantum dots are converging towards the demanding requirements of photonic quantum technologies. Among different systems, quantum dots with dimensions exceeding the free-exciton Bohr radius are appealing because of their high oscillator strengths. While this property has received much attention in the context of cavity quantum electrodynamics, little is known about the degree of indi… ▽ More Semiconductor quantum dots are converging towards the demanding requirements of photonic quantum technologies. Among different systems, quantum dots with dimensions exceeding the free-exciton Bohr radius are appealing because of their high oscillator strengths. While this property has received much attention in the context of cavity quantum electrodynamics, little is known about the degree of indistinguishability of single photons consecutively emitted by such dots and on the proper excitation schemes to achieve high indistinguishability. A prominent example is represented by GaAs quantum dots obtained by local droplet etching, which recently outperformed other systems as triggered sources of entangled photon pairs. On these dots, we compare different single-photon excitation mechanisms, and we find (i) a "phonon bottleneck" and poor indistinguishability for conventional excitation via excited states and (ii) photon indistinguishablilities above 90% for both strictly resonant and for incoherent acoustic- and optical-phonon-assisted excitation. Among the excitation schemes, optical phonon-assisted excitation enables straightforward laser rejection without a compromise on the source brightness together with a high photon indistinguishability. △ Less

Submitted 4 February, 2019; v1 submitted 31 January, 2019; originally announced January 2019.

Journal ref: Phys. Rev. B 100, 155420 (2019)

Anomalous Fermi arcs in a periodically driven Weyl system

Authors: Valerio Peri, Sebastian D. Huber

Abstract: Three dimensional Weyl semimetals exhibit open Fermi arcs on their sample surfaces connecting the projection of bulk Weyl points of opposite chirality. The canonical interpretation of these surfaces states is in terms of chiral edge modes of a layer quantum Hall effect: The two-dimensional momentum-space planes perpendicular to the momentum connecting the two Weyl points are characterized by a non… ▽ More Three dimensional Weyl semimetals exhibit open Fermi arcs on their sample surfaces connecting the projection of bulk Weyl points of opposite chirality. The canonical interpretation of these surfaces states is in terms of chiral edge modes of a layer quantum Hall effect: The two-dimensional momentum-space planes perpendicular to the momentum connecting the two Weyl points are characterized by a non-zero Chern number. It might be interesting to note, that in analogy to the known two-dimensional Floquet anomalous chiral edge states, one can realize open Fermi arcs in the absence of Chern numbers in periodically driven system. Here, we present a way to construct such anomalous Fermi arcs in a concrete model. △ Less

Submitted 17 December, 2018; originally announced December 2018.

Optimal Renormalization Group Transformation from Information Theory

Authors: Patrick M. Lenggenhager, Doruk Efe Gökmen, Zohar Ringel, Sebastian D. Huber, Maciej Koch-Janusz

Abstract: Recently a novel real-space RG algorithm was introduced, identifying the relevant degrees of freedom of a system by maximizing an information-theoretic quantity, the real-space mutual information (RSMI), with machine learning methods. Motivated by this, we investigate the information theoretic properties of coarse-graining procedures, for both translationally invariant and disordered systems. We p… ▽ More Recently a novel real-space RG algorithm was introduced, identifying the relevant degrees of freedom of a system by maximizing an information-theoretic quantity, the real-space mutual information (RSMI), with machine learning methods. Motivated by this, we investigate the information theoretic properties of coarse-graining procedures, for both translationally invariant and disordered systems. We prove that a perfect RSMI coarse-graining does not increase the range of interactions in the renormalized Hamiltonian, and, for disordered systems, suppresses generation of correlations in the renormalized disorder distribution, being in this sense optimal. We empirically verify decay of those measures of complexity, as a function of information retained by the RG, on the examples of arbitrary coarse-grainings of the clean and random Ising chain. The results establish a direct and quantifiable connection between properties of RG viewed as a compression scheme, and those of physical objects i.e. Hamiltonians and disorder distributions. We also study the effect of constraints on the number and type of coarse-grained degrees of freedom on a generic RG procedure. △ Less

Submitted 1 October, 2019; v1 submitted 25 September, 2018; originally announced September 2018.

Comments: Updated manuscript with new results on disordered systems

Journal ref: Phys. Rev. X 10, 011037 (2020)

Axial-field-induced chiral channels in an acoustic Weyl system

Authors: Valerio Peri, Marc Serra-Garcia, Roni Ilan, Sebastian D. Huber

Abstract: Condensed-matter and other engineered systems, such as cold atoms, photonic, or phononic metamaterials, have proven to be versatile platforms for the observation of low-energy counterparts of elementary particles from relativistic field theories. These include the celebrated Majorana modes, as well as Dirac and Weyl fermions. An intriguing feature of the Weyl equation is the chiral symmetry, where… ▽ More Condensed-matter and other engineered systems, such as cold atoms, photonic, or phononic metamaterials, have proven to be versatile platforms for the observation of low-energy counterparts of elementary particles from relativistic field theories. These include the celebrated Majorana modes, as well as Dirac and Weyl fermions. An intriguing feature of the Weyl equation is the chiral symmetry, where the two chiral sectors have an independent gauge freedom. While this freedom leads to a quantum anomaly, there is no corresponding axial background field coupling differently to opposite chiralities in quantum electrodynamics. Here, we provide the experimental characterization of the effect of such an axial field in an acoustic metamaterial. We implement the axial field through an inhomogeneous potential and observe the induced chiral Landau levels. From the metamaterials perspective these chiral channels open the possibility for the observation of non-local Weyl orbits and might enable unidirectional bulk transport in a time-reversal invariant system. △ Less

Submitted 26 August, 2020; v1 submitted 25 June, 2018; originally announced June 2018.

Journal ref: Nature Physics 15, 357 (2019)

Observation of Quadrupole Transitions and Edge Mode Topology in an LC network

Authors: Marc Serra-Garcia, Roman Süsstrunk, Sebastian D. Huber

Abstract: High-order topological insulators are a recent development extending the topological theory of charge polarization to higher multipole moments. Since their theoretical proposal, several experimental realizations of high-order topological insulators have been reported. However, high order topological transitions have not been observed. In this letter, we report on the observation of a high-order to… ▽ More High-order topological insulators are a recent development extending the topological theory of charge polarization to higher multipole moments. Since their theoretical proposal, several experimental realizations of high-order topological insulators have been reported. However, high order topological transitions have not been observed. In this letter, we report on the observation of a high-order topological transition in a quadrupole topological insulator implemented in an LC circuit with nonlinear couplings. This system presents the ability to confine electromagnetic energy in its corner states, with a localization length that can be tuned over a broad range through the use of an external bias voltage. Additionally, we provide an experimental characterization and an effective theory for the boundary states, further corroborating their topological nature by direct measurement of the winding number. △ Less

Submitted 19 June, 2018; originally announced June 2018.

Journal ref: Phys. Rev. B 99, 020304 (2019)

Preformed pairs in flat Bloch bands

Authors: Murad Tovmasyan, Sebastiano Peotta, Long Liang, Päivi Törmä, Sebastian D. Huber

Abstract: In a flat Bloch band the kinetic energy is quenched and single particles cannot propagate since they are localized due to destructive interference. Whether this remains true in the presence of interactions is a challenging question because a flat dispersion usually leads to highly correlated ground states. Here we compute numerically the ground state energy of lattice models with completely flat b… ▽ More In a flat Bloch band the kinetic energy is quenched and single particles cannot propagate since they are localized due to destructive interference. Whether this remains true in the presence of interactions is a challenging question because a flat dispersion usually leads to highly correlated ground states. Here we compute numerically the ground state energy of lattice models with completely flat band structure in a ring geometry. We find that the energy as a function of the magnetic flux threading the ring has a half-flux quantum $Φ_0/2 = hc/(2e)$ period, indicating that only bound pairs of particles with charge $2e$ are propagating, while single quasiparticles with charge $e$ remain localized. We show analytically in one dimension that in fact the whole many-body spectrum has the same periodicity. Our analytical arguments are valid for both bosons and fermions, for generic interactions respecting some symmetries of the lattice and at arbitrary temperatures. Moreover we construct an extensive number of exact conserved quantities for the one dimensional lattice models. These conserved quantities are associated to the occupation of localized single quasiparticle states. Our results imply that in lattice models with flat bands preformed pairs dominate transport even above the critical temperature of the transition to a superfluid state. △ Less

Submitted 1 March, 2019; v1 submitted 11 May, 2018; originally announced May 2018.

Comments: 24 pages, 10 figures. This version version is almost identical to the published one. Some minor errors present in the published version have been corrected here

Journal ref: Phys. Rev. B 98, 134513 (2018)

Semiconductor quantum dots as an ideal source of polarization entangled photon pairs on-demand: a review

Authors: Daniel Huber, Marcus Reindl, Johannes Aberl, Armando Rastelli, Rinaldo Trotta

Abstract: More than 80 years passed since the first publication on entangled quantum states. In this period of time the concept of spookily interacting quantum states became an emerging field of science. After various experiments proving the existence of such non-classical states, visionary ideas were put forward to exploit entanglement in quantum information science and technology. These novel concepts hav… ▽ More More than 80 years passed since the first publication on entangled quantum states. In this period of time the concept of spookily interacting quantum states became an emerging field of science. After various experiments proving the existence of such non-classical states, visionary ideas were put forward to exploit entanglement in quantum information science and technology. These novel concepts have not yet come out of the experimental stage, mostly because of the lack of suitable, deterministic sources of entangled quantum states. Among many systems under investigation, semiconductor quantum dots are particularly appealing emitters of on-demand, single polarization-entangled photon-pairs. Although, it was originally believed that quantum dots must exhibit a limited degree of entanglement related to numerous decoherence effects present in the solid-state. Recent studies invalidated the premise of unavoidable entanglement degrading effects. We review the relevant experiments which have led to these important discoveries and discuss the remaining challenges for the anticipated quantum technologies. △ Less

Submitted 10 October, 2018; v1 submitted 27 April, 2018; originally announced April 2018.

Journal ref: J. Opt. 20 (2018) 073002 (17pp)

Spiral-based phononic plates: From wave beaming to topological insulators

Authors: André Foehr, Osama R. Bilal, Sebastian D. Huber, Chiara Daraio

Abstract: Phononic crystals and metamaterials take advantage of pre-designed geometrical structures to sculpt elastic waves, controlling their dispersion using different mechanisms. These mechanisms revolve mostly around Bragg scattering (BS), local resonances (LR) and inertial amplification (IA), which employ ad-hoc, often problem-specific geometries. Here, we use parametrized, spiraling unit cells as buil… ▽ More Phononic crystals and metamaterials take advantage of pre-designed geometrical structures to sculpt elastic waves, controlling their dispersion using different mechanisms. These mechanisms revolve mostly around Bragg scattering (BS), local resonances (LR) and inertial amplification (IA), which employ ad-hoc, often problem-specific geometries. Here, we use parametrized, spiraling unit cells as building blocks for designing various types of phononic materials. We focus on planar spirals that are easy to fabricate, yet give rise to the desirable complex dynamics. By simple modifications of the spirals, we open full band gaps using BS, LR and IA. Moreover, we alter the underlying unit cell symmetry and lattice vectors, to create wave beaming and topologically protected band gaps, both affecting waves whose wavelength is much larger than the order of periodicity. △ Less

Submitted 4 December, 2017; originally announced December 2017.

Journal ref: Phys. Rev. Lett. 120, 205501 (2018)

High-yield fabrication of entangled photon emitters for hybrid quantum networking using high-temperature droplet epitaxy

Authors: Francesco Basso Basset, Sergio Bietti, Marcus Reindl, Luca Esposito, Alexey Fedorov, Daniel Huber, Armando Rastelli, Emiliano Bonera, Rinaldo Trotta, Stefano Sanguinetti

Abstract: Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching… ▽ More Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching the energy of the entangled photons to atomic transitions (a promising route towards quantum networking) remains an outstanding challenge. Here, we overcome these obstacles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared to previous reports. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations. △ Less

Submitted 10 October, 2018; v1 submitted 10 October, 2017; originally announced October 2017.

Comments: 14 pages, 3 figures

Observation of a phononic quadrupole topological insulator

Authors: Marc Serra-Garcia, Valerio Peri, Roman Süsstrunk, Osama R. Bilal, Tom Larsen, Luis Guillermo Villanueva, Sebastian D. Huber

Abstract: The modern theory of charge polarization in solids is based on a generalization of Berry's phase. Its possible quantization lies at the heart of our understanding of all systems with topological band structures that were discovered over the last decades. While based on the concept of the "charge" polarization, the same theory can be used as an elegant tool to characterize the Bloch bands of neutra… ▽ More The modern theory of charge polarization in solids is based on a generalization of Berry's phase. Its possible quantization lies at the heart of our understanding of all systems with topological band structures that were discovered over the last decades. While based on the concept of the "charge" polarization, the same theory can be used as an elegant tool to characterize the Bloch bands of neutral bosonic systems such as photonic or phononic crystals. Recently, the theory of this quantized polarization was extended from the dipole- to higher multipole-moments. In particular, a two-dimensional quantized quadrupole insulator is predicted to have gapped yet topological one-dimensional edge-modes, which in turn stabilize zero-dimensional in-gap corner states. However, such a state of matter has not been observed experimentally. Here, we provide the first measurements of a phononic quadrupole insulator. We experimentally characterize the bulk, edge, and corner physics of a mechanical metamaterial and find the predicted gapped edge and in-gap corner states. We further corroborate our findings by comparing the mechanical properties of a topologically non-trivial system to samples in other phases predicted by the quadrupole theory. From an application point of view, these topological corner states are an important stepping stone on the way to topologically protected wave-guides in higher dimensions and thereby open a new design path for metamaterials. △ Less

Submitted 27 August, 2020; v1 submitted 16 August, 2017; originally announced August 2017.

Comments: Submitted to journal on July 15, 2017

Journal ref: Nature 555, 342 (2018)

Intrinsically polar elastic metamaterials

Authors: Osama R. Bilal, Roman Süsstrunk, Chiara Daraio, Sebastian D. Huber

Abstract: The ability to design and fabricate materials with tailored mechanical properties, combined with immunity to damage, is a frontier of materials engineering. For example, materials which are characterized by elastic properties that depend on the position inside a medium are required in applications where structural stability has to be combined with a soft and compliant surface, like in impact prote… ▽ More The ability to design and fabricate materials with tailored mechanical properties, combined with immunity to damage, is a frontier of materials engineering. For example, materials which are characterized by elastic properties that depend on the position inside a medium are required in applications where structural stability has to be combined with a soft and compliant surface, like in impact protection and cushioning. A gradient in the elastic properties can be built from a single material, varying gradually the bulk porosity of the material or its geometrical structure. However, if such a gradient is built into the material at production, damage or wearing over time might expose unwanted elastic properties. Here, we implement a design principle for a spatially inhomogeneous material based on topological band-theory for mechanical systems. The resulting inhomogeneity is stable against wearing and even cutting the material in half. We show how, by creating a periodic elastic material with topological properties, one can create an intrinsically polar behavior, where a face with a given surface normal is stiff while its opposing face is soft. △ Less

Submitted 5 July, 2017; originally announced July 2017.

Journal ref: Adv. Mater. 1700540 (2017)

Phonon-assisted two-photon interference from remote quantum emitters

Authors: Marcus Reindl, Klaus D. Joens, Daniel Huber, Christian Schimpf, Yongheng Huo, Val Zwiller, Armando Rastelli, Rinaldo Trotta

Abstract: Photonic quantum technologies are on the verge of finding applications in everyday life with quantum cryptography and the quantum internet on the horizon. Extensive research has been carried out to determine suitable quantum emitters and single epitaxial quantum dots are emerging as near-optimal sources of bright, on-demand, highly indistinguishable single photons and entangled photon pairs. In or… ▽ More Photonic quantum technologies are on the verge of finding applications in everyday life with quantum cryptography and the quantum internet on the horizon. Extensive research has been carried out to determine suitable quantum emitters and single epitaxial quantum dots are emerging as near-optimal sources of bright, on-demand, highly indistinguishable single photons and entangled photon pairs. In order to build up quantum networks, it is now essential to interface remote quantum emitters. However, this is still an outstanding challenge, as the quantum states of dissimilar 'artificial atoms' have to be prepared on-demand with high fidelity, and the generated photons have to be made indistinguishable in all possible degrees of freedom. Here, we overcome this major obstacle and show an unprecedented two-photon interference (visibility of 51+/-5%) from remote strain-tunable GaAs quantum dots, emitting on-demand photon-pairs. We achieve this result by exploiting for the first time the full potential of the novel phonon-assisted two-photon excitation scheme, which allows for the generation of highly indistinguishable (visibility of 71+/-9%) entangled photon-pairs (fidelity of 90+/-2%), it enables push-to button biexciton state preparation (fidelity of 80+/-2%) and it outperforms conventional resonant two-photon excitation schemes in terms of robustness against environmental decoherence. Our results mark an important milestone for the practical realization of quantum repeaters and complex multi-photon entanglement experiments involving dissimilar artificial atoms. △ Less

Submitted 22 February, 2017; v1 submitted 26 January, 2017; originally announced January 2017.

Designing Perturbative Metamaterials from Discrete Models: From Veselago lenses to topological insulators

Authors: Kathryn H. Matlack, Marc Serra-Garcia, Antonio Palermo, Sebastian D. Huber, Chiara Daraio

Abstract: Discrete models provide concise descriptions of complex physical phenomena, such as negative refraction, topological insulators, and Anderson localization. While there are multiple tools to obtain discrete models that demonstrate particular phenomena, it remains a challenge to find metamaterial designs that replicate the behavior of desired nontrivial discrete models. Here we solve this problem by… ▽ More Discrete models provide concise descriptions of complex physical phenomena, such as negative refraction, topological insulators, and Anderson localization. While there are multiple tools to obtain discrete models that demonstrate particular phenomena, it remains a challenge to find metamaterial designs that replicate the behavior of desired nontrivial discrete models. Here we solve this problem by introducing a new class of metamaterial, which we term 'perturbative metamaterial', consisting of weakly interacting unit cells. The weak interaction allows us to associate each element of the discrete model (individual masses and springs) to individual geometric features of the metamaterial, thereby enabling a systematic design process. We demonstrate our approach by designing 2D mechanical metamaterials that realize Veselago lenses, zero-dispersion bands, and topological insulators. While our selected examples are within the mechanical domain, the same design principle can be applied to acoustic, thermal, and photonic metamaterials composed of weakly interacting unit cells. △ Less

Submitted 7 December, 2016; originally announced December 2016.

Journal ref: Nature Mat. 17, 323 (2018)

Switchable topological phonon channels

Authors: Roman Süsstrunk, Philipp Zimmermann, Sebastian D. Huber

Abstract: Guiding energy deliberately is one of the central elements in engineering and information processing. It is often achieved by designing specific transport channels in a suitable material. Topological metamaterials offer a way to construct stable and efficient channels of unprecedented versatility. However, due to their stability it can be tricky to terminate them or to temporarily shut them off wi… ▽ More Guiding energy deliberately is one of the central elements in engineering and information processing. It is often achieved by designing specific transport channels in a suitable material. Topological metamaterials offer a way to construct stable and efficient channels of unprecedented versatility. However, due to their stability it can be tricky to terminate them or to temporarily shut them off without changing the material properties massively. While a lot of effort was put into realizing mechanical topological metamaterials, almost no works deal with manipulating their edge channels in sight of applications. Here, we take a step in this direction, by taking advantage of local symmetry breaking potentials to build a switchable topological phonon channel. △ Less

Submitted 31 October, 2016; originally announced November 2016.

Comments: 6 pages, 4 figures; Submitted to New Journal of Physics focus issue on Topological Mechanics

Journal ref: New Journal of Phys. 19, 015013 (2017)

Learning phase transitions by confusion

Authors: Evert P. L. van Nieuwenburg, Ye-Hua Liu, Sebastian D. Huber

Abstract: Classifying phases of matter is a central problem in physics. For quantum mechanical systems, this task can be daunting owing to the exponentially large Hilbert space. Thanks to the available computing power and access to ever larger data sets, classification problems are now routinely solved using machine learning techniques. Here, we propose to use a neural network based approach to find phase t… ▽ More Classifying phases of matter is a central problem in physics. For quantum mechanical systems, this task can be daunting owing to the exponentially large Hilbert space. Thanks to the available computing power and access to ever larger data sets, classification problems are now routinely solved using machine learning techniques. Here, we propose to use a neural network based approach to find phase transitions depending on the performance of the neural network after training it with deliberately incorrectly labelled data. We demonstrate the success of this method on the topological phase transition in the Kitaev chain, the thermal phase transition in the classical Ising model, and the many-body-localization transition in a disordered quantum spin chain. Our method does not depend on order parameters, knowledge of the topological content of the phases, or any other specifics of the transition at hand. It therefore paves the way to a generic tool to identify unexplored phase transitions. △ Less

Submitted 6 October, 2016; originally announced October 2016.

Comments: 5 pages, 3 figures

Journal ref: Nature Physics (2017)

Temperature dependence of the electron spin resonance linewidth in magnetic insulators

Authors: M. Acikgoz, D. L. Huber

Abstract: We analyze the temperature dependence of the electron spin resonance linewidth above the critical region in exchange-coupled magnetic insulators. The focus is on separating the contributions to the linewidth from spin-spin interactions, spin-one-phonon interactions and spin-two-phonon interactions at temperatures where the spin-spin term is constant and the one- and two-phonon terms vary as T and… ▽ More We analyze the temperature dependence of the electron spin resonance linewidth above the critical region in exchange-coupled magnetic insulators. The focus is on separating the contributions to the linewidth from spin-spin interactions, spin-one-phonon interactions and spin-two-phonon interactions at temperatures where the spin-spin term is constant and the one- and two-phonon terms vary as T and T^2, respectively. Taking Co3O4 as an example, we use a least squares fit over the temperature range 50 K < T < 500 K to obtain values of the three components. It is found that the spin-spin mechanism is dominant below 100 K, while the two-phonon mechanism is most important above 250 K. In the intermediate region, all three mechanisms make significant contributions. △ Less

Submitted 24 June, 2017; v1 submitted 19 August, 2016; originally announced August 2016.

Comments: 8 pages 2 figures

Effective theory and emergent $SU(2)$ symmetry in the flat bands of attractive Hubbard models

Authors: Murad Tovmasyan, Sebastiano Peotta, Päivi Törmä, Sebastian D. Huber

Abstract: In a partially filled flat Bloch band electrons do not have a well defined Fermi surface and hence the low-energy theory is not a Fermi liquid. Neverethless, under the influence of an attractive interaction, a superconductor well described by the Bardeen-Cooper-Schrieffer (BCS) wave function can arise. Here we study the low-energy effective Hamiltonian of a generic Hubbard model with a flat band.… ▽ More In a partially filled flat Bloch band electrons do not have a well defined Fermi surface and hence the low-energy theory is not a Fermi liquid. Neverethless, under the influence of an attractive interaction, a superconductor well described by the Bardeen-Cooper-Schrieffer (BCS) wave function can arise. Here we study the low-energy effective Hamiltonian of a generic Hubbard model with a flat band. We obtain an effective Hamiltonian for the flat band physics by eliminating higher lying bands via perturbative Schrieffer-Wolff transformation. At first order in the interaction energy we recover the usual procedure of projecting the interaction term onto the flat band Wannier functions. We show that the BCS wave function is the exact ground state of the projected interaction Hamiltonian and that the compressibility is diverging as a consequence of an emergent $SU(2)$ symmetry. This symmetry is broken by second order interband transitions resulting in a finite compressibility, which we illustrate for a one-dimensional ladder with two perfectly flat bands. These results motivate a further approximation leading to an effective ferromagnetic Heisenberg model. The gauge-invariant result for the superfluid weight of a flat band can be obtained from the ferromagnetic Heisenberg model only if the maximally localized Wannier functions in the Marzari-Vanderbilt sense are used. Finally, we prove an important inequality $D \geq \mathcal{W}^2$ between the Drude weight $D$ and the winding number $\mathcal{W}$, which guarantees ballistic transport for topologically nontrivial flat bands in one dimension. △ Less

Submitted 2 August, 2016; originally announced August 2016.

Journal ref: Phys. Rev. B 94, 245149 (2016)

Single spin probe of Many-Body Localization

Authors: Evert P. L. van Nieuwenburg, Sebastian D. Huber, R. Chitra

Abstract: We use an external spin as a dynamical probe of many body localization. The probe spin is coupled to an interacting and disordered environment described by a Heisenberg spin chain in a random field. The spin-chain environment can be tuned between a thermalizing delocalized phase and non-thermalizing localized phase, both in its ground- and high-energy states. We study the decoherence of the probe… ▽ More We use an external spin as a dynamical probe of many body localization. The probe spin is coupled to an interacting and disordered environment described by a Heisenberg spin chain in a random field. The spin-chain environment can be tuned between a thermalizing delocalized phase and non-thermalizing localized phase, both in its ground- and high-energy states. We study the decoherence of the probe spin when it couples to the environment prepared in three states: the ground state, the infinite temperature state and a high energy Néel state. In the non-thermalizing many body localized regime, the coherence shows scaling behaviour in the disorder strength. The long-time dynamics of the probe spin shows a logarithmic dephasing in analogy with the logarithmic growth of entanglement entropy for a bi-partition of a many-body localized system. In summary, we show that decoherence of the probe spin provides clear signatures of many-body localization. △ Less

Submitted 24 November, 2016; v1 submitted 28 July, 2016; originally announced July 2016.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 94, 180202 (2016)

The effect of anisotropy on the absorption spectrum and the density of states of two-dimensional Frenkel exciton systems with Gaussian diagonal disorder

Authors: I. Avgin, D. L. Huber

Abstract: On the optical absorption and the density of states of Frenkel exciton systems on square, rectangular, and triangular lattices with nearest-neighbor interactions and a Gaussian distribution of transition frequencies. The analysis is based on an elliptic integral approach that gives results over the entire spectrum. It is found that the absorption is weakly affected by the anisotropy in contrast to… ▽ More On the optical absorption and the density of states of Frenkel exciton systems on square, rectangular, and triangular lattices with nearest-neighbor interactions and a Gaussian distribution of transition frequencies. The analysis is based on an elliptic integral approach that gives results over the entire spectrum. It is found that the absorption is weakly affected by the anisotropy in contrast to the density of states where the effects can be much stronger. The results for the square lattice are in good agreement with the finite array calculations of Schreiber and Toyozawa. Our findings suggest that the coherent potential approximation can be useful in interpreting the optical properties of two-dimensional systems with dominant nearest-neighbor interactions and Gaussian diagonal disorder where the optically excited states are Frenkel excitons. △ Less

Submitted 3 June, 2016; originally announced June 2016.

Comments: one pdf file

Classification of topological phonons in linear mechanical metamaterials

Authors: Roman Süsstrunk, Sebastian D. Huber

Abstract: Topological phononic crystals, alike their electronic counterparts, are characterized by a bulk-edge correspondence where the interior of a material dictates the existence of stable surface or boundary modes. In the mechanical setup, such surface modes can be used for various applications such as wave-guiding, vibration isolation, or the design of static properties such as stable floppy modes wher… ▽ More Topological phononic crystals, alike their electronic counterparts, are characterized by a bulk-edge correspondence where the interior of a material dictates the existence of stable surface or boundary modes. In the mechanical setup, such surface modes can be used for various applications such as wave-guiding, vibration isolation, or the design of static properties such as stable floppy modes where parts of a system move freely. Here, we provide a classification scheme of topological phonons based on local symmetries. We import and adapt the classification of non-interacting electron systems and embed it into the mechanical setup. Moreover, we provide an extensive set of examples that illustrate our scheme and can be used to generate new models in unexplored symmetry classes. Our works unifies the vast recent literature on topological phonons and paves the way to future applications of topological surface modes in mechanical metamaterials. △ Less

Submitted 4 April, 2016; originally announced April 2016.

Comments: 14 pages, 8 figures

Journal ref: Proc. Natl. Acad. Sci. USA 113, E4767 (2016)