Showing 1–50 of 95 results for author: Sim, H

Observation of Time-Domain Braiding of Non-Abelian Anyons at $ν= 5/2$ State

Authors: Tomer Alkalay, Jinhong Park, Minseong Oh, Changki Hong, Jun-Young M. Lee, Priya Tiwari, Tapas Senapati, Vladimir Umansky, Moty Heiblum, H. -S. Sim

Abstract: Unlike elementary particles, which obey either bosonic or fermionic exchange statistics, certain quasiparticles, known as anyons, are predicted to exhibit Abelian or non-Abelian braiding statistics. While braiding Abelian anyons modifies the wavefunction by a 'statistical phase', braiding non-Abelian anyons implements a unitary transformation of the state within a degenerate subspace of states. Ex… ▽ More Unlike elementary particles, which obey either bosonic or fermionic exchange statistics, certain quasiparticles, known as anyons, are predicted to exhibit Abelian or non-Abelian braiding statistics. While braiding Abelian anyons modifies the wavefunction by a 'statistical phase', braiding non-Abelian anyons implements a unitary transformation of the state within a degenerate subspace of states. Experimental evidence of non-Abelian braiding has thus far remained elusive. Here, we report a 'time-domain braiding' signature of non-Abelian anyons in the $ν= 5/2$ fractional quantum Hall state, by extending our previously demonstrated approach with Abelian anyons at $ν= 1/3$. Our approach is based on measurements of the current fluctuations arising from weak partitioning of a highly dilute one-dimensional edge mode. We independently probe the partition noise of the downstream charged mode and also that of the upstream neutral mode. These independent measurements agree with our theoretical predictions for 'time-domain braiding' of the downstream Abelian and the upstream non-Abelian anyons, respectively, in the 'particle-hole Pfaffian' topological order. Together, these results provide evidence for the presence of non-Abelian anyons. △ Less

Submitted 13 August, 2026; originally announced August 2026.

Comments: 15 pages, 3 figures

Boundary-induced Majorana coupling in a planar topological Josephson junction

Authors: Hyeongseop Kim, Sang-Jun Choi, H. -S. Sim, Sunghun Park

Abstract: Understanding environmental effects in a topological Josephson junction is vital for identifying signatures of Majorana modes. We consider a planar Josephson junction formed on the surface of a three-dimensional topological insulator, which possesses Majorana modes inside the junction and boundary modes outside. We find that tunneling between the inner and outer modes gives rise to effective coupl… ▽ More Understanding environmental effects in a topological Josephson junction is vital for identifying signatures of Majorana modes. We consider a planar Josephson junction formed on the surface of a three-dimensional topological insulator, which possesses Majorana modes inside the junction and boundary modes outside. We find that tunneling between the inner and outer modes gives rise to effective coupling between the inner Majorana modes, and hence induces energy splitting of their states even in the absence of the direct spatial overlap of their wave functions. The energy splitting is obtained analytically in the weak tunneling limit and is numerically investigated for an arbitrary tunneling strength. We discuss in detail the evolution of the energy splitting with an external perpendicular magnetic field and its effect on the shape of the Fraunhofer pattern. △ Less

Submitted 14 January, 2025; originally announced January 2025.

Comments: 11 pages, 6 figures

Journal ref: Physical Review B 111, 045414 (2025)

Coulomb sensing of single ballistic electrons

Authors: J. D. Fletcher, W. Park, P. See, J. P. Griffiths, G. A. C. Jones, I. Farrer, D. A. Ritchie, H. -S. Sim, M. Kataoka

Abstract: While ballistic electrons are a key tool for applications in sensing and flying qubits, sub-nanosecond propagation times and complicated interactions make control of ballistic single electrons challenging. Recent experiments have revealed Coulomb collisions of counterpropagating electrons in a beam splitter, giving time resolved control of interactions between single electrons. Here we use remote… ▽ More While ballistic electrons are a key tool for applications in sensing and flying qubits, sub-nanosecond propagation times and complicated interactions make control of ballistic single electrons challenging. Recent experiments have revealed Coulomb collisions of counterpropagating electrons in a beam splitter, giving time resolved control of interactions between single electrons. Here we use remote Coulomb interactions to demonstrate a scheme for sensing single ballistic electrons. We show that interactions are highly controllable via electron energy and emission timing. We use a weakly-coupled `sensing' regime to characterise the nanoscale potential landscape of the beam splitter and the strength of the Coulomb interaction, and show multi-electron sensing with picosecond resolution. △ Less

Submitted 20 December, 2024; originally announced December 2024.

Comments: 6 pages, 5 figures (main) + 11 pages, 10 figures (supplementary)

Emission and Coherent Control of Levitons in Graphene

Authors: A. Assouline, L. Pugliese, H. Chakraborti, Seunghun Lee, L. Bernabeu, M. Jo, K. Watanabe, T. Taniguchi, D. C. Glattli, N. Kumada, H. -S. Sim, F. D. Parmentier, P. Roulleau

Abstract: Flying qubits encode quantum information in propagating modes instead of stationary discrete states. Although photonic flying qubits are available, the weak interaction between photons limits the efficiency of conditional quantum gates. Conversely, electronic flying qubits can use Coulomb interactions, but the weaker quantum coherence in conventional semiconductors has hindered their realization.… ▽ More Flying qubits encode quantum information in propagating modes instead of stationary discrete states. Although photonic flying qubits are available, the weak interaction between photons limits the efficiency of conditional quantum gates. Conversely, electronic flying qubits can use Coulomb interactions, but the weaker quantum coherence in conventional semiconductors has hindered their realization. In this work, we engineered on-demand injection of a single electronic flying qubit state and its manipulation over the Bloch sphere. The flying qubit is a Leviton propagating in quantum Hall edge channels of a high-mobility graphene monolayer. Although single-shot qubit readout and two-qubit operations are still needed for a viable manipulation of flying qubits, the coherent manipulation of an itinerant electronic state at the single-electron level presents a highly promising alternative to conventional qubits. △ Less

Submitted 13 December, 2024; originally announced December 2024.

Comments: 5 Figures, Comments are welcome

Journal ref: Science 382, 1260, 2023

Universal Spin Screening Clouds in Local Moment Phases

Authors: Minsoo L. Kim, Jeongmin Shim, H. -S. Sim, Donghoon Kim

Abstract: When a local impurity spin interacts with conduction electrons whose density of states (DOS) has a (pseudo)gap or diverges at the Fermi energy, a local moment (LM) phase can be favored over a Kondo phase. Theoretically studying quantum entanglement between the impurity and conduction electrons, we demonstrate that conduction electrons form an ''LM spin cloud'' in general LM phases, which correspon… ▽ More When a local impurity spin interacts with conduction electrons whose density of states (DOS) has a (pseudo)gap or diverges at the Fermi energy, a local moment (LM) phase can be favored over a Kondo phase. Theoretically studying quantum entanglement between the impurity and conduction electrons, we demonstrate that conduction electrons form an ''LM spin cloud'' in general LM phases, which corresponds to, but has fundamental difference from, the Kondo cloud screening the impurity spin in the Kondo phase. The LM cloud algebraically decays over the distance from the impurity when the DOS has a pseudogap or divergence, and exponentially when it has a hard gap. We find an ''LM cloud length'', a single length scale characterizing a universal form of the LM cloud. The findings are supported by both of analytic theories and numerical computations. △ Less

Submitted 6 November, 2024; v1 submitted 4 November, 2024; originally announced November 2024.

Comments: 21 pages, 4 figures for the manuscript; Supplementary material included

Magnetoelectric domain engineering from micrometer to Ångstrøm scales

Authors: Marcela Giraldo, Arkadiy Simonov, Hasung Sim, Ahmed Samir Lotfy, Martin Lilienblum, Lea Forster, Elzbieta Gradauskaite, Morgan Trassin, Je-Geun Park, Thomas Lottermoser, Manfred Fiebig

Abstract: The functionality of magnetoelectric multiferroics depends on the formation, size, and coupling of their magnetic and electric domains. Knowing the parameters guiding these criteria is a key effort in the emerging field of magnetoelectric domain engineering. Here we show, using a combination of piezoresponse-force microscopy, non-linear optics, and x-ray scattering, that the correlation length set… ▽ More The functionality of magnetoelectric multiferroics depends on the formation, size, and coupling of their magnetic and electric domains. Knowing the parameters guiding these criteria is a key effort in the emerging field of magnetoelectric domain engineering. Here we show, using a combination of piezoresponse-force microscopy, non-linear optics, and x-ray scattering, that the correlation length setting the size of the ferroelectric domains in the multiferroic hexagonal manganites can be engineered from the micron range down to a few unit cells under the substitution of Mn$^{3+}$ ions with Al$^{3+}$ ions. The magnetoelectric coupling mechanism between the antiferromagnetic Mn$^{3+}$ order and the distortive-ferroelectric order remains intact even at substantial replacement of Mn$^{3+}$ by Al$^{3+}$. Hence, chemical substitution proves to be an effective tool for domain-size engineering in one of the most studied classes of multiferroics. △ Less

Submitted 13 May, 2024; originally announced May 2024.

Comments: 10 pages, 8 figures

Electrical control of a Kondo spin screening cloud

Authors: Ngoc Han Tu, Donghoon Kim, Minsoo L. Kim, Jeongmin Shim, Ryo Ito, David Pomaranski, Ivan V. Borzenets, Arne Ludwig, Andreas D. Wieck, Heung-Sun Sim, Michihisa Yamamoto

Abstract: Quantitative analysis of quantum many-body systems, consisting of numerous itinerant electrons that interact with localized spins or electrons, is a long-standing issue. The Kondo cloud, a quantum many-body object of conduction electrons that screens a single localized spin, is the building block of such strongly correlated electronic systems. While quantitative analysis of the Kondo cloud associa… ▽ More Quantitative analysis of quantum many-body systems, consisting of numerous itinerant electrons that interact with localized spins or electrons, is a long-standing issue. The Kondo cloud, a quantum many-body object of conduction electrons that screens a single localized spin, is the building block of such strongly correlated electronic systems. While quantitative analysis of the Kondo cloud associated with a single magnetic impurity is well established for uniform conduction electrons, the fundamental properties of a deformed Kondo cloud influenced by conduction electrons with a modulated density of states remain unsolved. Here we report engineering of the Kondo cloud deformation by confining a part of the cloud into a quantum box called the Kondo box that mimics realistic material systems. We demonstrate quantitative control of the Kondo cloud by developing a way of tuning quantum interference in the box and monitoring the Kondo entanglement. The temperature dependence of the entanglement reveals counterintuitively that the cloud shape is altered mainly outside the box although the quantum interference in the box is tuned. Our work provides a way to simulate various strongly correlated systems by integrating the Kondo cloud, which is not possible in the current theoretical framework. △ Less

Submitted 25 March, 2025; v1 submitted 18 April, 2024; originally announced April 2024.

Measuring the arrival time of an electron wave packet using a dynamical potential barrier

Authors: Wanki Park, H. -S. Sim, Sungguen Ryu

Abstract: A time-dependent potential barrier has been used to probe the arrival-time distribution of the wave packet of a hot electron by raising the barrier to block the packet upon arrival of the packet at the barrier. To see whether the barrier precisely detects the distribution, it is necessary to study an error caused by a finite rising speed of the barrier. For this purpose, we study transmission of a… ▽ More A time-dependent potential barrier has been used to probe the arrival-time distribution of the wave packet of a hot electron by raising the barrier to block the packet upon arrival of the packet at the barrier. To see whether the barrier precisely detects the distribution, it is necessary to study an error caused by a finite rising speed of the barrier. For this purpose, we study transmission of an electron wave packet through the dynamical barrier, and identify two regimes, the semiclassical regime and the quasistatic regime. In each regime, we calculate the arrival-time distribution reconstructed by using the barrier and quantify the error in the detection, the difference of the temporal uncertainty between the wave-packet distribution and the reconstructed distribution. Our finding suggests that for precise detection, the time scale, in which the barrier height rises over the energy distribution of the wave packet and the tunneling energy window of the barrier, has to be much shorter than the temporal uncertainty of the wave packet. The analytical results are confirmed with numerical calculations. △ Less

Submitted 6 December, 2023; originally announced December 2023.

Comments: 11 pages, 6 figures

Journal ref: Phys. Rev. B 108, 195309 (2023)

Thermal Hall effects due to topological spin fluctuations in YMnO$_3$

Authors: Ha-Leem Kim, Takuma Saito, Heejun Yang, Hiroaki Ishizuka, Matthew John Coak, Jun Han Lee, Hasung Sim, Yoon Seok Oh, Naoto Nagaosa, Je-Geun Park

Abstract: The thermal Hall effect in magnetic insulators has been considered a powerful method for examining the topological nature of charge-neutral quasiparticles such as magnons. Yet, unlike the kagome system, the triangular lattice has received less attention for studying the thermal Hall effect because the scalar spin chirality cancels out between adjacent triangles. However, such cancellation cannot b… ▽ More The thermal Hall effect in magnetic insulators has been considered a powerful method for examining the topological nature of charge-neutral quasiparticles such as magnons. Yet, unlike the kagome system, the triangular lattice has received less attention for studying the thermal Hall effect because the scalar spin chirality cancels out between adjacent triangles. However, such cancellation cannot be perfect if the triangular lattice is distorted, which could open the possibility of a non-zero thermal Hall effect. Here, we report that the trimerized triangular lattice of multiferroic hexagonal manganite YMnO$_3$ produces a highly unusual thermal Hall effect due to topological spin fluctuations with the additional intricacy of a Dzyaloshinskii-Moriya interaction under an applied magnetic field. We conclude the thermal Hall conductivity arises from the system's topological nature of spin fluctuations. Our theoretical calculations demonstrate that the thermal Hall conductivity is also related in this material to the splitting of the otherwise degenerate two chiralities, left and right, of its 120$^{\circ}$ magnetic structure. Our result is one of the most unusual cases of topological physics due to this broken $Z_2$ symmetry of the chirality in the supposedly paramagnetic state of YMnO$_3$, with strong topological spin fluctuations. These new mechanisms in this important class of materials are crucial in exploring new thermal Hall physics and exotic excitations. △ Less

Submitted 19 November, 2023; originally announced November 2023.

Comments: 14 pages, 3 figures; accepted for publication in Nature Communications

Hierarchical entanglement shells of multichannel Kondo clouds

Authors: Jeongmin Shim, Donghoon Kim, H. -S. Sim

Abstract: Impurities or boundaries often impose nontrivial boundary conditions on a gapless bulk, resulting in distinct boundary universality classes for a given bulk, phase transitions, and non-Fermi liquids in diverse systems. The underlying boundary states however remain largely unexplored. This is related with a fundamental issue how a Kondo cloud spatially forms to screen a magnetic impurity in a metal… ▽ More Impurities or boundaries often impose nontrivial boundary conditions on a gapless bulk, resulting in distinct boundary universality classes for a given bulk, phase transitions, and non-Fermi liquids in diverse systems. The underlying boundary states however remain largely unexplored. This is related with a fundamental issue how a Kondo cloud spatially forms to screen a magnetic impurity in a metal. Here we predict the quantum-coherent spatial and energy structure of multichannel Kondo clouds, representative boundary states involving competing non-Fermi liquids, by studying quantum entanglement between the impurity and the channels. Entanglement shells of distinct non-Fermi liquids coexist in the structure, depending on the channels. As temperature increases, the shells become suppressed one by one from the outside, and the remaining outermost shell determines the thermal phase of each channel. Detection of the entanglement shells is experimentally feasible. Our findings suggest a guide to studying other boundary states and boundary-bulk entanglement. △ Less

Submitted 18 June, 2023; originally announced June 2023.

Comments: 21 pages, 4 figures for the manuscript; Supplementary information included

Journal ref: Nat Commun 14, 3521 (2023)

Observation of Electronic Modes in Open Cavity Resonator

Authors: Hwanchul Jung, Dongsung T. Park, Seokyeong Lee, Uhjin Kim, Chanuk Yang, Jehyun Kim, V. Umansky, Dohun Kim, H. -S. Sim, Yunchul Chung, Hyoungsoon Choi, Hyung Kook Choi

Abstract: The resemblance between electrons and optical waves has strongly driven the advancement of mesoscopic physics. However, electron waves have yet to be understood in open cavity structures which have provided contemporary optics with rich insight towards non-Hermitian systems and complex interactions between resonance mode. Here, we report the realization of an open cavity resonator in a two-dimensi… ▽ More The resemblance between electrons and optical waves has strongly driven the advancement of mesoscopic physics. However, electron waves have yet to be understood in open cavity structures which have provided contemporary optics with rich insight towards non-Hermitian systems and complex interactions between resonance mode. Here, we report the realization of an open cavity resonator in a two-dimensional electronic system. We studied the resonant electron modes within the cavity and resolved the signatures of longitudinal and transverse quantization, showing that the modes are robust despite the openness of the cavity being highly open to the background continuum. The transverse modes were investigated by applying a controlled deformation to the cavity, and their spatial distributions were further analyzed using magnetoconductance measurements and numerical simulation. These results lay the groundwork to exploring electronic wavefunctions in the context of modern optical systems, such as the dielectric microcavity. △ Less

Submitted 15 March, 2023; originally announced March 2023.

Journal ref: Nat Commun 14, 415 (2023)

Time-resolved Coulomb collision of single electrons

Authors: J. D. Fletcher, W. Park, S. Ryu, P. See, J. P. Griffiths, G. A. C. Jones, I. Farrer, D. A. Ritchie, H. -S. Sim, M. Kataoka

Abstract: Precise control over interactions between ballistic electrons will enable us to exploit Coulomb interactions in novel ways, to develop high-speed sensing, to reach a non-linear regime in electron quantum optics and to realise schemes for fundamental two-qubit operations on flying electrons. Time-resolved collisions between electrons have been used to probe the indistinguishability, Wigner function… ▽ More Precise control over interactions between ballistic electrons will enable us to exploit Coulomb interactions in novel ways, to develop high-speed sensing, to reach a non-linear regime in electron quantum optics and to realise schemes for fundamental two-qubit operations on flying electrons. Time-resolved collisions between electrons have been used to probe the indistinguishability, Wigner function and decoherence of single electron wavepackets. Due to the effects of screening, none of these experiments were performed in a regime where Coulomb interactions were particularly strong. Here we explore the Coulomb collision of two high energy electrons in counter-propagating ballistic edge states. We show that, in this kind of unscreened device, the partitioning probabilities at different electron arrival times and barrier height are shaped by Coulomb repulsion between the electrons. This prevents the wavepacket overlap required for the manifestation of fermionic exchange statistics but suggests a new class of devices for studying and manipulating interactions of ballistic single electrons. △ Less

Submitted 7 October, 2022; originally announced October 2022.

Coulomb-mediated antibunching of an electron pair surfing on sound

Authors: Junliang Wang, Hermann Edlbauer, Aymeric Richard, Shunsuke Ota, Wanki Park, Jeongmin Shim, Arne Ludwig, Andreas Wieck, Heung-Sun Sim, Matias Urdampilleta, Tristan Meunier, Tetsuo Kodera, Nobu-Hisa Kaneko, Hermann Sellier, Xavier Waintal, Shintaro Takada, Christopher Bäuerle

Abstract: Electron flying qubits are envisioned as potential information link within a quantum computer, but also promise -- alike photonic approaches -- a self-standing quantum processing unit. In contrast to its photonic counterpart, electron-quantum-optics implementations are subject to Coulomb interaction, which provide a direct route to entangle the orbital or spin degree of freedom. However, the contr… ▽ More Electron flying qubits are envisioned as potential information link within a quantum computer, but also promise -- alike photonic approaches -- a self-standing quantum processing unit. In contrast to its photonic counterpart, electron-quantum-optics implementations are subject to Coulomb interaction, which provide a direct route to entangle the orbital or spin degree of freedom. However, the controlled interaction of flying electrons at the single particle level has not yet been established experimentally. Here we report antibunching of a pair of single electrons that is synchronously shuttled through a circuit of coupled quantum rails by means of a surface acoustic wave. The in-flight partitioning process exhibits a reciprocal gating effect which allows us to ascribe the observed repulsion predominantly to Coulomb interaction. Our single-shot experiment marks an important milestone on the route to realise a controlled-phase gate for in-flight quantum manipulations. △ Less

Submitted 7 October, 2022; originally announced October 2022.

Journal ref: Nature Nanotechnology 18, 721-726 (2023)

Partitioning of Diluted Anyons Reveals their Braiding Statistics

Authors: June-Young M. Lee, Changki Hong, Tomer Alkalay, Noam Schiller, Vladimir Umansky, Moty Heiblum, Yuval Oreg, H. -S. Sim

Abstract: Correlations of partitioned particles carry essential information about their quantumness. Partitioning full beams of charged particles leads to current fluctuations, with their autocorrelation (namely, shot noise) revealing the particle' charge. This is not the case when the partitioned particle beams are diluted. Bosons or fermions will exhibit particles antibunching (due to their sparsity and d… ▽ More Correlations of partitioned particles carry essential information about their quantumness. Partitioning full beams of charged particles leads to current fluctuations, with their autocorrelation (namely, shot noise) revealing the particle' charge. This is not the case when the partitioned particle beams are diluted. Bosons or fermions will exhibit particles antibunching (due to their sparsity and discreteness). However, when diluted anyons, such as the quasiparticles in fractional quantum Hall states, are partitioned in a narrow constriction, their autocorrelation reveals an essential aspect of their exchange statistics: their braiding phase. Here, we describe detailed measurements of weak partitioned, highly diluted, one-dimension-like edge modes of the one-third filling fractional quantum Hall state. The measured autocorrelation agrees with our theory of braiding anyons in the time-domain (instead of braiding in space); with a braiding phase 2$θ$=2$π$/3, without any fitting parameters. Our work offers a relatively straightforward and simple method to observe the braiding statistics of other exotic anyonic states, such as non-abelian states, without resorting to complex interference experiments. △ Less

Submitted 17 May, 2023; v1 submitted 30 September, 2022; originally announced September 2022.

Comments: 9 pages, 5 figures

Journal ref: Nature 617, 277-281 (2023)

Partition of Two Interacting Electrons by a Potential Barrier

Authors: Sungguen Ryu, H. -S. Sim

Abstract: Scattering or tunneling of an electron at a potential barrier is a fundamental quantum effect. Electron-electron interactions often affect the scattering, and understanding of the interaction effect is crucial in detection of various phenomena of electron transport and their application to electron quantum optics. We theoretically study the partition and collision of two interacting hot electrons… ▽ More Scattering or tunneling of an electron at a potential barrier is a fundamental quantum effect. Electron-electron interactions often affect the scattering, and understanding of the interaction effect is crucial in detection of various phenomena of electron transport and their application to electron quantum optics. We theoretically study the partition and collision of two interacting hot electrons at a potential barrier in the quantum Hall regime. We predict their kinetic energy change by their Coulomb interaction during the scattering delay time inside the barrier. The energy change results in characteristic deviation of the partition probabilities from the noninteracting case. The derivation includes nonmonotonic dependence of the probabilities on the barrier height, which agrees with recent experiments, and reduction of the fermionic antibunching. △ Less

Submitted 9 November, 2022; v1 submitted 4 July, 2022; originally announced July 2022.

Comments: 7 pages (including references)+ 7 pages (supplemental material), more discussions included

Journal ref: Phys. Rev. Lett. 129, 166801 (2022)

Coexisting Z-type charge and bond order in metallic NaRu$_2$O$_4$

Authors: Arvind Kumar Yogi, Alexander Yaresko, C. I. Sathish, Hasung Sim, Daisuke Morikawa, J. Nuss, Kenji Tsuda, Y. Noda, Daniel I. Khomskii, Je-Geun Park

Abstract: How particular bonds form in quantum materials has been a long-standing puzzle. Two key concepts dealing with charge degrees of freedom are dimerization (forming metal-metal bonds) and charge ordering (CO). Since the 1930s, these two concepts have been frequently invoked to explain numerous exciting quantum materials, typically insulators. Here we report dimerization and CO within the dimers coexi… ▽ More How particular bonds form in quantum materials has been a long-standing puzzle. Two key concepts dealing with charge degrees of freedom are dimerization (forming metal-metal bonds) and charge ordering (CO). Since the 1930s, these two concepts have been frequently invoked to explain numerous exciting quantum materials, typically insulators. Here we report dimerization and CO within the dimers coexisting in metallic NaRu$_2$O$_4$. By combining high-resolution x-ray diffraction studies and theoretical calculations, we demonstrate that this unique phenomenon occurs through a new type of bonding, which we call Z-type ordering. The low-temperature superstructure has strong dimerization in legs of zigzag ladders, with short dimers in legs connected by short zigzag bonds, forming Z-shape clusters: simultaneously, site-centered charge ordering also appears. Our results demonstrate the yet unknown flexibility of quantum materials with the intricate interplay among orbital, charge, and lattice degrees of freedom. △ Less

Submitted 13 February, 2022; originally announced February 2022.

Comments: 37 pages, 9 figures, 4 tables

Journal ref: Communications Materials 3, 3 (2022) (the Nature journal)

Non-Abelian Anyon Collider

Authors: June-Young M. Lee, H. -S. Sim

Abstract: A collider, where particles are injected to a beam splitter from opposite sides, has been used for identifying quantum statistics of identical particles. The collision leads to bunching of the particles for bosons and antibunching for fermions. In recent experiments, a collider was applied to a fractional quantum Hall regime hosting Abelian anyons. The observed negative cross correlation of electr… ▽ More A collider, where particles are injected to a beam splitter from opposite sides, has been used for identifying quantum statistics of identical particles. The collision leads to bunching of the particles for bosons and antibunching for fermions. In recent experiments, a collider was applied to a fractional quantum Hall regime hosting Abelian anyons. The observed negative cross correlation of electrical currents cannot be understood with fermionic antibunching. Here we predict, based on a conformal field theory and non-perturbative treatment of non-equilibrium anyon injection, that the collider provides a tool for direct observation of the braiding statistics of various Abelian and non-Abelian anyons. Its dominant process is not direct collision between injected anyons, contrary to common expectation, but braiding between injected anyons and an anyon excited at the collider. The dependence of the resulting negative cross correlation on the injection currents distinguishes non-Abelian SU(2)$_k$ anyons, Ising anyons, and Abelian Laughlin anyons. △ Less

Submitted 8 February, 2022; originally announced February 2022.

Comments: Main text: 7 pages, 3 figures, Supplementary text: 8 pages, 3 figures

Journal ref: Nat. Commun. 13, 6660 (2022)

Universal Thermal Entanglement of Multichannel Kondo Effects

Authors: Donghoon Kim, Jeongmin Shim, H. -S. Sim

Abstract: Quantum entanglement between an impurity and its environment is expected to be central in quantum impurity problems. We develop a method to compute the entanglement in spin-1/2 impurity problems, based on the entanglement negativity and the boundary conformal field theory (BCFT). Using the method, we study the thermal decay of the entanglement in the multichannel Kondo effects. At zero temperature… ▽ More Quantum entanglement between an impurity and its environment is expected to be central in quantum impurity problems. We develop a method to compute the entanglement in spin-1/2 impurity problems, based on the entanglement negativity and the boundary conformal field theory (BCFT). Using the method, we study the thermal decay of the entanglement in the multichannel Kondo effects. At zero temperature, the entanglement has the maximal value independent of the number of the screening channels. At low temperature, the entanglement exhibits a power-law thermal decay. The power-law exponent equals two times of the scaling dimension of the BCFT boundary operator describing the impurity spin, and it is attributed to the energy-dependent scaling behavior of the entanglement in energy eigenstates. These agree with numerical renormalization group results, unveiling quantum coherence inside the Kondo screening length. △ Less

Submitted 2 December, 2021; originally announced December 2021.

Journal ref: Phys. Rev. Lett. 127, 226801 (2021)

Fractional Mutual Statistics on Integer Quantum Hall Edges

Authors: June-Young M. Lee, Cheolhee Han, H. -S. Sim

Abstract: Fractional charge and statistics are hallmarks of low-dimensional interacting systems such as fractional quantum Hall (QH) systems. Integer QH systems are regarded noninteracting, yet they can have fractional charge excitations when they couple to another interacting system or time-dependent voltages. Here, we notice Abelian fractional mutual statistics between such a fractional excitation and an… ▽ More Fractional charge and statistics are hallmarks of low-dimensional interacting systems such as fractional quantum Hall (QH) systems. Integer QH systems are regarded noninteracting, yet they can have fractional charge excitations when they couple to another interacting system or time-dependent voltages. Here, we notice Abelian fractional mutual statistics between such a fractional excitation and an electron, and propose a setup for detection of the statistics, in which a fractional excitation is generated at a source and injected to a Mach-Zehnder interferometer (MZI) in the integer QH regime. In a parameter regime, the dominant interference process involves braiding, via double exchange, between an electron excited at an MZI beam splitter and the fractional excitation. The braiding results in the interference phase shift by the phase angle of the mutual statistics. This proposal for directly observing the fractional mutual statistics is within experimental reach. △ Less

Submitted 2 November, 2021; v1 submitted 1 November, 2021; originally announced November 2021.

Comments: Main text: 6 pages, 2 figures, Supplemental material: 7 pages, 3 figures

Journal ref: Phys. Rev. Lett. 125, 196802 (2020)

Fractionalization and anyonic statistics in the integer quantum Hall collider

Authors: Tom Morel, June-Young M. Lee, H. -S. Sim, Christophe Mora

Abstract: One remarkable feature of strongly correlated systems is the phenomenon of fractionalization where quasiparticles carry only a fraction of the charge or spin of the elementary constituents. Such quasiparticles often present anyonic statistics in two dimensions and lie at the heart of the fractional quantum Hall effect. We discuss the observation of fractionalization and anyonic statistics already… ▽ More One remarkable feature of strongly correlated systems is the phenomenon of fractionalization where quasiparticles carry only a fraction of the charge or spin of the elementary constituents. Such quasiparticles often present anyonic statistics in two dimensions and lie at the heart of the fractional quantum Hall effect. We discuss the observation of fractionalization and anyonic statistics already in the integer quantum Hall effect coupled to a metallic island. A continuous fractional emitter is proposed, which sends dilute beams of non-integer charges, and its full counting statistics is obtained. The fractional charge is governed solely by the number of ballistic channels covered by the island and it is one half of the electron charge for a single ballistic channel. We further characterize the mixing of two such fractional beams through a quantum point contact beam splitter. We predict negative cross-correlations, in strong contrast with free electrons, that depend on the double exchange phase between electrons and the fractional charges emulating anyons. The result is similar to a genuine fractional edge state as recently measured at filling $ν= 1/3$. We revisit the physical interpretation of this experiment and point towards a direct braiding measurement rather than a deviation from fermionic antibunching. △ Less

Submitted 16 May, 2022; v1 submitted 26 October, 2021; originally announced October 2021.

Comments: 19 pages, 7 figures

Journal ref: Phys. Rev. B 105, 075433 (2022)

Asymmetric arms maximise visibility in hot-electron interferometers

Authors: Clarissa J. Barratt, Sungguen Ryu, Lewis A. Clark, H. -S. Sim, Masaya Kataoka, Clive Emary

Abstract: We consider theoretically an electronic Mach-Zehnder interferometer constructed from quantum Hall edge channels and quantum point contacts, fed with single electrons from a dynamic quantum dot source. By considering the energy dependence of the edge-channel guide centres, we give an account of the phase averaging in this set up that is particularly relevant for the short, high-energy wavepackets i… ▽ More We consider theoretically an electronic Mach-Zehnder interferometer constructed from quantum Hall edge channels and quantum point contacts, fed with single electrons from a dynamic quantum dot source. By considering the energy dependence of the edge-channel guide centres, we give an account of the phase averaging in this set up that is particularly relevant for the short, high-energy wavepackets injected by this type of electron source. We present both analytic and numerical results for the energy-dependent arrival time distributions of the electrons and also give an analysis of the delay times associated with the quantum point contacts and their effects on the interference patterns. A key finding is that, contrary to expectation, maximum visibility requires the interferometer arms to be different in length, with an offset of up to a micron for typical parameters. By designing interferometers that incorporate this asymmetry in their geometry, phase-averaging effects can be overcome such that visibility is only limited by other incoherent mechanisms. △ Less

Submitted 15 September, 2021; v1 submitted 4 April, 2021; originally announced April 2021.

Comments: 12 pages; 6 figures

Journal ref: Phys. Rev. B 104, 035436 (2021)

Spin texture induced by non-magnetic doping and spin dynamics in 2D triangular lattice antiferromagnet h-Y(Mn,Al)O3

Authors: Pyeongjae Park, Kisoo Park, Joosung Oh, Ki Hoon Lee, Jonathan C. Leiner, Hasung Sim, Taehun Kim, Jaehong Jeong, Kirrily C. Rule, Kazuya Kamazawa, Kazuki Iida, T. G. Perring, Hyungje Woo, S. -W. Cheong, M. E. Zhitomirsky, A. L. Chernyshev, Je-Geun Park

Abstract: Novel effects induced by nonmagnetic impurities in frustrated magnets and quantum spin liquid represent a highly nontrivial and interesting problem. A theoretical proposal of extended modulated spin structures induced by doping of such magnets, distinct from the well-known skyrmions has attracted significant interest. Here, we demonstrate that nonmagnetic impurities can produce such extended spin… ▽ More Novel effects induced by nonmagnetic impurities in frustrated magnets and quantum spin liquid represent a highly nontrivial and interesting problem. A theoretical proposal of extended modulated spin structures induced by doping of such magnets, distinct from the well-known skyrmions has attracted significant interest. Here, we demonstrate that nonmagnetic impurities can produce such extended spin structures in h-YMnO3, a triangular antiferromagnet with noncollinear magnetic order. Using inelastic neutron scattering (INS), we measured the full dynamical structure factor in Al-doped h-YMnO3 and confirmed the presence of magnon damping with a clear momentum dependence. Our theoretical calculations can reproduce the key features of the INS data, supporting the formation of the proposed spin textures. As such, our study provides the first experimental confirmation of the impurity-induced spin textures. It offers new insights and understanding of the impurity effects in a broad class of noncollinear magnetic systems. △ Less

Submitted 15 March, 2021; v1 submitted 10 March, 2021; originally announced March 2021.

Comments: 18 pages, 4 figures and supplementary Information. Accepted for publication in Nature Communications

Journal ref: Nature Communications 12, 2306 (2021)

Quantum Hall valley splitters and tunable Mach-Zehnder interferometer in graphene

Authors: M. Jo, P. Brasseur, A. Assouline, G. Fleury, H. -S. Sim, K. Watanabe, T. Taniguchi, W. Dumnernpanich, P. Roche, D. C. Glattli, N. Kumada, F. D. Parmentier, P. Roulleau

Abstract: Graphene is a very promising test-bed for the field of electron quantum optics. However, a fully tunable and coherent electronic beam splitter is still missing. We report the demonstration of electronic beam splitters in graphene that couple quantum Hall edge channels having opposite valley polarizations. The electronic transmission of our beam splitters can be tuned from zero to near unity. By in… ▽ More Graphene is a very promising test-bed for the field of electron quantum optics. However, a fully tunable and coherent electronic beam splitter is still missing. We report the demonstration of electronic beam splitters in graphene that couple quantum Hall edge channels having opposite valley polarizations. The electronic transmission of our beam splitters can be tuned from zero to near unity. By independently setting the beam splitters at the two corners of a graphene PN junction to intermediate transmissions, we realize a fully tunable electronic Mach-Zehnder interferometer. This tunability allows us to unambiguously identify the quantum interferences due to the Mach-Zehnder interferometer, and to study their dependence with the beam-splitter transmission and the interferometer bias voltage. The comparison with conventional semiconductor interferometers points towards universal processes driving the quantum decoherence in those two different 2D systems, with graphene being much more robust to their effect. △ Less

Submitted 19 July, 2021; v1 submitted 10 November, 2020; originally announced November 2020.

Journal ref: Phys. Rev. Lett. 126, 146803 (2021)

Adjustable quantum interference oscillations in Sb-doped Bi2Se3 topological insulator nanoribbons

Authors: Hong-Seok Kim, Tae-Ha Hwang, Nam-Hee Kim, Yasen Hou, Dong Yu, H. -S. Sim, Yong-Joo Doh

Abstract: Topological insulator (TI) nanoribbons (NRs) provide a unique platform for investigating quantum interference oscillations combined with topological surface states. One-dimensional subbands formed along the perimeter of a TI NR can be modulated by an axial magnetic field, exhibiting Aharonov-Bohm (AB) and Altshuler-Aronov-Spivak (AAS) oscillations of magnetoconductance (MC). Using Sb-doped Bi2Se3… ▽ More Topological insulator (TI) nanoribbons (NRs) provide a unique platform for investigating quantum interference oscillations combined with topological surface states. One-dimensional subbands formed along the perimeter of a TI NR can be modulated by an axial magnetic field, exhibiting Aharonov-Bohm (AB) and Altshuler-Aronov-Spivak (AAS) oscillations of magnetoconductance (MC). Using Sb-doped Bi2Se3 TI NRs, we found that the relative amplitudes of the two quantum oscillations can be tuned by varying the channel length, exhibiting crossover from quasi-ballistic to diffusive transport regimes. The AB and AAS oscillations were discernible even for a 70 micrometer long channel, while only the AB oscillations were observed for a short channel. Analyses based on ensemble-averaged fast Fourier transform of MC curves revealed exponential temperature dependences of the AB and AAS oscillations, from which the circumferential phase-coherence length and thermal length were obtained. Our observations indicate that the channel length in a TI NR can be a useful control knob for tailored quantum interference oscillations, especially for developing topological hybrid quantum devices. △ Less

Submitted 19 August, 2020; originally announced August 2020.

Comments: 22 pages, 3 figures for main text

Journal ref: ACS Nano (2020)

Ultrafast energy relaxation of quantum dot-generated 2D hot electrons

Authors: Dongsung T. Park, Dongkun Kim, Uhjin Kim, Hwanchul Jung, Juho Choi, Cheolhee Han, Yunchul Chung, H. -S. Sim, V. Umansky, Hyoungsoon Choi, Hyung Kook Choi

Abstract: Through a series of transverse magnetic focusing experiments, we show that hot electrons in a two-dimensional electron gas system undergo an ultrafast relaxation when generated by a quantum dot (QD) instead of a quantum point contact (QPC). We find here that QPC hot electrons were well described by the non-interacting Fermi gas model for excitations up to 1.5 meV above the Fermi level of 7.44 meV,… ▽ More Through a series of transverse magnetic focusing experiments, we show that hot electrons in a two-dimensional electron gas system undergo an ultrafast relaxation when generated by a quantum dot (QD) instead of a quantum point contact (QPC). We find here that QPC hot electrons were well described by the non-interacting Fermi gas model for excitations up to 1.5 meV above the Fermi level of 7.44 meV, whereas QD hot electrons exhibited an energy loss quadratic to the excitation. The energy relaxation was a sizeable fraction of the tested excitations, up to about 55%. With the proposal that the hot electrons are relaxed by the QD immediately after emission, we present a toy model in which a capacitive coupling between the QD and its leads results in a finite, ultrafast energy relaxation. △ Less

Submitted 23 April, 2020; originally announced April 2020.

Comments: 11 pages, 4 figures (main article), 9 pages, 6 figures (supplementary materials)

Journal ref: Phys. Rev. Research 3, 033015 (2021)

Josephson junction of finite-size superconductors on a topological insulator under a magnetic field

Authors: Sang-Jun Choi, H. -S. Sim

Abstract: We theoretically study a Josephson junction formed by two finite-size $s$-wave SCs on a topological insulator under a magnetic field. At certain conditions, the junction hosts the chiral Majorana modes enclosing the two finite-size SCs. The interplay of the extended chiral Majorana modes and the states inside the junction can results in nontrivial topological effects such as the $2n π$ fractional… ▽ More We theoretically study a Josephson junction formed by two finite-size $s$-wave SCs on a topological insulator under a magnetic field. At certain conditions, the junction hosts the chiral Majorana modes enclosing the two finite-size SCs. The interplay of the extended chiral Majorana modes and the states inside the junction can results in nontrivial topological effects such as the $2n π$ fractional AC Josephson effects predicted in Ref.~\cite{ChoiSim} We show that the $2n π$ fractional AC Josephson effects can occur in a realistic situation, such as the presence of the midgap states, without requiring fine tuning of the parameters of the junction. We also find that the Shapiro spikes of the junction show a rich structure in a wide range of the AC voltage bias, facilitating experimental identification of the $2n π$ fractional AC Josephson effects. Moreover, we discuss how to observe the non-commutativity of the operations that braid the Majorana fermions of the junction, by measuring the Josephson current. Finally, we study the state evolution of the junction when the junction hosts a different number of Majorana zero modes from the case of the $2n π$ fractional AC Josephson effects. △ Less

Submitted 29 August, 2019; originally announced August 2019.

Comments: 19 pages, 9 figures

Negative Excess Shot Noise by Anyon Braiding

Authors: Byeongmok Lee, Cheolhee Han, H. -S. Sim

Abstract: Anyonic fractional charges $e^*$ have been detected by autocorrelation shot noise at a quantum point contact (QPC) between two fractional quantum Hall edges. We find that the autocorrelation noise can also show a fingerprint of Abelian anyonic fractional statistics. We predict the noise of electrical tunneling current $I$ at the QPC of the fractional-charge detection setup, when anyons are dilutel… ▽ More Anyonic fractional charges $e^*$ have been detected by autocorrelation shot noise at a quantum point contact (QPC) between two fractional quantum Hall edges. We find that the autocorrelation noise can also show a fingerprint of Abelian anyonic fractional statistics. We predict the noise of electrical tunneling current $I$ at the QPC of the fractional-charge detection setup, when anyons are dilutely injected, from an additional edge biased by a voltage, to the setup in equilibrium. At large voltages, the nonequilibrium noise is {\it reduced} below the thermal equilibrium noise by the value $2 e^* I$. This negative excess noise is opposite to the positive excess noise $2e^* I$ of the conventional fractional-charge detection and also to usual positive autocorrelation noises of electrical currents. This is a signature of the Abelian fractional statistics, resulting from the effective braiding of an anyon thermally excited at the QPC around another anyon injected from the additional edge. △ Less

Submitted 4 July, 2019; v1 submitted 1 July, 2019; originally announced July 2019.

Comments: 13 pages, 5 figures

Journal ref: Phys. Rev. Lett. 123, 016803, 2019

Observation of the Kondo Screening Cloud of Micron Lengths

Authors: I. V. Borzenets, J. Shim, J. Chen, A. Ludwig, A. Wieck, S. Tarucha, H. -S. Sim, M. Yamamoto

Abstract: When a magnetic impurity exists in a metal, conduction electrons form a spin cloud that screens the impurity spin. This basic phenomenon is called the Kondo effect. Contrary to electric charge screening, the spin screening cloud occurs quantum coherently, forming spin-singlet entanglement with the impurity. Although the spins interact locally around the impurity, the cloud can spread out over micr… ▽ More When a magnetic impurity exists in a metal, conduction electrons form a spin cloud that screens the impurity spin. This basic phenomenon is called the Kondo effect. Contrary to electric charge screening, the spin screening cloud occurs quantum coherently, forming spin-singlet entanglement with the impurity. Although the spins interact locally around the impurity, the cloud can spread out over micrometers. The Kondo cloud has never been detected to date, and its existence, a fundamental aspect of the Kondo effect, remains as a long-standing controversial issue. Here we present experimental evidence of a Kondo cloud extending over a length of micrometers comparable to the theoretical length $ξ_\mathrm{K}$. In our device, a Kondo impurity is formed in a quantum dot (QD), one-sided coupling to a quasi-one dimensional channel~\cite{Theory_Proposal_HS} that houses a Fabry-Perot (FP) interferometer of various gate-defined lengths $L > 1 \, μ$m. When we sweep a voltage on the interferometer end gate separated from the QD by the length $L$ to induce FP oscillations in conductance, we observe oscillations in measured Kondo temperature $T_\mathrm{K}$, a sign of the cloud at distance $L$. For $L \lesssim ξ_\mathrm{K}$ the $T_\mathrm{K}$ oscillation amplitude becomes larger for the smaller $L$, obeying a scaling function of a single parameter $L/ ξ_\mathrm{K}$, while for $L>ξ_\mathrm{K}$ the oscillation is much weaker. The result reveals that $ξ_\mathrm{K}$ is the only length parameter associated with the Kondo effect, and that the cloud lies mostly inside the length $ξ_\mathrm{K}$ which reaches microns. Our experimental method of using electron interferometers offers a way of detecting the spatial distribution of exotic non-Fermi liquids formed by multiple magnetic impurities or multiple screening channels and solving long-standing issues of spin-correlated systems. △ Less

Submitted 20 June, 2019; originally announced June 2019.

Comments: Main text: 13 pages, 3 figures. Supplementary text: 14 pages, 6 figures

Journal ref: Nature (London) 579, 210 (2020)

Picosecond coherent electron motion in a silicon single-electron source

Authors: Gento Yamahata, Sungguen Ryu, Nathan Johnson, H. -S. Sim, Akira Fujiwara, Masaya Kataoka

Abstract: Understanding ultrafast coherent electron dynamics is necessary for application of a single-electron source to metrological standards, quantum information processing, including electron quantum optics, and quantum sensing. While the dynamics of an electron emitted from the source has been extensively studied, there is as yet no study of the dynamics inside the source. This is because the speed of… ▽ More Understanding ultrafast coherent electron dynamics is necessary for application of a single-electron source to metrological standards, quantum information processing, including electron quantum optics, and quantum sensing. While the dynamics of an electron emitted from the source has been extensively studied, there is as yet no study of the dynamics inside the source. This is because the speed of the internal dynamics is typically higher than 100 GHz, beyond state-of-the-art experimental bandwidth. Here, we theoretically and experimentally demonstrate that the internal dynamics in a silicon singleelectron source comprising a dynamic quantum dot can be detected, utilising a resonant level with which the dynamics is read out as gate-dependent current oscillations. Our experimental observation and simulation with realistic parameters show that an electron wave packet spatially oscillates quantum-coherently at $\sim$ 200 GHz inside the source. Our results will lead to a protocol for detecting such fast dynamics in a cavity and offer a means of engineering electron wave packets. This could allow high-accuracy current sources, high-resolution and high-speed electromagnetic-field sensing, and high-fidelity initialisation of flying qubits. △ Less

Submitted 18 March, 2019; originally announced March 2019.

Journal ref: Nature Nanotechnology 14, 1019-1023 (2019)

Electron-Tunneling-Assisted Non-Abelian Braiding of Rotating Majorana Bound States

Authors: Sunghun Park, H. -S. Sim, Patrik Recher

Abstract: It has been argued that fluctuations of fermion parity are harmful for the demonstration of non-Abelian anyonic statistics. Here, we demonstrate a striking exception in which such fluctuations are actively used. We present a theory of coherent electron transport from a tunneling tip into a Corbino geometry Josephson junction where four Majorana bound states (MBSs) rotate. While the MBSs rotate, el… ▽ More It has been argued that fluctuations of fermion parity are harmful for the demonstration of non-Abelian anyonic statistics. Here, we demonstrate a striking exception in which such fluctuations are actively used. We present a theory of coherent electron transport from a tunneling tip into a Corbino geometry Josephson junction where four Majorana bound states (MBSs) rotate. While the MBSs rotate, electron tunneling happens from the tip to one of the MBSs thereby changing the fermion parity of the MBSs. The tunneling events in combination with the rotation allow us to identify a novel braiding operator that does not commute with the braiding cycles in the absence of tunneling, revealing the non-Abelian nature of MBSs. The time-averaged tunneling current exhibits resonances as a function of the tip voltage with a period that is a direct consequence of the interference between the non-commuting braiding operations. Our work opens up a possibility for utilizing parity non-conserving processes to control non-Abelian states. △ Less

Submitted 6 November, 2020; v1 submitted 22 December, 2018; originally announced December 2018.

Comments: Published version: 7 pages, 3 figures + Supplemental material (10 pages, 3 figures)

Journal ref: Phys. Rev. Lett. 125, 187702 (2020)

Detecting Kondo Entanglement by Electron Conductance

Authors: Gwangsu Yoo, Seung-Sup B. Lee, H. -S. Sim

Abstract: Quantum entanglement between an impurity spin and electrons nearby is a key property of the single-channel Kondo effects. We show that the entanglement can be detected by measuring electron conductance through a double quantum dot in an orbital Kondo regime. We derive a relation between the entanglement and the conductance, when the SU(2) spin symmetry of the regime is weakly broken. The relation… ▽ More Quantum entanglement between an impurity spin and electrons nearby is a key property of the single-channel Kondo effects. We show that the entanglement can be detected by measuring electron conductance through a double quantum dot in an orbital Kondo regime. We derive a relation between the entanglement and the conductance, when the SU(2) spin symmetry of the regime is weakly broken. The relation reflects the universal form of many-body states near the Kondo fixed point. Using it, the spatial distribution of the entanglement, hence, the Kondo cloud, can be detected, with breaking the symmetry spatially nonuniformly by electrical means. △ Less

Submitted 27 August, 2018; originally announced August 2018.

Journal ref: Phys. Rev. Lett. 120, 146801 (2018)

Non-Abelian Evolution of a Majorana Train in a Single Josephson Junction

Authors: Sang-Jun Choi, H. -S. Sim

Abstract: Demonstration of non-Abelian anyon statistics often requires dynamical controls of a complicated device that are challenging in realistic situations. We propose a {\it single} Josephson junction to detect a non-Abelian statistics effect of Majorana fermions, formed by two finite-size $s$-wave superconductors on a topological insulator under a magnetic field. At certain field strengths, a train of… ▽ More Demonstration of non-Abelian anyon statistics often requires dynamical controls of a complicated device that are challenging in realistic situations. We propose a {\it single} Josephson junction to detect a non-Abelian statistics effect of Majorana fermions, formed by two finite-size $s$-wave superconductors on a topological insulator under a magnetic field. At certain field strengths, a train of three localized Majorana fermions appears along the junction, while an extended chiral Majorana fermion encircles the train and the superconductors. A DC voltage bias across the junction causes the train to move and collide with the extended Majorana fermion. This involves interchange of fusion partners among the four Majorana fermions, leading to non-Abelian state evolution. The evolution gives rise to a $2nπ$ fractional AC Josephson effect with an arbitrary integer $n\ge2$ tunable by the voltage. △ Less

Submitted 29 August, 2019; v1 submitted 27 August, 2018; originally announced August 2018.

Numerical renormalization group method for entanglement negativity at finite temperature

Authors: Jeongmin Shim, H. -S. Sim, Seung-Sup B. Lee

Abstract: We develop a numerical method to compute the negativity, an entanglement measure for mixed states, between the impurity and the bath in quantum impurity systems at finite temperature. We construct a thermal density matrix by using the numerical renormalization group (NRG), and evaluate the negativity by implementing the NRG approximation that reduces computational cost exponentially. We apply the… ▽ More We develop a numerical method to compute the negativity, an entanglement measure for mixed states, between the impurity and the bath in quantum impurity systems at finite temperature. We construct a thermal density matrix by using the numerical renormalization group (NRG), and evaluate the negativity by implementing the NRG approximation that reduces computational cost exponentially. We apply the method to the single-impurity Kondo model and the single-impurity Anderson model. In the Kondo model, the negativity exhibits a power-law scaling at temperature much lower than the Kondo temperature and a sudden death at high temperature. In the Anderson model, the charge fluctuation of the impurity contribute to the negativity even at zero temperature when the on-site Coulomb repulsion of the impurity is finite, while at low temperature the negativity between the impurity spin and the bath exhibits the same power-law scaling behavior as in the Kondo model. △ Less

Submitted 26 August, 2018; originally announced August 2018.

Journal ref: Physical Review B 97, 155123 (2018)

Nonlocal Entanglement of 1D Thermal States Induced by Fermion Exchange Statistics

Authors: YeJe Park, Jeongmin Shim, S. -S. B. Lee, H. -S. Sim

Abstract: When two identical fermions exchange their positions, their wave function gains phase factor $-1$. We show that this distance-independent effect can induce nonlocal entanglement in one-dimensional (1D) electron systems having Majorana fermions at the ends. It occurs in the system bulk and has nontrivial temperature dependence. In a system having a single Majorana at each end, the nonlocal entangle… ▽ More When two identical fermions exchange their positions, their wave function gains phase factor $-1$. We show that this distance-independent effect can induce nonlocal entanglement in one-dimensional (1D) electron systems having Majorana fermions at the ends. It occurs in the system bulk and has nontrivial temperature dependence. In a system having a single Majorana at each end, the nonlocal entanglement has a Bell-state form at zero temperature and decays as temperature increases, vanishing suddenly at certain finite temperature. In a system having two Majoranas at each end, it is in a cluster-state form and its nonlocality is more noticeable at finite temperature. By contrast, thermal states of corresponding 1D spins do not have nonlocal entanglement. △ Less

Submitted 26 August, 2018; originally announced August 2018.

Journal ref: Physical Review Letters 119, 210501 (2017)

Attractive Coulomb interactions in a triple quantum dot

Authors: Changki Hong, Gwangsu Yoo, Jinhong Park, Min-Kyun Cho, Yunchul Chung, H. -S. Sim, Dohun Kim, Hyungkook Choi, Vladimir Umansky, Diana Mahalu

Abstract: Electron pairing due to a repulsive Coulomb interaction in a triple quantum dot (TQD) is experimentally studied. It is found that electron pairing in two dots of a TQD is mediated by the third dot, when the third dot strongly couples with the other two via Coulomb repulsion so that the TQD is in the twofold degenerate ground states of (1, 0, 0) and (0, 1, 1) charge configurations. Using the transp… ▽ More Electron pairing due to a repulsive Coulomb interaction in a triple quantum dot (TQD) is experimentally studied. It is found that electron pairing in two dots of a TQD is mediated by the third dot, when the third dot strongly couples with the other two via Coulomb repulsion so that the TQD is in the twofold degenerate ground states of (1, 0, 0) and (0, 1, 1) charge configurations. Using the transport spectroscopy that monitors electron transport through each individual dot of a TQD, we analyze how to achieve the degeneracy in experiments, how the degeneracy is related to electron pairing, and the resulting nontrivial behavior of electron transport. Our findings may be used to design a system with nontrivial electron correlations and functionalities. △ Less

Submitted 23 July, 2018; originally announced July 2018.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. B 97, 241115(R), 2018

Renormalization of spin excitations in hexagonal HoMnO3 by magnon-phonon coupling

Authors: Taehun Kim, Jonathan C. Leiner, Kisoo Park, Joosung Oh, Hasung Sim, Kazuki Iida, Kazuya Kamazawa, Je-Geun Park

Abstract: Hexagonal HoMnO3, a two-dimensional Heisenberg antiferromagnet, has been studied via inelastic neutron scattering. A simple Heisenberg model with a single-ion anisotropy describes most features of the spin-wave dispersion curves. However, there is shown to be a renormalization of the magnon energies located at around 11 meV. Since both the magnon-magnon interaction and magnon-phonon coupling can a… ▽ More Hexagonal HoMnO3, a two-dimensional Heisenberg antiferromagnet, has been studied via inelastic neutron scattering. A simple Heisenberg model with a single-ion anisotropy describes most features of the spin-wave dispersion curves. However, there is shown to be a renormalization of the magnon energies located at around 11 meV. Since both the magnon-magnon interaction and magnon-phonon coupling can affect the renormalization in a noncollinear magnet, we have accounted for both of these couplings by using a Heisenberg XXZ model with 1=S expansions [1] and the Einstein site phonon model [13], respectively. This quantitative analysis leads to the conclusion that the renormalization effect primarily originates from the magnon-phonon coupling, while the spontaneous magnon decay due to the magnon-magnon interaction is suppressed by strong two-ion anisotropy. △ Less

Submitted 22 May, 2018; originally announced May 2018.

Comments: 5 pages, 4 figures

Symmetry breaking and unconventional charge ordering in single crystal Na$_{2.7}$Ru$_4$O$_9$

Authors: Arvind Yogi, C. I. Sathish, Hasung Sim, Matthew J. Coak, Y. Noda, Je-Geun Park

Abstract: The interplay of charge, spin, and lattice degrees of freedom in matter leads to various forms of ordered states through phase transitions. An important subclass of these phenomena of complex materials is charge ordering (CO), mainly driven by mixed-valence states. We discovered by combining the results of electrical resistivity ($ρ$), specific heat, susceptibility $χ$ (\textit{T}), and single cry… ▽ More The interplay of charge, spin, and lattice degrees of freedom in matter leads to various forms of ordered states through phase transitions. An important subclass of these phenomena of complex materials is charge ordering (CO), mainly driven by mixed-valence states. We discovered by combining the results of electrical resistivity ($ρ$), specific heat, susceptibility $χ$ (\textit{T}), and single crystal x-ray diffraction (SC-XRD) that Na$_{2.7}$Ru$_4$O$_9$ with the monoclinic tunnel type lattice (space group $C$2/$m$) exhibits an unconventional CO at room temperature while retaining metallicity. The temperature-dependent SC-XRD results show successive phase transitions with super-lattice reflections at \textbf{q}$_1$=(0, $\frac{1}{2}$, 0) and \textbf{q}$_2$=(0, $\frac{1}{3}$, $\frac{1}{3}$) below $T_{\textrm{C2}}$ (365 K) and only at \textbf{q}$_1$=(0, $\frac{1}{2}$, 0) between $T_{\textrm{C2}}$ and $T_{\textrm{C1}}$ (630 K). We interpreted these as an evidence for the formation of an unconventional CO. It reveals a strong first-order phase transition in the electrical resistivity at $T_{\textrm{C2}}$ (cooling) = 345 K and $T_{\textrm{C2}}$ (heating) = 365 K. We argue that the origin of the phase transition is due to the localized 4$d$ Ru-electrons. The results of our finding reveal an unique example of Ru$^{3+}$/Ru$^{4+}$ mixed valance heavy \textit{d}$^4$ ions. △ Less

Submitted 5 April, 2018; originally announced April 2018.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. B 98, 085113 (2018)

Studies on the high-temperature ferroelectric transition of multiferroic hexagonal manganite RMnO3

Authors: Hasung Sim, Jaehong Jeong, Haeri Kim, S-W Cheong, Je-Geun Park

Abstract: Hexagonal manganites are multiferroic materials with two highly-dissimilar phase transitions: a ferroelectric transition (from P63/mmc to P63cm) at a temperature higher than 1000 K and an antiferromagnetic transition at TN=65 - 130 K. Despite its critical relevance to the intriguing ferroelectric domain physics, the details of the ferroelectric transition are yet not well known to date primarily b… ▽ More Hexagonal manganites are multiferroic materials with two highly-dissimilar phase transitions: a ferroelectric transition (from P63/mmc to P63cm) at a temperature higher than 1000 K and an antiferromagnetic transition at TN=65 - 130 K. Despite its critical relevance to the intriguing ferroelectric domain physics, the details of the ferroelectric transition are yet not well known to date primarily because of the ultra-high transition temperature. Using high-temperature X-ray diffraction experiments, we show that the ferroelectric transition is a single transition of abrupt order and R-Op displacement is the primary order parameter. This structural transition is then simultaneously accompanied by MnO5 tilting and the subsequent development of electric polarization. △ Less

Submitted 18 February, 2018; v1 submitted 4 February, 2018; originally announced February 2018.

Comments: 15 pages, 6 figures

Journal ref: J. Phys.: Condens. Matter 30 (2018) 105601 (8pp)

LO-phonon emission rate of hot electrons from an on-demand single-electron source in a GaAs/AlGaAs heterostructure

Authors: N. Johnson, C. Emary, S. Ryu, H. -S. Sim, P. See, J. D. Fletcher, J. P. Griffiths, G. A. C. Jones, I. Farrer, D. A. Ritchie, M. Pepper, T. J. B. M. Janssen, M. Kataoka

Abstract: Using a recently-developed time-of-flight measurement technique with 1 ps time resolution and electron-energy spectroscopy, we developed a method to measure the longitudinal-optical-phonon emission rate of hot electrons travelling along a depleted edge of a quantum Hall bar. A comparison of the experimental results to a single-particle model implies that the main scattering mechanism involves a tw… ▽ More Using a recently-developed time-of-flight measurement technique with 1 ps time resolution and electron-energy spectroscopy, we developed a method to measure the longitudinal-optical-phonon emission rate of hot electrons travelling along a depleted edge of a quantum Hall bar. A comparison of the experimental results to a single-particle model implies that the main scattering mechanism involves a two-step process via intra-Landau-level transition. We show this scattering can be suppressed by controlling the edge potential profile, and a scattering length > 1 mm can be achieved, allowing the use of this system for scalable single-electron device applications. △ Less

Submitted 25 December, 2017; originally announced December 2017.

Comments: 5 pages and supplementary

Journal ref: Phys. Rev. Lett. 121, 137703 (2018)

Frustrated antiferromagnetic honeycomb-tunnel-like lattice CuRE2Ge2O8 (RE=Pr, Nd, Sm, and Eu)

Authors: Hwanbeom Cho, Marie Kratochvilova, Nahyun Lee, Hasung Sim, Je-Geun Park

Abstract: New frustrated antiferromagnetic compounds CuRE2Ge2O8 (RE=Pr, Nd, Sm, Eu) have been investigated using high-resolution x-ray diffraction, magnetic and heat capacity measurements. These systems show different magnetic lattices depending on rare-earth element. The nonmagnetic Eu compound is a S=1/2 two-dimensional triangular antiferromagnetic lattice oriented in the ac plane with geometrical frustra… ▽ More New frustrated antiferromagnetic compounds CuRE2Ge2O8 (RE=Pr, Nd, Sm, Eu) have been investigated using high-resolution x-ray diffraction, magnetic and heat capacity measurements. These systems show different magnetic lattices depending on rare-earth element. The nonmagnetic Eu compound is a S=1/2 two-dimensional triangular antiferromagnetic lattice oriented in the ac plane with geometrical frustration. On the other hand, the Pr, Nd, and Sm compounds show a three-dimensional honeycomb-tunnel-like lattice made of RE^3+ running along the a axis with the characteristic behavior of frustrated antiferromagnets. △ Less

Submitted 30 November, 2017; originally announced December 2017.

Comments: 9 pages, 5 figures

Journal ref: Phys. Rev. B 96, 224427 (2017)

Transport spectroscopy for Paschen-Back splitting of Landau levels in InAs nanowires

Authors: Bum-Kyu Kim, Sang-Jun Choi, Jae Cheol Shin, Minsoo Kim, Ye-Hwan Ahn, H. -S. Sim, Ju-Jin Kim, Myung-Ho Bae

Abstract: The coupling of electron orbital motion and spin leads to nontrivial changes in energy-level structures, leading to various spectroscopies and applications. In atoms, such spin-orbit coupling (SOC) causes anomalous Zeeman splitting, known as the Paschen-Back (PB) effect, in the pres-ence of a strong magnetic field. In solids, SOC generates energy-band inversion or splitting, a prerequisite for top… ▽ More The coupling of electron orbital motion and spin leads to nontrivial changes in energy-level structures, leading to various spectroscopies and applications. In atoms, such spin-orbit coupling (SOC) causes anomalous Zeeman splitting, known as the Paschen-Back (PB) effect, in the pres-ence of a strong magnetic field. In solids, SOC generates energy-band inversion or splitting, a prerequisite for topological phases or Majorana fermions, at zero or weak magnetic fields. Here, we present the first observation of PB splitting of Landau levels (LLs) in indium arsenide nan-owires in a strong-field regime. Our energy-resolved transport spectroscopy results indicated the presence of LL-dependent anomalous Zeeman splitting in these nanowires, analogous to the atomic PB effect. This result was found to be in good agreement with a theoretical analysis based on Rashba SOC. Our findings also suggested a way of generating spin-resolved electron transport in nanowires. △ Less

Submitted 18 November, 2017; v1 submitted 15 November, 2017; originally announced November 2017.

Comments: 23 pages, 5 figures, SI

Low-energy spin dynamics of orthoferrites AFeO$_3$ (A = Y, La, Bi)

Authors: Kisoo Park, Hasung Sim, Jonathan C. Leiner, Yoshiyuki Yoshida, Jaehong Jeong, Shin-ichiro Yano, Jason Gardner, Philippe Bourges, Milan Klicpera, Vladimír Sechovský, Martin Boehm, Je-Geun Park

Abstract: YFeO$_3$ and LaFeO$_3$ are members of the rare-earth orthoferrites family with \textit{Pbnm} space group. Using inelastic neutron scattering, the low-energy spin excitations have been measured around magnetic Brillouin zone center. Splitting of magnon branches and non-zero magnon gap is observed for both compounds, which is similar to the behavior observed in multiferroic BiFeO$_3$. Spin wave calc… ▽ More YFeO$_3$ and LaFeO$_3$ are members of the rare-earth orthoferrites family with \textit{Pbnm} space group. Using inelastic neutron scattering, the low-energy spin excitations have been measured around magnetic Brillouin zone center. Splitting of magnon branches and non-zero magnon gap is observed for both compounds, which is similar to the behavior observed in multiferroic BiFeO$_3$. Spin wave calculations which include both Dzyaloshinsky-Moriya interactions and single-ion anisotropy in the spin-Hamiltonian comprehensively accounts for all the experimentally observed features. Our results offer insight into the unifying physics underlying the Fe$^{3+}$-based perovskites as well as their distinguishing characteristics. △ Less

Submitted 11 May, 2018; v1 submitted 26 May, 2017; originally announced May 2017.

Comments: 7 pages, 4 figures

Properties of spin 1/2 triangular lattice antiferromagnets: CuRE2Ge2O8 (RE=Y, La)

Authors: Hwanbeom Cho, Marie Kratochvilova, Hasung Sim, Ki-Young Choi, Choong Hyun Kim, Carley Paulsen, Maxim Avdeev, Darren C. Peets, Younghun Jo, Sanghyun Lee, Yukio Noda, Michael J. Lawler, Je-Geun Park

Abstract: We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-ran… ▽ More We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg = 2 eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering. △ Less

Submitted 4 May, 2017; v1 submitted 13 March, 2017; originally announced March 2017.

Comments: 15 pages, 6 figures, and 1 table

Spin glass behavior in frustrated quantum spin system CuAl2O4 with a possible orbital liquid state

Authors: R. Nirmala, Kwang-Hyun Jang, Hasung Sim, Hwanbeom Cho, Junghwan Lee, Nam-Geun Yang, Seongsu Lee, R. M. Ibberson, K. Kakurai, M. Matsuda, S. -W. Cheong, V. V. Gapontsev S. V. Streltsov, Je-Geun Park

Abstract: CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu2+. It is a rather unique feature that the Cu2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc mag… ▽ More CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu2+. It is a rather unique feature that the Cu2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu2+ is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f = Theta_CW/Tm ~67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4 should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state. △ Less

Submitted 28 January, 2017; originally announced January 2017.

Comments: J. Phys. Condens Matter (accepted): 20 pages, 4 figures, 1 Table

Doping effects on trimerization and magnetoelectric coupling of single crystal multiferroic (Y,Lu)MnO3

Authors: Seongil Choi, Hasung Sim, Sunmin Kang, Ki-Young Choi, Je-Geun Park

Abstract: Hexagonal RMnO3 is a multiferroic compound with a giant spin-lattice coupling at an antiferromagnetic transition temperature [1]. Despite extensive studies over the past two decades, however, the origin and underlying microscopic mechanism of the strong spin-lattice coupling remain still very much elusive. In this study, we have tried to address this problem by measuring the thermal expansion and… ▽ More Hexagonal RMnO3 is a multiferroic compound with a giant spin-lattice coupling at an antiferromagnetic transition temperature [1]. Despite extensive studies over the past two decades, however, the origin and underlying microscopic mechanism of the strong spin-lattice coupling remain still very much elusive. In this study, we have tried to address this problem by measuring the thermal expansion and dielectric constant of doped single crystals Y1-xLuxMnO3 with x = 0, 0.25, 0.5, 0.75, and 1.0. From these measurements, we confirm that there is a progressive change in the physical properties with doping. At the same time, all our samples exhibit clear anomalies at TN, even in the samples with x = 0.5 and 0.75 as opposed to some earlier ideas, which suggests an unusual doping dependence of the anomaly. Our work reveals yet another interesting facet of the spin lattice coupling issue in hexagonal RMnO3. △ Less

Submitted 28 January, 2017; originally announced January 2017.

Comments: 11 pages, 5 figures

Journal ref: J. Phys.: Condens. Matter 29, 095602 (2017)

The $3d$-Electron Heisenberg Pyrochlore Mn$_2$Sb$_2$O$_7$

Authors: Darren C. Peets, Hasung Sim, Maxim Avdeev, Je-Geun Park

Abstract: In frustrated magnetic systems, geometric constraints or the competition amongst interactions introduce extremely high degeneracy and prevent the system from readily selecting a low-temperature ground state. The most frustrated known spin arrangement is on the pyrochlore lattice, but nearly all magnetic pyrochlores have unquenched orbital angular momentum, constraining the spin directions through… ▽ More In frustrated magnetic systems, geometric constraints or the competition amongst interactions introduce extremely high degeneracy and prevent the system from readily selecting a low-temperature ground state. The most frustrated known spin arrangement is on the pyrochlore lattice, but nearly all magnetic pyrochlores have unquenched orbital angular momentum, constraining the spin directions through spin-orbit coupling. Pyrochlore Mn$_2$Sb$_2$O$_7$ is an extremely rare Heisenberg pyrochlore system, with directionally-unconstrained spins and low chemical disorder. We show that it undergoes a spin-glass transition at 5.5K, which is suppressed by disorder arising from Mn vacancies, indicating this ground state to be a direct consequence of the spins' interactions. The striking similarities to $3d$ transition metal pyrochlores with unquenched angular momentum suggests that the low spin-orbit coupling in the $3d$ block makes Heisenberg pyrochlores far more accessible than previously imagined. △ Less

Submitted 8 November, 2016; originally announced November 2016.

Comments: 6 pages, 6 figues, including Supplementary

Journal ref: Phys. Rev. B 94, 174431 (2016)

Magnetic Transitions in the Chiral Armchair-Kagome System Mn$_2$Sb$_2$O$_7$

Authors: Darren C. Peets, Hasung Sim, Seongil Choi, Maxim Avdeev, Seongsu Lee, Su Jae Kim, Hoju Kang, Docheon Ahn, Je-Geun Park

Abstract: The competition between interactions in frustrated magnets allows a wide variety of new ground states, often exhibiting emergent physics and unique excitations. Expanding the suite of lattices available for study enhances our chances of finding exotic physics. Mn$_2$Sb$_2$O$_7$ forms in a chiral, kagome-based structure in which a fourth member is added to the kagome-plane triangles to form an armc… ▽ More The competition between interactions in frustrated magnets allows a wide variety of new ground states, often exhibiting emergent physics and unique excitations. Expanding the suite of lattices available for study enhances our chances of finding exotic physics. Mn$_2$Sb$_2$O$_7$ forms in a chiral, kagome-based structure in which a fourth member is added to the kagome-plane triangles to form an armchair unit and link adjacent kagome planes. This structural motif may be viewed as intermediate between the triangles of the kagome network and the tetrahedra in the pyrochlore lattice. Mn$_2$Sb$_2$O$_7$ exhibits two distinct magnetic phase transitions, at 11.1 and 14.2K, at least one of which has a weak ferromagnetic component. The magnetic propagation vector does not change through the lower transition, suggesting a metamagnetic transition or a transition involving a multi-component order parameter. Although previously reported in the $P3_121$ space group, Mn$_2$Sb$_2$O$_7$ actually crystallizes in $P2$, which allows ferroelectricity, and we show clear evidence of magnetoelectric coupling indicative of multiferroic order. The quasi-two-dimensional `armchair-kagome' lattice presents a promising platform for probing chiral magnetism and the effect of dimensionality in highly frustrated systems. △ Less

Submitted 17 January, 2017; v1 submitted 7 November, 2016; originally announced November 2016.

Comments: 11 pages, 10 figures

Journal ref: Phys. Rev. B 95, 014424 (2017)

Spontaneous structural distortion of metallic Shastry-Sutherland system DyB4 by quadrupole-spin-lattice coupling

Authors: Hasung Sim, Seongsu Lee, Kun-Pyo Hong, Jaehong Jeong, J. R. Zhang, T. Kamiyama, D. T. Adroja, C. A. Murray, S. P. Thompson, F. Iga, S. Ji, D. Khomskii, Je-Geun Park

Abstract: DyB4 has a two-dimensional Shastry-Sutherland (Sh-S) lattice with strong Ising character of the Dy ions. Despite the intrinsic frustrations, surprisingly, it undergoes two successive transitions: a magnetic ordering at TN = 20K, and a quadrupole ordering at TQ=12.5 K. From high-resolution neutron and synchrotron X-ray powder diffraction studies, we have obtained full structural information on this… ▽ More DyB4 has a two-dimensional Shastry-Sutherland (Sh-S) lattice with strong Ising character of the Dy ions. Despite the intrinsic frustrations, surprisingly, it undergoes two successive transitions: a magnetic ordering at TN = 20K, and a quadrupole ordering at TQ=12.5 K. From high-resolution neutron and synchrotron X-ray powder diffraction studies, we have obtained full structural information on this material in all phases, and demonstrate that structural modifications occurring at quadrupolar transition lead to the lifting of frustrations inherent in the Sh-S model. Our study thus provides a complete experimental picture of how the intrinsic frustration of the Sh-S lattice can be lifted by the coupling to quadrupole moments. We show that two other factors, i.e. strong spin-orbit coupling and long-range RKKY interaction in metallic DyB4, play an important role in this behavior. △ Less

Submitted 26 October, 2016; originally announced October 2016.

Comments: 12 pages, 3 figures Physical Review B, accepted

Ultrafast Emission and Detection of a Single-Electron Gaussian Wave Packet: A Theoretical Study

Authors: Sungguen Ryu, M. Kataoka, H. -S. Sim

Abstract: Generating and detecting a prescribed single-electron state is an important step towards solid-state fermion optics. We propose how to generate an electron in a Gaussian state, using a quantum-dot pump with gigahertz operation and realistic parameters. With the help of a strong magnetic field, the electron occupies a coherent state in the pump, insensitive to the details of nonadiabatic evolution.… ▽ More Generating and detecting a prescribed single-electron state is an important step towards solid-state fermion optics. We propose how to generate an electron in a Gaussian state, using a quantum-dot pump with gigahertz operation and realistic parameters. With the help of a strong magnetic field, the electron occupies a coherent state in the pump, insensitive to the details of nonadiabatic evolution. The state changes during the emission from the pump, governed by competition between the Landauer-Buttiker traversal time and the passage time. When the former is much shorter than the latter, the emitted state is a Gaussian wave packet. The Gaussian packet can be identified by using a dynamical potential barrier, with a resolution reaching the Heisenberg minimal uncertainty $\hbar/2$. △ Less

Submitted 24 October, 2016; originally announced October 2016.

Journal ref: Phys. Rev. Lett. 117, 146802 (2016)

Spontaneous decays of magneto-elastic excitations in noncollinear antiferromagnet (Y,Lu)MnO3

Authors: Joosung Oh, Manh Duc Le, Ho-Hyun Nahm, Hasung Sim, Jaehong Jeong, T. G. Perring, Hyungje Woo, Kenji Nakajima, Seiko Ohira-Kawamura, Zahra Yamani, Y. Yoshida, H. Eisaki, S. -W. Cheong, A. L. Chernyshev, Je-Geun Park

Abstract: When magnons and phonons, the fundamental quasiparticles of the solid, are coupled to one another, they form a new hybrid quasi-particle, leading to novel phenomena and interesting applications. Despite its wide-ranging importance, however, detailed experimental studies on the underlying Hamiltonian is rare for actual materials. Moreover, the anharmonicity of such magnetoelastic excitations remain… ▽ More When magnons and phonons, the fundamental quasiparticles of the solid, are coupled to one another, they form a new hybrid quasi-particle, leading to novel phenomena and interesting applications. Despite its wide-ranging importance, however, detailed experimental studies on the underlying Hamiltonian is rare for actual materials. Moreover, the anharmonicity of such magnetoelastic excitations remains largely unexplored although it is essential for a proper understanding of their diverse thermodynamic behaviour as well as intrinsic zero-temperature decay. Here we show that in noncollinear antiferromagnets, a strong magnon-phonon coupling can significantly enhance the anharmonicity, resulting in the creation of magnetoelastic excitations and their spontaneous decay. By measuring the spin waves over the full Brillouin zone and carrying out anharmonic spin wave calculations using a Hamiltonian with an explicit magnon-phonon coupling, we have identified a hybrid magnetoelastic mode in (Y,Lu)MnO3 and quantified its decay rate and the exchange-striction coupling term required to produce it. Our work has wide implications for understanding of the spin-phonon coupling and the resulting excitations of the broad classes of noncollinear magnets. △ Less

Submitted 12 September, 2016; originally announced September 2016.

Journal ref: Nat. Commun. 7, 13146 (2016)