Showing 1–50 of 55 results for author: Joshi, R

Entropy Transport in Programmable Quantum Junctions

Authors: Radhika Joshi, Yuli V. Nazarov, Mohammad H. Ansari

Abstract: We show that driven qubit junctions enable programmable control of physical entropy transport, with entropy conductance governed by quantum dynamics rather than by reservoir parameters alone. By comparing two simple quantum architectures -- a driven single-qubit junction and a driven two-qubit junction -- we find that the two-qubit junction enhances entropy transfer while requiring substantially l… ▽ More We show that driven qubit junctions enable programmable control of physical entropy transport, with entropy conductance governed by quantum dynamics rather than by reservoir parameters alone. By comparing two simple quantum architectures -- a driven single-qubit junction and a driven two-qubit junction -- we find that the two-qubit junction enhances entropy transfer while requiring substantially lower driving power than its single-qubit counterpart. We further reveal two non-intuitive effects in both junctions: a sizable coherent contribution to the entropy current that emerges only under resonant driving, and negative differential entropy conductance, where increasing the thermal bias suppresses entropy flow into the probe reservoir. These results identify quantum logic architectures as programmable devices for entropy transport and suggest routes toward quantum feedback control, reservoir protection and refrigeration in driven quantum circuits. △ Less

Submitted 14 July, 2026; originally announced July 2026.

Comments: 15 pages, 8 figures

Synthesis of Bulk Superconducting LiNbO$_2$ Crystals through CaH$_2$ Reduction

Authors: Ryan Paxson, Stephanie J. Hong, Bicky S. Moirangthem, Parham Kabirifar, Saya Takeuchi, Tianyu Li, Chih-Yu Lee, Haotong Liang, Keenan Avers, Kamal R. Joshi, Amlan Datta, Makariy A. Tanatar, Shanta Saha, Joseph A. Dura, Peter Zavalij, Johnpierre Paglione, Ruslan Prozorov, Alexander J. Grutter, Efrain E. Rodriguez, Ichiro Takeuchi

Abstract: We have synthesized layered superconducting LiNbO$_2$ crystals through a bulk phase transformation from LiNbO$_3$ single crystals via CaH$_2$ reduction. As the Nb valence is reduced from 5+ to 3+, the material undergoes a structural transformation to the resulting product, LiNbO$_2$, which is accompanied by metallic behavior and a superconducting transition, Tc onset, as high as 14.4 K. Secondary… ▽ More We have synthesized layered superconducting LiNbO$_2$ crystals through a bulk phase transformation from LiNbO$_3$ single crystals via CaH$_2$ reduction. As the Nb valence is reduced from 5+ to 3+, the material undergoes a structural transformation to the resulting product, LiNbO$_2$, which is accompanied by metallic behavior and a superconducting transition, Tc onset, as high as 14.4 K. Secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) show that the resulting phase is hole-doped through de-lithiation during the reduction. Magnetization and AC susceptibility measurements from a tunnel diode resonator confirm the bulk nature of superconductivity with a superconducting volume fraction of approximately 77% and an upper critical field approaching 26 T. Our study demonstrates extreme hydride reduction as an effective method to induce phase transformations with non-topotactic pathways and can be used to synthesize bulk materials with exotic properties. △ Less

Submitted 10 August, 2026; v1 submitted 7 July, 2026; originally announced July 2026.

Comments: 17 main text pages, with 9 figures. Supporting information section which is 3 pages and 3 figures. v3 corrected minor typographical errors

20 Second Parity Lifetime in an InAs--Pb Tetron Device

Authors: Morteza Aghaee, Zulfi Alam, Mariusz Andrzejczuk, Andrey Antipov, Theodora Asimakidis, Mikhail Astafev, Lukas Avilovas, Ahmad Azizimanesh, Amin Barzegar, Bela Bauer, Jonathan Becker, Umesh Kumar Bhaskar, Andrea G. Boa, Srini Boddapati, Nichlaus Bohac, Jouri Bommer, Jan Borovsky, Léo Bourdet, Samuel Boutin, Srivatsa Chakravarthi, Benjamin J. Chapman, Nikolaos Chatzaras, Tzu-Chiao Chien, Jason Cho, Patrick T. Codd , et al. (140 additional authors not shown)

Abstract: A central promise of topological quantum computing is that increasing the excitation gap improves device performance significantly. Here, we experimentally validate this principle in an InAs--Pb tetron device via interferometric single-shot parity measurements. By replacing aluminum with the higher-gap superconductor lead in our superconductor-semiconductor hybrid devices, we have improved the rob… ▽ More A central promise of topological quantum computing is that increasing the excitation gap improves device performance significantly. Here, we experimentally validate this principle in an InAs--Pb tetron device via interferometric single-shot parity measurements. By replacing aluminum with the higher-gap superconductor lead in our superconductor-semiconductor hybrid devices, we have improved the robustness of our topological phase. In addition, to enable fast and precise bring-up at scale, we have developed an rf measurement technique that resolves low-energy wire-end states and directly measures their energy splitting with $μ\text{eV}$ precision. We employ this technique to bring up a device in a multi-tetron array and perform parity measurements of one of the tetron's hybrid nanowires (NWs). By controllably switching the wire parity, we observe $h/2e$-periodic bimodal shifts in the quantum capacitance of a quantum dot coupled to the hybrid nanowire in an interference loop. Further time-resolved measurements reveal a characteristic parity switching time of $\sim 20$ s with some instances reaching minute-scale. Such extremely long parity lifetimes are orders of magnitude longer than typical qubit operation times, which are on the order of $μ\text{s}$. Finally, we discuss potential implications for the fidelity of Pauli measurements. △ Less

Submitted 2 June, 2026; v1 submitted 2 June, 2026; originally announced June 2026.

Observation of genuine $2+1$D string dynamics in a U$(1)$ lattice gauge theory with a tunable plaquette term on a trapped-ion quantum computer

Authors: Rohan Joshi, Yizhuo Tian, Kevin Hemery, N. S. Srivatsa, Jesse J. Osborne, Henrik Dreyer, Enrico Rinaldi, Jad C. Halimeh

Abstract: Quantum simulations of high-energy physics in $2+1$D can probe dynamical phenomena nonexistent in one spatial dimension and access regimes that are challenging for existing classical simulation methods. For string dynamics -- relevant to hadronization -- a plaquette term is required to realize genuine $2+1$D behavior, as it endows the gauge field with dynamics and enables the propagation of photon… ▽ More Quantum simulations of high-energy physics in $2+1$D can probe dynamical phenomena nonexistent in one spatial dimension and access regimes that are challenging for existing classical simulation methods. For string dynamics -- relevant to hadronization -- a plaquette term is required to realize genuine $2+1$D behavior, as it endows the gauge field with dynamics and enables the propagation of photon-like excitations. Here, we realize a U$(1)$ quantum link model of quantum electrodynamics in two spatial dimensions with a tunable plaquette term on a \texttt{Quantinuum System Model H2} quantum computer. We implement, to our knowledge, the largest quantum simulation of string-breaking dynamics reported to date, on a $5 \times 4$ matter-site square lattice using $51$ qubits. The simulation uses a shallow circuit design with a two-qubit gate depth of $28$ per Trotter step and up to $1540$ entangling gates. Starting from far-from-equilibrium string configurations, we measure the probability for the string to propagate within the lattice plane and find signatures of genuine $2+1$D dynamics only when the plaquette term is present. In a resonant regime, we observe the annihilation of string segments accompanied by the production of electron--positron pairs that screen them. We further find that, only with a nonzero plaquette term, matter creation extends across the lattice plane rather than remaining confined to the initial string path. These results experimentally realize string breaking and demonstrate the emergence of dynamical gauge fields in two spatial dimensions, establishing a route to photon-like propagation in programmable quantum simulators of gauge theories. △ Less

Submitted 8 April, 2026; originally announced April 2026.

Comments: $12+13$ pages, $4+12$ figures, $0+1$ table. See parallel submission by K. Xu et al., "Observation of glueball excitations and string breaking in a $2+1$D $\mathbb{Z}_2$ lattice gauge theory on a trapped-ion quantum computer''

Observation of glueball excitations and string breaking in a $2+1$D $\mathbb{Z}_2$ lattice gauge theory on a trapped-ion quantum computer

Authors: Kaidi Xu, Umberto Borla, Kevin Hemery, Rohan Joshi, Henrik Dreyer, Enrico Rinaldi, Jad C. Halimeh

Abstract: A major goal of the quantum simulation of high-energy physics (HEP) is to probe real-time nonperturbative far-from-equilibrium quantum processes underlying phenomena such as hadronization in quantum chromodynamics (QCD). The quantum simulation of the dynamics of confining strings and glueballs, both essential aspects of quark confinement, in a controllable first-principles way is an important step… ▽ More A major goal of the quantum simulation of high-energy physics (HEP) is to probe real-time nonperturbative far-from-equilibrium quantum processes underlying phenomena such as hadronization in quantum chromodynamics (QCD). The quantum simulation of the dynamics of confining strings and glueballs, both essential aspects of quark confinement, in a controllable first-principles way is an important step towards this goal. Here, we realize a $\mathbb{Z}_2$ lattice gauge theory in $2+1$D with a tunable plaquette term on a \texttt{Quantinuum System Model H2} trapped-ion quantum computer. We implement a shallow depth-6 Trotter circuit on a $6 \times 5$ matter-site square lattice utilizing all $56$ available qubits to execute over $1000$ entangling gates. We prepare far-from-equilibrium initial string configurations that we quench across a range of parameters to observe rich dynamical phenomena, such as the formation of gauge-invariant closed-loop excitations reminiscent of glueballs in QCD and multi-order string breaking accompanied by spontaneous matter creation. We further demonstrate experimentally that the system displays genuine $2+1$D dynamics, as evidenced by string snapshots over time that cannot be trivially mapped to $1+1$D physics. Our results demonstrate digital quantum simulations of nonequilibrium dynamics in a higher-dimensional lattice gauge theory and provide an experimentally accessible setting for phenomena related to confinement physics. △ Less

Submitted 8 April, 2026; originally announced April 2026.

Comments: $12+7$ pages, $4+6$ figures, $0+1$ table. See parallel submission by R. Joshi et al., "Observation of genuine $2+1$D string dynamics in a U$(1)$ lattice gauge theory with a tunable plaquette term on a trapped-ion quantum computer''

Tunable Floquet selection rules in a driven Ising chain

Authors: Rishi Paresh Joshi, Sanchayan Banerjee, Sneha Narasimha Moorthy, Tapan Mishra

Abstract: We study a periodically driven spin-$1/2$ Ising chain with a nearest-neighbour coupling and longitudinal field while a weak transverse field induces single-spin flips. Through Floquet perturbation theory (FPT), we obtain signatures of Hilbert space fragmentation (HSF) and an unconventional form of dynamical localisation which we call the Floquet freezing. Our analysis suggests that these observati… ▽ More We study a periodically driven spin-$1/2$ Ising chain with a nearest-neighbour coupling and longitudinal field while a weak transverse field induces single-spin flips. Through Floquet perturbation theory (FPT), we obtain signatures of Hilbert space fragmentation (HSF) and an unconventional form of dynamical localisation which we call the Floquet freezing. Our analysis suggests that these observations emerge due to a single Floquet selection rule that dictates the prethermal dynamics. For a special value of the field-to-interaction strength ratio together with commensurate drive periods, this rule permits only a constrained subset of bulk spin flips, leading to prethermal HSF in the full spin-$1/2$ Hilbert space. Under open boundary conditions, the same rule suppresses boundary spin flips up to higher order in perturbation and produces long-lived prethermal edge memory, which is neither topological in origin nor is a strong zero mode. Furthermore, under periodic boundary conditions, the largest surviving fragment is exactly the PXP sector at leading order and therefore exhibits Floquet-inherited scar phenomenology in the prethermal window. At higher commensurate ratios of field strength to interaction strength, all first-order single-spin-flip channels are suppressed and the system enters a regime of Floquet freezing. Hence, our study leverages the selection rules obtained through Floquet perturbation theory to obtain exotic prethermal phenomena at different parameter regimes. △ Less

Submitted 24 March, 2026; originally announced March 2026.

Comments: 18 pages, 10 figures

Magnetic flux distribution, quasiparticle spectroscopy, and quality factors in Nb films for superconducting qubits

Authors: Amlan Datta, Bicky S. Moirangthem, Kamal R. Joshi, Anthony P. Mcfadden, Florent Lecocq, Raymond W. Simmonds, Makariy A. Tanatar, Matthew J. Kramer, Ruslan Prozorov

Abstract: Niobium is a practical material platform for superconducting microwave circuits; however, device-level performance can vary significantly depending on film growth and processing conditions. We compare three epitaxial Nb films grown on $c-$plane sapphire substrates under nominally identical conditions, except for the deposition temperature. To correlate internal quality factors, $Q_{\mathrm {i}}$,… ▽ More Niobium is a practical material platform for superconducting microwave circuits; however, device-level performance can vary significantly depending on film growth and processing conditions. We compare three epitaxial Nb films grown on $c-$plane sapphire substrates under nominally identical conditions, except for the deposition temperature. To correlate internal quality factors, $Q_{\mathrm {i}}$, with material properties, we combine magneto-optical imaging of magnetic flux distribution with quasiparticle spectroscopy via measurements of the London penetration depth, $λ(T)$. In the low-$Q_{\mathrm i}$ film, there is a lesser ability to screen the magnetic field and an irregular temperature variation of $λ(T)$, implying the existence of localized in-gap states. High $Q_{\mathrm i}$ films show the opposite trend. We conclude that our measurements provide an efficient method for characterizing and optimizing superconducting films for quantum informatics applications. △ Less

Submitted 24 March, 2026; originally announced March 2026.

Quasiparticle spectroscopy in tantalum films with different Ta/sapphire interfaces

Authors: Bicky S. Moirangthem, Kamal R. Joshi, Anthony P. Mcfadden, Jin-Su Oh, Amlan Datta, Makariy A. Tanatar, Florent Lecocq, Raymond W. Simmonds, Lin Zhou, Matthew J. Kramer, Ruslan Prozorov

Abstract: One of the crucial aspects of current research in quantum information science is the identification and control of loss mechanisms in superconducting circuits. Although microwave measurements directly quantify device performance, additional techniques that probe quasiparticle excitations in superconducting films are needed to understand the microscopic mechanisms underlying dissipation and decoher… ▽ More One of the crucial aspects of current research in quantum information science is the identification and control of loss mechanisms in superconducting circuits. Although microwave measurements directly quantify device performance, additional techniques that probe quasiparticle excitations in superconducting films are needed to understand the microscopic mechanisms underlying dissipation and decoherence. Here, we present results from quasiparticle spectroscopy of Ta/sapphire films by measuring the Meissner-state magnetic susceptibility using a precision frequency-domain resonator specifically designed for thin films. We find direct evidence for additional low-energy excitations in samples with lower internal quality factors. These excitations are consistent with deep subgap states due to two-level systems, Yu-Shiba-Rusinov states near the gap edge, and perhaps other pair-breaking mechanisms. The developed non-destructive frequency-domain quasiparticle spectroscopy is a valuable addition to the quantum materials toolbox. △ Less

Submitted 6 March, 2026; originally announced March 2026.

Magneto-optical study of Nb thin films for superconducting qubits

Authors: Amlan Datta, Kamal R. Joshi, Sunil Ghimire, Makariy A. Tanatar, Cameron J. Kopas, Jayss Marshall, Josh Y. Mutus, David P. Pappas, Matthew J. Kramer, Ruslan Prozorov

Abstract: Among the recognized sources of decoherence in superconducting qubits, the spatial inhomogeneity of the superconducting state and the possible presence of magnetic-flux vortices remain comparatively underexplored. Niobium is commonly used as a structural material in transmon qubits that host Josephson junctions, and excess dissipation anywhere in the transmon can become a bottleneck that limits ov… ▽ More Among the recognized sources of decoherence in superconducting qubits, the spatial inhomogeneity of the superconducting state and the possible presence of magnetic-flux vortices remain comparatively underexplored. Niobium is commonly used as a structural material in transmon qubits that host Josephson junctions, and excess dissipation anywhere in the transmon can become a bottleneck that limits overall quantum performance. The metal/substrate interfacial layer may simultaneously host pair-breaking loss channels (e.g., two-level systems, TLS) and control thermal transport, thereby affecting dissipation and temperature stability. Here, we use quantitative magneto-optical imaging of the magnetic-flux distribution to characterize the homogeneity of the superconducting state and the critical current density, $j_{c}$, in niobium films fabricated under different sputtering conditions. The imaging reveals distinct flux-penetration regimes, ranging from a nearly ideal Bean critical state to strongly nonuniform thermo-magnetic dendritic avalanches. By fitting the measured magnetic-induction profiles, we extract $j_{c}$ and correlate it with film physical properties and with measured qubit internal quality factors. Our results indicate that the Nb/Si interlayer can be a significant contributor to decoherence and should be considered an important factor that must be optimized. △ Less

Submitted 10 February, 2026; originally announced February 2026.

Effect of metal encapsulation on bulk superconducting properties of niobium thin films used in qubits

Authors: Amlan Datta, Kamal R. Joshi, Sunil Ghimire, Bicky S. Moirangthem, Makariy A. Tanatar, Mustafa Bal, Zuhawn Sung, Sabrina Garattoni, Francesco Crisa, Akshay Murthy, David A. Garcia-Wetten, Dominic P. Goronzy, Mark C. Hersam, Michael J. Bedzyk, Shaojiang Zhu, David Olaya, Peter Hopkins, Matthew J. Kramer, Alexander Romanenko, Anna Grassellino, Ruslan Prozorov

Abstract: Niobium metal occupies nearly 100\% of the volume of a typical 2D transmon device. While the aluminum Josephson junction is of utmost importance, maintaining quantum coherence across the entire device means that pair-breaking in Nb leads, capacitive pads, and readout resonators can be a major source of decoherence. The established contributors are surface oxides and hydroxides, as well as absorbed… ▽ More Niobium metal occupies nearly 100\% of the volume of a typical 2D transmon device. While the aluminum Josephson junction is of utmost importance, maintaining quantum coherence across the entire device means that pair-breaking in Nb leads, capacitive pads, and readout resonators can be a major source of decoherence. The established contributors are surface oxides and hydroxides, as well as absorbed hydrogen and oxygen. Metal encapsulation of freshly grown surfaces with non-oxidizing metals, preferably without breaking the vacuum, is a successful strategy to mitigate these issues. While the positive effects of encapsulation are undeniable, it is important to understand its impact on the macroscopic behavior of niobium films. We present a comprehensive study of the bulk superconducting properties of Nb thin films encapsulated with gold and palladium/gold, and compare them to those of bare Nb films. Magneto-optical imaging, magnetization, resistivity, and London and Campbell penetration depth measurements reveal significant differences in encapsulated samples. Both sputtered, and epitaxial Au-capped films exhibit the highest residual resistivity ratio and superconducting transition temperature, as well as the lowest upper critical field, London penetration depth, and critical current. These results are in good agreement with the microscopic theory of anisotropic normal and superconducting states of Nb. We conclude that pair-breaking in the bulk of niobium films, driven by disorder throughout the film rather than just at the surface, is a significant source of quantum decoherence in transmons. We also conclude that gold capping not only passivates the surface but also affects the properties of the entire film, significantly reducing the scattering rate due to defects likely induced by surface diffusion if the film is not protected immediately after fabrication. △ Less

Submitted 7 February, 2026; originally announced February 2026.

String Breaking and Glueball Dynamics in $2+1$D Quantum Link Electrodynamics

Authors: Jiahao Cao, Rohan Joshi, Yizhuo Tian, N. S. Srivatsa, Jad C. Halimeh

Abstract: At the heart of quark confinement and hadronization, the physics of flux strings has recently become a focal point in the field of quantum simulation of high-energy physics (HEP). Despite considerable progress, a detailed understanding of the behavior of flux strings in quantum simulation-relevant lattice formulations of gauge theories has remained limited to the lowest truncations of the gauge fi… ▽ More At the heart of quark confinement and hadronization, the physics of flux strings has recently become a focal point in the field of quantum simulation of high-energy physics (HEP). Despite considerable progress, a detailed understanding of the behavior of flux strings in quantum simulation-relevant lattice formulations of gauge theories has remained limited to the lowest truncations of the gauge field, which are severely limited in their ability to draw conclusions about the quantum field theory limit. Here, we employ tensor network simulations to investigate the behavior of flux strings in a quantum link formulation of $2+1$D quantum electrodynamics (QED) with a spin-$1$ representation of the gauge field. We first map out the ground-state phase diagram of this model in the presence of two spatially separated static charges, revealing distinct microscopic processes responsible for string breaking, including a two-stage breaking mechanism not possible in the spin-$\frac{1}{2}$ formulation. Starting in different initial product state string configurations, we then explore far-from-equilibrium quench dynamics across various parameter regimes, demonstrating genuine $2+1$D real-time string breaking and glueball-like bound state formation, with the latter not possible in the spin-$\frac{1}{2}$ formulation. In and out of equilibrium, we consider different values and placements of the static charges. Finally, we provide efficient qudit circuits for a quantum simulation experiment in which our results can be observed in state-of-the-art ion-trap setups. Our findings lay the groundwork for quantum simulations of flux strings towards the quantum field theory limit. △ Less

Submitted 22 January, 2026; originally announced January 2026.

Comments: $21$ pages, $11$ figures, $2$ tables

Information Transport in Classical-Quantum Hybrid System

Authors: Julian Rapp, Radhika H. Joshi, Alwin van Steensel, Yuli V. Nazarov, Mohammad H. Ansari

Abstract: Many important quantities in quantum information science, such as entropy and entanglement, are non-linear functions of the density matrix and cannot be expressed as operator observables. Standard open-system approaches evolve only a single copy of the density matrix, making it impossible to track the dynamics of such quantities. A formalism proposed by some of the present authors addressed this c… ▽ More Many important quantities in quantum information science, such as entropy and entanglement, are non-linear functions of the density matrix and cannot be expressed as operator observables. Standard open-system approaches evolve only a single copy of the density matrix, making it impossible to track the dynamics of such quantities. A formalism proposed by some of the present authors addressed this challenge by evolving multiple virtual replicas, but was limited to the weak-coupling regime. Here, we extend this approach to strong coupling between a quantum system and classical environments. The resulting multi-replica master equation enables direct evaluation of entropy flow and related metrics in strongly hybridized quantum-classical systems. Our results show that quantum coherence and hybridization jointly suppress net entropy transfer, creating a thermodynamic bottleneck. This framework provides a general tool for studying entropy dynamics and guiding the design of more robust, resource-efficient quantum hardware. △ Less

Submitted 18 August, 2025; v1 submitted 11 August, 2025; originally announced August 2025.

Comments: 16 pages, 11 figures, References updated

Probing Hadron Scattering in Lattice Gauge Theories on Qudit Quantum Computers

Authors: Rohan Joshi, Jan C. Louw, Michael Meth, Jesse J. Osborne, Kevin Mato, Guo-Xian Su, Martin Ringbauer, Jad C. Halimeh

Abstract: An overarching goal in the flourishing field of quantum simulation for high-energy physics is the first-principles study of the microscopic dynamics of scattering processes on a quantum computer. Currently, this is hampered by small system sizes and a restriction to two-level representations of the gauge fields in state-of-the-art quantum simulators. Here, we propose efficient experimentally feasi… ▽ More An overarching goal in the flourishing field of quantum simulation for high-energy physics is the first-principles study of the microscopic dynamics of scattering processes on a quantum computer. Currently, this is hampered by small system sizes and a restriction to two-level representations of the gauge fields in state-of-the-art quantum simulators. Here, we propose efficient experimentally feasible digital qudit quantum circuits for far-from-equilibrium quench dynamics of a $\mathrm{U}(1)$ quantum link lattice gauge theory, where the electric and gauge fields are represented as spin-$1$ operators. Using dedicated numerical simulations, we probe scattering processes in this model on these proposed circuits, focusing on meson-meson and meson-antimeson collisions. The latter are not possible with a two-level representation of the fields, highlighting the suitability of qudits in exploring scattering processes relevant to quantum electrodynamics. The probed scattering dynamics showcases rich physics, including meson flipping and a reflection-transmission transition in meson-antimeson collisions as a function of the gauge coupling strength. Our simulations, which include realistic noise models of dephasing and depolarization, show very good agreement with the exact noiseless dynamics, signaling the readiness of current qudit platforms to observe microscopic scattering dynamics with significantly shallower circuit depths than their qubit counterparts. △ Less

Submitted 16 July, 2025; originally announced July 2025.

Comments: $21$ pages, $15$ figures

Efficient Qudit Circuit for Quench Dynamics of $2+1$D Quantum Link Electrodynamics

Authors: Rohan Joshi, Michael Meth, Jan C. Louw, Jesse J. Osborne, Kevin Mato, Martin Ringbauer, Jad C. Halimeh

Abstract: A major challenge in the burgeoning field of quantum simulation for high-energy physics is the realization of scalable $2+1$D lattice gauge theories on state-of-the-art quantum hardware, which is an essential step towards the overarching goal of probing $3+1$D quantum chromodynamics on a quantum computer. Despite great progress, current experimental implementations of $2+1$D lattice gauge theories… ▽ More A major challenge in the burgeoning field of quantum simulation for high-energy physics is the realization of scalable $2+1$D lattice gauge theories on state-of-the-art quantum hardware, which is an essential step towards the overarching goal of probing $3+1$D quantum chromodynamics on a quantum computer. Despite great progress, current experimental implementations of $2+1$D lattice gauge theories are mostly restricted to relatively small system sizes and two-level representations of the gauge and electric fields. Here, we propose a resource-efficient method for quantum simulating $2+1$D spin-$S$ $\mathrm{U}(1)$ quantum link lattice gauge theories with dynamical matter using qudit-based quantum processors. By integrating out the matter fields through Gauss's law, we reformulate the quantum link model in a purely spin picture compatible with qudit encoding across arbitrary spatial dimensions, eliminating the need for ancillary qubits and reducing resource overhead. Focusing first on the spin-$1/2$ case, we construct explicit circuits for the full Hamiltonian and demonstrate through numerical simulations that the first-order Trotterized circuits accurately capture the quench dynamics even in the presence of realistic noise levels. Additionally, we introduce a general method for constructing coupling-term circuits for higher-spin representations $S>1/2$. Compared to conventional qubit encodings, our framework significantly reduces the number of quantum resources and gate count. Our approach significantly enhances scalability and fidelity for probing nonequilibrium phenomena in higher-dimensional lattice gauge theories, and is readily amenable to implementation on state-of-the-art qudit platforms. △ Less

Submitted 16 July, 2025; originally announced July 2025.

Comments: $19$ pages, $8$ figures, $3$ tables

Full characterization of measurement-induced transitions of a superconducting qubit

Authors: Thomas Connolly, Pavel D. Kurilovich, Vladislav D. Kurilovich, Charlotte G. L. Bøttcher, Sumeru Hazra, Wei Dai, Andy Z. Ding, Vidul R. Joshi, Heekun Nho, Spencer Diamond, Daniel K. Weiss, Valla Fatemi, Luigi Frunzio, Leonid I. Glazman, Michel H. Devoret

Abstract: Repeated quantum non-demolition measurement is a cornerstone of quantum error correction protocols. In superconducting qubits, the speed of dispersive state readout can be enhanced by increasing the power of the readout tone. However, such an increase has been found to result in additional qubit state transitions that violate the desired quantum non-demolition character of the measurement. Recentl… ▽ More Repeated quantum non-demolition measurement is a cornerstone of quantum error correction protocols. In superconducting qubits, the speed of dispersive state readout can be enhanced by increasing the power of the readout tone. However, such an increase has been found to result in additional qubit state transitions that violate the desired quantum non-demolition character of the measurement. Recently, the readout of a transmon superconducting qubit was improved by using a tone with frequency much larger than the qubit frequency. Here, we experimentally identify the mechanisms of readout-induced transitions in this regime. In the dominant mechanism, the energy of an incoming readout photon is partially absorbed by the transmon and partially returned to the transmission line as a photon with lower frequency. Other mechanisms involve the excitation of unwanted package modes, decay via material defects, and, at higher qubit frequencies, the activation of undesired resonances in the transmon spectrum. Our work provides a comprehensive characterization of superconducting qubit state transitions caused by a strong drive. △ Less

Submitted 5 June, 2025; originally announced June 2025.

Comments: 30 pages, 16 figures

Electrical- and magneto-transport across the thermo-elastic martensitic transformation in anti-site-disordered off-stoichiometric Co-Fe-Ti-Si Heusler alloy thin films

Authors: Mainur Rahaman, Lanuakum A Longchar, Rajeev Joshi, R. Rawat, M. Manivel Raja, S. N. Kaul, S. Srinath

Abstract: In this work, we systematically investigate the effect of Anti-site Disorder (ASD) on electrical resistivity $ρ(T)$ and transverse magnetoresistance $MR_{\perp}$ in off-stoichiometric Co-Fe-Ti-Si (CFTS) thin films across the thermo-elastic martensitic phase transformation (MPT). The CFTS films with A2 ASD exhibit a negative temperature coefficient of resistivity (n-TCR) and an upturn below… ▽ More In this work, we systematically investigate the effect of Anti-site Disorder (ASD) on electrical resistivity $ρ(T)$ and transverse magnetoresistance $MR_{\perp}$ in off-stoichiometric Co-Fe-Ti-Si (CFTS) thin films across the thermo-elastic martensitic phase transformation (MPT). The CFTS films with A2 ASD exhibit a negative temperature coefficient of resistivity (n-TCR) and an upturn below $\sim 30\,$K. In sharp contrast, the partially L2$_1$-ordered films are metallic in nature, characterized by a resistivity minimum at low temperatures ($T_{\min} \cong 30\,$K) and a positive TCR for $T > T_{\min}$. The change in the sign of TCR finds a straightforward explanation in terms of the competition between the quantum corrections (weak localization, electron-diffuson scattering) and the ballistic scattering mechanisms (electron-magnon and electron-phonon). We find that, stronger the atomic ASD, more prominent the quantum corrections and the weaker the scattering of $e-m$ and $e-p$ scattering. All the CFTS films exhibit a distinct thermal hysteresis and a significant drop in resistivity, symptomatic of a MPT, near the characteristic temperatures: martensite-end $T_{Me} \cong 300\,$K and austenite-begin $T_{\mathrm{Ab}} \cong 325\,$K. Regardless of the strength of ASD, in the martensite phase the anti-symmetric (ASMR) component of $\mathrm{MR}_{\perp}(H)$ dominates over the symmetric (SMR) counterpart, whereas the reverse is true (i.e. SMR $\gg$ ASMR) for the austenite phase at temperatures $T_{\mathrm{Ab}} \cong 325\,$K $\le T \le 375\,$K, where $\mathrm{MR}_{\perp}$ increases very sharply with temperature as the austenite phase grows rapidly at the expense of the martensite phase. The present results assert that the CFTS Heusler alloy thin films are promising candidates for shape-memory devices and for spintronic applications such as spin valves. △ Less

Submitted 25 November, 2025; v1 submitted 8 May, 2025; originally announced May 2025.

Quantum Monte Carlo assessment of embedding for for strongly correlated defects: interplay between mean-field starting point and interactions

Authors: Kevin G. Kleiner, Sonali Joshi, Rohan Joshi, Woncheol Lee, Alexander Hampel, Malte Rösner, Cyrus E. Dreyer, Lucas K. Wagner

Abstract: Point defects are of interest for many applications, from quantum sensing to modifying bulk properties of materials. Because of their localized orbitals, the electronic states are often strongly correlated, which has led to a proliferation of quantum embedding techniques to treat this correlation. In these techniques, most of the one-body states are treated with a weakly correlated theory such as… ▽ More Point defects are of interest for many applications, from quantum sensing to modifying bulk properties of materials. Because of their localized orbitals, the electronic states are often strongly correlated, which has led to a proliferation of quantum embedding techniques to treat this correlation. In these techniques, most of the one-body states are treated with a weakly correlated theory such as density functional theory, and certain one-body states are singled out as an active space to be treated using an effective interaction. We assess these techniques for iron and chromium defects in aluminum nitride using quantum Monte Carlo (QMC) calculations on identical Hamiltonians. For these systems, we find the dominant errors in the embedding arise from the one-body crystal-field splitting in the d orbitals inherited from density functional theory (DFT), rather than double counting corrections, with the screened interactions also affected by the DFT orbitals. Strikingly, the best double counting recipe is opposite in these two cases. Because excitation energies can agree while the underlying wave functions do not, diagnosing these errors requires detailed information about the many-body wave functions, which QMC provides. △ Less

Submitted 29 July, 2026; v1 submitted 1 May, 2025; originally announced May 2025.

Comments: includes Supplementary Material (15 pages, 6 figures, 9 tables)

Evidence of magnetoelastic coupling and magnetic phase coexistence in Mn$_{1.7}$Fe$_{1.3}$Si Heusler Alloy

Authors: Kulbhushan Mishra, Elaine T. Dias, Rajeev Joshi, A. D. Fortes, Christopher M. Howard, Rajeev Rawat, P. A. Bhobe

Abstract: Noncollinear metallic antiferromagnets, with their rapid spin dynamics, efficient spin transport, and distinctive spin textures, play a pivotal role in advancing the field of spintronics. In this study, we report a comprehensive investigation of the structural, magnetic, and transport properties of cubic Mn$_{1.7}$Fe$_{1.3}$Si Heusler compound. Temperature-dependent magnetization measurement revea… ▽ More Noncollinear metallic antiferromagnets, with their rapid spin dynamics, efficient spin transport, and distinctive spin textures, play a pivotal role in advancing the field of spintronics. In this study, we report a comprehensive investigation of the structural, magnetic, and transport properties of cubic Mn$_{1.7}$Fe$_{1.3}$Si Heusler compound. Temperature-dependent magnetization measurement reveals a paramagnetic to ferromagnetic transition at $T_C$ = 85 K, followed by a spin reorientation transition. Neutron diffraction data, analyzed as a function of temperature, demonstrates that the occurrence of a spin-reorientation transition is accompanied by magnetoelastic coupling, as evidenced by a change in unit cell volume below $T_C$. Magnetic structure refinement of the low-temperature neutron powder diffraction data confirms the canted antiferromagnetic ordering below 55 K. The metallic nature of the sample is confirmed by the gradual decrease in the $ρ$(T) with decreasing temperature. At low temperatures, a field-induced metamgnetic transition is observed in both, magnetization and magneto-transport measurements. The $H-T$ phase diagram shows a phase-coexistence region emerging at low temperatures for H $<$ 2.5 T. These findings provide valuable insights into the magnetic and transport behavior of the Heusler compounds, underscoring their potential for spintronic applications. △ Less

Submitted 13 February, 2025; originally announced February 2025.

Comments: 11 pages, 10 figures

High-frequency readout free from transmon multi-excitation resonances

Authors: Pavel D. Kurilovich, Thomas Connolly, Charlotte G. L. Bøttcher, Daniel K. Weiss, Sumeru Hazra, Vidul R. Joshi, Andy Z. Ding, Heekun Nho, Spencer Diamond, Vladislav D. Kurilovich, Wei Dai, Valla Fatemi, Luigi Frunzio, Leonid I. Glazman, Michel H. Devoret

Abstract: Quantum computation will rely on quantum error correction to counteract decoherence. Successfully implementing an error correction protocol requires the fidelity of qubit operations to be well-above error correction thresholds. In superconducting quantum computers, measurement of the qubit state remains the lowest-fidelity operation. For the transmon, a prototypical superconducting qubit, measurem… ▽ More Quantum computation will rely on quantum error correction to counteract decoherence. Successfully implementing an error correction protocol requires the fidelity of qubit operations to be well-above error correction thresholds. In superconducting quantum computers, measurement of the qubit state remains the lowest-fidelity operation. For the transmon, a prototypical superconducting qubit, measurement is carried out by scattering a microwave tone off the qubit. Conventionally, the frequency of this tone is of the same order as the transmon frequency. The measurement fidelity in this approach is limited by multi-excitation resonances in the transmon spectrum which are activated at high readout power. These resonances excite the qubit outside of the computational basis, violating the desired quantum non-demolition character of the measurement. Here, we find that strongly detuning the readout frequency from that of the transmon exponentially suppresses the strength of spurious multi-excitation resonances. By increasing the readout frequency up to twelve times the transmon frequency, we achieve a quantum non-demolition measurement fidelity of 99.93% with a residual probability of leakage to non-computational states of only 0.02%. △ Less

Submitted 15 January, 2025; originally announced January 2025.

Comments: 27 pages, 14 figures

Fermion as a non-local particle-hole excitation

Authors: Alok Kushwaha, Rishi Paresh Joshi, Girish Sampath Setlur

Abstract: We show that the fermion, in the context of a system that comprises many such entities - which, by virtue of the Pauli exclusion principle, possesses a Fermi surface at zero temperature - may itself be thought of as a collection of non-local particle-hole excitations across this Fermi surface. This result is purely kinematical and completely general - not being restricted to any specific dimension… ▽ More We show that the fermion, in the context of a system that comprises many such entities - which, by virtue of the Pauli exclusion principle, possesses a Fermi surface at zero temperature - may itself be thought of as a collection of non-local particle-hole excitations across this Fermi surface. This result is purely kinematical and completely general - not being restricted to any specific dimension, applicable to both continuum and lattice systems. There is also no implication that it is applicable only to low-energy phenomena close to the Fermi surface. We are able to derive the full single-particle dynamical Green function of this fermion at finite temperature by viewing it as a collection of these non-local particle-hole excitations. The Green function of the fermion then manifests itself as a solution to a first-order differential equation in a parameter that controls the number of particle-hole pairs across the Fermi surface, and this equation itself reveals variable coefficients that may be identified with a Bose-Einstein distribution - implying that there is a sense in which the non-local particle-hole excitations have bosonic qualities while not being exact bosons at the level of operators. We also recall the definition of the non-local particle-hole operator that may be used to diagonalize the kinetic energy of free fermions of the sort mentioned above. Number-conserving products of creation and annihilation operators of fermions are expressible as a (rather complicated) combination of these non-local particle-hole operators. △ Less

Submitted 11 January, 2025; originally announced January 2025.

Conventional s-wave superconductivity and hidden peak effect in single crystals of Mo$_8$Ga$_41$ superconductor

Authors: Sunil Ghimire, Kyuil Cho, Kamal R. Joshi, Makariy A. Tanatar, Zhixiang Hu, Cedomir Petrovic, Ruslan Prozorov

Abstract: London and Campbell penetration depths were measured in single crystals of the endohedral gallide cluster superconductor, Mo$_{8}$Ga$_{41}$. The full temperature range superfluid density is consistent with the clean isotropic $s-$wave weak-coupling BCS theory without any signs of the second gap or strong coupling. The temperature dependence of the Campbell length is hysteretic between zero-field c… ▽ More London and Campbell penetration depths were measured in single crystals of the endohedral gallide cluster superconductor, Mo$_{8}$Ga$_{41}$. The full temperature range superfluid density is consistent with the clean isotropic $s-$wave weak-coupling BCS theory without any signs of the second gap or strong coupling. The temperature dependence of the Campbell length is hysteretic between zero-field cooling (ZFC) and field-cooling (FC) protocols, indicating an anharmonic vortex pinning potential. The field dependence of the effective critical current density, $j_{c}\left(H\right)$, reveals an unusual result. While in the ZFC protocol, $j_{c}\left(H\right)$ is monotonically suppressed by the magnetic field, it exhibits a profound ``hidden'' peak effect in the FC protocol, that is, without a vortex density gradient. We suggest a possible novel mechanism for the formation of the peak effect, which involves both static and dynamic aspects. △ Less

Submitted 7 July, 2024; originally announced July 2024.

Journal ref: SciPost Physics 17, 117 (2024)

Single-gap Isotropic $s-$wave Superconductivity in Single Crystals $\text{AuSn}_4$

Authors: Sunil Ghimire, Kamal R. Joshi, Elizabeth H. Krenkel, Makariy A. Tanatar, Marcin Konczykowski, Romain Grasset, Paul C. Canfield, Ruslan Prozorov

Abstract: London, $λ_L (T)$, and Campbell, $λ_{C} (T)$, penetration depths were measured in single crystals of a topological superconductor candidate $\text{AuSn}_4$. At low temperatures, $λ_L (T)$ is exponentially attenuated and, if fitted with the power law, $λ(T) \sim T^n$, gives exponents $n>4$, indistinguishable from the isotropic single $s-$wave gap Bardeen-Cooper-Schrieffer (BCS) asymptotic. The supe… ▽ More London, $λ_L (T)$, and Campbell, $λ_{C} (T)$, penetration depths were measured in single crystals of a topological superconductor candidate $\text{AuSn}_4$. At low temperatures, $λ_L (T)$ is exponentially attenuated and, if fitted with the power law, $λ(T) \sim T^n$, gives exponents $n>4$, indistinguishable from the isotropic single $s-$wave gap Bardeen-Cooper-Schrieffer (BCS) asymptotic. The superfluid density fits perfectly in the entire temperature range to the BCS theory. The superconducting transition temperature, $T_c = 2.40 \pm 0.05\:\text{K}$, does not change after 2.5 MeV electron irradiation, indicating the validity of the Anderson theorem for isotropic $s-$wave superconductors. Campbell penetration depth before and after electron irradiation shows no hysteresis between the zero-field cooling (ZFC) and field cooling (FC) protocols, consistent with the parabolic pinning potential. Interestingly, the critical current density estimated from the original Campbell theory decreases after irradiation, implying that a more sophisticated theory involving collective effects is needed to describe vortex pinning in this system. In general, our thermodynamic measurements strongly suggest that the bulk response of the $\text{AuSn}_4$ crystals is fully consistent with the isotropic $s-$wave weak-coupling BCS superconductivity. △ Less

Submitted 3 July, 2024; originally announced July 2024.

Journal ref: SciPost Physics 17, 116 (2024)

The nontrivial effects of annealing on superconducting properties of Nb single crystals

Authors: Amlan Datta, Kamal R. Joshi, Giulia Berti, Sunil Ghimire, Aidan Goerdt, Makariy A. Tanatar, Deborah L. Schlagel, Matthew F. Besser, Dapeng Jing, Matthew Kramer, Maria Iavarone, Ruslan Prozorov

Abstract: The effect of annealing on the superconducting properties of niobium single crystals cut from the same master boule was studied by local and global magnetic measurements, as well as scanning tunneling microscopy (STM). The formation of large hydride precipitates was observed in unannealed samples. The variation in structural and magnetic properties was studied after annealing under high vacuum at… ▽ More The effect of annealing on the superconducting properties of niobium single crystals cut from the same master boule was studied by local and global magnetic measurements, as well as scanning tunneling microscopy (STM). The formation of large hydride precipitates was observed in unannealed samples. The variation in structural and magnetic properties was studied after annealing under high vacuum at 800 C, 1400 C, and near the melting point of niobium (2477 C) for a few seconds. The initial samples had a high hydrogen content. Polarized optics and magneto-optical studies show that the formation of large niobium hydride precipitates is suppressed already by 800 C annealing. However, the overall superconducting properties in the annealed samples did not improve after annealing, and in fact, worsened. The superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. Parallel studies were conducted using STM, where the sample was annealed initially at 400 C, measured, annealed again at 1700 C, and measured again. These studies revealed a ``dirty'' superconducting gap with a significant spatial variation of tunneling conductance after annealing at 400 C. The clean gap was recovered after annealing at 1700 C. It is likely that these results are due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. Overall, the results indicate that vacuum annealing at least up to 1400 C, while expected to remove a large amount of hydrogen, introduces additional nanosized defects, perhaps hydride precipitates, that act as efficient pair-breaking and pinning centers. △ Less

Submitted 24 March, 2024; originally announced March 2024.

Journal ref: Superconductor Science and Technology 37, 95006 (2024)

Creep-enhanced vortex pinning revealed through nonmonotonic relaxation of the Campbell length

Authors: Sunil Ghimire, Filippo Gaggioli, Kamal R. Joshi, Marcin Konczykowski, Romain Grasset, Elizabeth H. Krenkel, Amlan Datta, Makariy A. Tanatar, Shuzhang Chen, Cedomir Petrovic, Vadim B. Geshkenbein, Ruslan Prozorov

Abstract: We study the effects of flux creep on the linear AC response of the vortex lattice in single crystals Ca$_3$Ir$_4$Sn$_{13}$ by measuring the Campbell penetration depth, $λ_{\rm \scriptscriptstyle C}(T,H,t)$. Thermal fluctuations release vortices from shallow pinning sites, only for them to become re-trapped by deeper potential wells, causing an initial increase of the effective Labusch parameter,… ▽ More We study the effects of flux creep on the linear AC response of the vortex lattice in single crystals Ca$_3$Ir$_4$Sn$_{13}$ by measuring the Campbell penetration depth, $λ_{\rm \scriptscriptstyle C}(T,H,t)$. Thermal fluctuations release vortices from shallow pinning sites, only for them to become re-trapped by deeper potential wells, causing an initial increase of the effective Labusch parameter, which is proportional to the pinning well curvature. This effect cannot be detected in conventional magnetic relaxation measurements but is revealed by our observation of a nonmonotonic time evolution of $λ_{\rm \scriptscriptstyle C}(T,H,t)$, which directly probes the average curvature of the occupied pinning centers. The time evolution of $λ_{\rm \scriptscriptstyle C}(T,H,t)$ was measured at different temperatures in samples with different densities of pinning centers produced by electron irradiation. The curves can be collapsed together when plotted on a logarithmic time scale $t \to T\ln{(t/t_0)}$ confirming that the time evolution is driven by flux creep. The $λ_{\rm \scriptscriptstyle C}(T,H,t)$ is hysteretic with a noticeable nonmonotonic relaxation in the presence of a vortex density gradient (after zero-field cooling), but is monotonic after field cooling, where the vortex density is uniform. This result quantitatively corroborates the novel picture of vortex creep based on the strong pinning theory. △ Less

Submitted 31 May, 2024; v1 submitted 21 March, 2024; originally announced March 2024.

Signatures of quantum phases in a dissipative system

Authors: Rohan Joshi, Saikat Mondal, Souvik Bandyopadhyay, Sourav Bhattacharjee, Adhip Agarwala

Abstract: Lindbladian formalism, as tuned to dissipative and open systems, has been all-pervasive to interpret non-equilibrium steady states of quantum many-body systems. We study the fate of free fermionic and superconducting phases in a dissipative one-dimensional Kitaev model - where the bath acts both as a source and a sink of fermionic particles with different coupling rates. As a function of these two… ▽ More Lindbladian formalism, as tuned to dissipative and open systems, has been all-pervasive to interpret non-equilibrium steady states of quantum many-body systems. We study the fate of free fermionic and superconducting phases in a dissipative one-dimensional Kitaev model - where the bath acts both as a source and a sink of fermionic particles with different coupling rates. As a function of these two couplings, we investigate the steady state, its entanglement content, and its approach from varying initial states. Interestingly, we find that the steady state phase diagram retains decipherable signatures of ground state critical physics. We also show that early-time fidelity is a useful marker to find a subclass of phase transitions in such situations. Moreover, we show that the survival of critical signatures at late-times, strongly depend on the thermal nature of the steady state. This connection hints at a correspondence between quantum observables and classical magnetism in the steady state of such systems. Our work uncovers interesting connections between dissipative quantum many-body systems, thermalization of a classical spin and many-body quantum critical phenomena. △ Less

Submitted 16 April, 2024; v1 submitted 28 December, 2023; originally announced December 2023.

Comments: 14 pages, 10 figures

Journal ref: Journal of Physics: Condensed Matter 36 (2024) 275601

Electron irradiation reveals robust fully gapped superconductivity in LaNiGa$_{2}$

Authors: S. Ghimire, K. R. Joshi, E. H. Krenkel, M. A. Tanatar, Yunshu Shi, M. Konczykowski, R. Grasset, V. Taufour, P. P. Orth, M. S. Scheurer, R. Prozorov

Abstract: The effects of 2.5 MeV electron irradiation were studied in the superconducting phase of single crystals of LaNiGa$_2$, using measurements of electrical transport and radio-frequency magnetic susceptibility. The London penetration depth is found to vary exponentially with temperature, suggesting a fully gapped Fermi surface. The inferred superfluid density is close to that of a single-gap weak-cou… ▽ More The effects of 2.5 MeV electron irradiation were studied in the superconducting phase of single crystals of LaNiGa$_2$, using measurements of electrical transport and radio-frequency magnetic susceptibility. The London penetration depth is found to vary exponentially with temperature, suggesting a fully gapped Fermi surface. The inferred superfluid density is close to that of a single-gap weak-coupling isotropic $s-$wave superconductor. Superconductivity is extremely robust against nonmagnetic point-like disorder induced by electron irradiation. Our results place strong constraints on the previously proposed triplet pairing state by requiring fine-tuned impurity scattering amplitudes and are most naturally explained by a sign-preserving, weak-coupling, and approximately momentum independent singlet superconducting state in LaNiGa$_2$, which does not break time-reversal symmetry. We discuss how our findings could be reconciled with previous measurements indicating magnetic moments in the superconducting phase. △ Less

Submitted 20 November, 2023; originally announced November 2023.

Journal ref: Phys. Rev. B 109, 024515 (2024)

Robust superconductivity and the suppression of charge-density wave in $\text{Ca}_{3}(\text{Ir}_{1-x}\text{Rh}_{x})_{4}\text{Sn}_{13}$ single crystals at ambient pressure

Authors: Elizabeth H. Krenkel, Makariy A. Tanatar, Sunil Ghimire, Kamal R. Joshi, Shuzhang Chen, Cedomir Petrovic, Ruslan Prozorov

Abstract: Single crystals of Ca$_3$(Ir$_{1-x}$Rh$_x$)$_4$Sn$_{13}$ (3-4-13) were synthesized by flux growth and characterized by X-ray diffraction, EDX, magnetization, resistivity and radio frequency magnetic susceptibility tunnel diode resonator (TDR) techniques. Compositional variation of the Rh/Ir ratio was used to study the coexistence and competition between the charge density wave (CDW) and supercondu… ▽ More Single crystals of Ca$_3$(Ir$_{1-x}$Rh$_x$)$_4$Sn$_{13}$ (3-4-13) were synthesized by flux growth and characterized by X-ray diffraction, EDX, magnetization, resistivity and radio frequency magnetic susceptibility tunnel diode resonator (TDR) techniques. Compositional variation of the Rh/Ir ratio was used to study the coexistence and competition between the charge density wave (CDW) and superconductivity. The superconducting transition temperature varies from approximately 7 K in pure Ir ($x=0$) to approximately 8.3 K in pure Rh ($x=1$). Temperature-dependent electrical resistivity reveals monotonic suppression of the CDW transition temperature, $T_{\text{CDW}}(x)$. The CDW starts in pure Ir, $x=0$, with $T_{\text{CDW}}\approx40$~K and extrapolates roughly linearly to zero at $x_c=0.58$ under the dome of superconductivity. Magnetization and transport measurements show a significant influence of CDW on the superconducting and normal state. Vortex pinning is substantially enhanced in the CDW region, and the resistivity is larger in this part of the phase diagram. The London penetration depth is attenuated exponentially upon cooling at low temperatures for all compositions, indicating a fully-gapped Fermi surface. We conclude that a novel $\text{Ca}_3(\text{Ir}_{1-x}\text{Rh}_x)_4\text{Sn}_{13}$ alloy with coexisting/competing CDW and superconductivity, is a good candidate to look for a composition-driven quantum critical point at ambient pressure. △ Less

Submitted 16 August, 2023; originally announced August 2023.

Journal ref: Journal of Physics: Condensed Matter 36, 385702 (2024)

Anisotropic multiband superconductivity in 2M-WS$_{2}$ probed by controlled disorder

Authors: Sunil Ghimire, Kamal R. Joshi, Marcin Konczykowski, Romain Grasset, Amlan Datta, Makariy A. Tanatar, Damien Berube, Su-Yang Xu, Yuqiang Fang, Fuqiang Huang, Peter P. Orth, Mathias S. Scheurer, Ruslan Prozorov

Abstract: The intrinsically superconducting Dirac semimetal 2M-WS$_{2}$ is a promising candidate to realize proximity-induced topological superconductivity in its protected surface states. A precise characterization of the bulk superconducting state is essential for understanding the nature of surface superconductivity in the system. Here, we perform a detailed experimental study of the temperature and nonm… ▽ More The intrinsically superconducting Dirac semimetal 2M-WS$_{2}$ is a promising candidate to realize proximity-induced topological superconductivity in its protected surface states. A precise characterization of the bulk superconducting state is essential for understanding the nature of surface superconductivity in the system. Here, we perform a detailed experimental study of the temperature and nonmagnetic disorder dependence of the London penetration depth $λ$, the upper critical field $H_{c2}$, and the superconducting transition temperature $T_c$ in 2M-WS$_{2}$. We observe a power-law dependence $λ(T) - λ(0) \propto T^{3}$ at temperatures below $0.35~T_c$, which is remarkably different from the expected exponential attenuation of a fully gapped isotropic $s$-wave superconductor. We then probe the effect of controlled nonmagnetic disorder induced by 2.5 MeV electron irradiation at various doses and find a significant $T_c$ suppression rate. Together with the observed increase of the slope $dH_{c2}/dT|_{T=T_c}$ with irradiation, our results reveal a strongly anisotropic $s^{++}$ multiband superconducting state that takes the same sign on different Fermi sheets. Our results have direct consequences for the expected proximity-induced superconductivity of the topological surface states. △ Less

Submitted 27 July, 2023; originally announced July 2023.

Journal ref: Phys. Rev. Res. 6, 013124 (2024)

Quasiparticle spectroscopy in technologically-relevant niobium using London penetration depth measurements

Authors: Sunil Ghimire, Kamal R. Joshi, Amlan Datta, Aidan Goerdt, Makariy A. Tanatar, Deborah Schlagel, Matthew J. Kramer, Jayss Marshall, Cameron J. Copas, Joshua Y. Mutus, Alexander Romanenko, Anna Grassellino, Ruslan Prozorov

Abstract: London penetration depth was measured in niobium foils, thin films, single crystals, and superconducting radio-frequency (SRF) cavity pieces cut out from different places. The low-temperature (T<Tc/3) variation, sensitive to the low-energy quasiparticles with states inside the superconducting gap, differs dramatically between different types of samples. With the help of phenomenological modeling,… ▽ More London penetration depth was measured in niobium foils, thin films, single crystals, and superconducting radio-frequency (SRF) cavity pieces cut out from different places. The low-temperature (T<Tc/3) variation, sensitive to the low-energy quasiparticles with states inside the superconducting gap, differs dramatically between different types of samples. With the help of phenomenological modeling, we correlate these different behaviors with known pair-breaking mechanisms and show that such measurements may help distinguish between different pair-breaking mechanisms, such as niobium hydrides and two-level systems (TLS). The conclusions also apply to SRF cavities when tracking the temperature-dependent quality factor and the resonant frequency. △ Less

Submitted 23 June, 2023; originally announced June 2023.

Journal ref: Materials for Quantum Technology 4, 45201 (2024)

Molecular Dynamics in Rydberg Tweezer Arrays: Spin-Phonon Entanglement and Jahn-Teller Effect

Authors: Matteo Magoni, Radhika Joshi, Igor Lesanovsky

Abstract: Atoms confined in optical tweezer arrays constitute a platform for the implementation of quantum computers and simulators. State-dependent operations are realized by exploiting electrostatic dipolar interactions that emerge, when two atoms are simultaneously excited to high-lying electronic states, so-called Rydberg states. These interactions also lead to state-dependent mechanical forces, which c… ▽ More Atoms confined in optical tweezer arrays constitute a platform for the implementation of quantum computers and simulators. State-dependent operations are realized by exploiting electrostatic dipolar interactions that emerge, when two atoms are simultaneously excited to high-lying electronic states, so-called Rydberg states. These interactions also lead to state-dependent mechanical forces, which couple the electronic dynamics of the atoms to their vibrational motion. We explore these vibronic couplings within an artificial molecular system in which Rydberg states are excited under so-called facilitation conditions. This system, which is not necessarily self-bound, undergoes a structural transition between an equilateral triangle and an equal-weighted superposition of distorted triangular states (Jahn-Teller regime) exhibiting spin-phonon entanglement on a micrometer distance. This highlights the potential of Rydberg tweezer arrays for the study of molecular phenomena at exaggerated length scales. △ Less

Submitted 25 August, 2023; v1 submitted 15 March, 2023; originally announced March 2023.

Comments: Accepted version in PRL. 7+4 pages, 4 figures

Hamiltonian Extrema of an Arbitrary Flux-Biased Josephson Circuit

Authors: A. Miano, V. R. Joshi, G. Liu, W. Dai, P. D. Parakh, L. Frunzio, M. H. Devoret

Abstract: Flux-biased loops including one or more Josephson junctions are ubiquitous elements in quantum information experiments based on superconducting circuits. These quantum circuits can be tuned to implement a variety of Hamiltonians, with applications ranging from decoherence-protected qubits to quantum limited converters and amplifiers. The extrema of the Hamiltonian of these circuits are of special… ▽ More Flux-biased loops including one or more Josephson junctions are ubiquitous elements in quantum information experiments based on superconducting circuits. These quantum circuits can be tuned to implement a variety of Hamiltonians, with applications ranging from decoherence-protected qubits to quantum limited converters and amplifiers. The extrema of the Hamiltonian of these circuits are of special interest because they govern their low-energy dynamics. However, the theory of superconducting quantum circuits so far lacks a systematic method to find these extrema and compute the series expansion of the Hamiltonian in their vicinity for an arbitrary nonlinear superconducting circuit. We present such a method, which can aid the synthesis of new functionalities in quantum devices. △ Less

Submitted 27 September, 2023; v1 submitted 6 February, 2023; originally announced February 2023.

Journal ref: PRX Quantum 4, 030324 (2023)

Magnetism and $T-x$ phase diagrams of Na and Ag substituted EuCd$_2$As$_2$

Authors: Brinda Kuthanazhi, Kamal R. Joshi, Sunil Ghimire, Erik Timmons, Lin-Lin Wang, Elena Gati, Li Xiang, Ruslan Prozorov, Sergey L. Bud'ko, Paul C. Canfield

Abstract: EuCd$_2$As$_2$ is an antiferromagnetic semimetal, that can host non-trivial topological properties, depending upon its magnetic state and excitations. Here, we report the synthesis and characterization of Eu(Cd$_{1-x}$Ag$_x$)$_2$As$_2$ and Eu$_{1-y}$Na$_y$Cd$_2$As$_2$, and study the evolution and nature of magnetic order with doping. Temperature-substitution phase diagrams are constructed from the… ▽ More EuCd$_2$As$_2$ is an antiferromagnetic semimetal, that can host non-trivial topological properties, depending upon its magnetic state and excitations. Here, we report the synthesis and characterization of Eu(Cd$_{1-x}$Ag$_x$)$_2$As$_2$ and Eu$_{1-y}$Na$_y$Cd$_2$As$_2$, and study the evolution and nature of magnetic order with doping. Temperature-substitution phase diagrams are constructed from the electrical resistance and magnetic susceptibility data. We observe a splitting of the magnetic transition into two different transitions, and the gradual increase in one of the transition temperatures with Ag- and Na-substitution. The other transition remains more or less independent of doping. We further show that a magnetic state with a net ferromagnetic moment is stabilized by both Ag and Na doping and this can be explained by considering the changes in band filling due to substitution as suggested by density functional theory (DFT) calculations. We thus show that chemical substitution and the subsequent changes in band filling could be a pathway to tune the magnetic ground state and to stabilize a ferromagnetic phase in EuCd$_2$As$_2$. △ Less

Submitted 16 November, 2022; originally announced November 2022.

Triclinic BiFeO3: A room-temperature multiferroic phase with enhanced magnetism and resistivity

Authors: Md Sariful Sheikh, Tushar Kanti Bhowmik, Alo Dutta, Sujoy Saha, Chhatra R. Joshi, T. P. Sinha

Abstract: The magnetic and transport properties of BiFeO3/La2NiMnO6 (BFO/LNMO) composite have been investigated both experimentally and theoretically. Unlike the normal rhombohedral (R3c) phase, BFO in the composites is crystallized in the triclinic phase (P1). Interestingly, the composites demonstrate a significant enhancement in the magnetization, magnetoelectric coupling and show higher resistivity than… ▽ More The magnetic and transport properties of BiFeO3/La2NiMnO6 (BFO/LNMO) composite have been investigated both experimentally and theoretically. Unlike the normal rhombohedral (R3c) phase, BFO in the composites is crystallized in the triclinic phase (P1). Interestingly, the composites demonstrate a significant enhancement in the magnetization, magnetoelectric coupling and show higher resistivity than that of the regular BFO (R3c). As LNMO has its Curie temperature at 280 K, the room temperature and above room temperature magnetic contribution in the composites is expected to be from the triclinic BFO phase. Experimentally observed enhancement in magnetization is validated using classical Monte Carlo simulation and density functional theory (DFT) calculations. The calculations reveal higher magnetic moments in triclinic BFO as compared to the rhombohedral BFO. Overall, this study reveals triclinic BFO as the promising room temperature multiferroic phase which is helpful to optimize the multiferroicity of BFO and achieve wider applications in future. △ Less

Submitted 9 December, 2023; v1 submitted 6 November, 2022; originally announced November 2022.

Anisotropic superconductivity of niobium based on its response to non-magnetic disorder

Authors: Makariy A. Tanatar, Daniele Torsello, Kamal R. Joshi, Sunil Ghimire, Cameron J. Kopas, Jayss Marshall, Josh Y. Mutus, Gianluca Ghigo, Mehdi Zarea, James A. Sauls, Ruslan Prozorov

Abstract: Niobium is one of the most studied superconductors, both theoretically and experimentally. It is tremendously important for applications, and it has the highest superconducting transition temperature, $T_{c}=9.33$ K, of all pure metals. In addition to power applications in alloys, pure niobium is used for sensitive magneto-sensing, radio-frequency cavities, and, more recently, as circuit metalliza… ▽ More Niobium is one of the most studied superconductors, both theoretically and experimentally. It is tremendously important for applications, and it has the highest superconducting transition temperature, $T_{c}=9.33$ K, of all pure metals. In addition to power applications in alloys, pure niobium is used for sensitive magneto-sensing, radio-frequency cavities, and, more recently, as circuit metallization layers in superconducting qubits. A detailed understanding of its electronic and superconducting structure, especially its normal and superconducting state anisotropies, is crucial for mitigating the loss of quantum coherence in such devices. Recently, a microscopic theory of the anisotropic properties of niobium with the disorder was put forward. To verify theoretical predictions, we studied the effect of disorder produced by 3.5 MeV proton irradiation of thin Nb films grown by the same team and using the same protocols as those used in transmon qubits. By measuring the superconducting transition temperature and upper critical fields, we show a clear suppression of $T_{c}$ by potential (non-magnetic) scattering, which is directly related to the anisotropic order parameter. We obtain a very close quantitative agreement between the theory and the experiment. △ Less

Submitted 9 October, 2022; v1 submitted 28 July, 2022; originally announced July 2022.

Journal ref: Phys. Rev. B 106, 224511 (2022)

Quasiparticle spectroscopy, transport, and magnetic properties of Nb films used in superconducting transmon qubits

Authors: Kamal R. Joshi, Sunil Ghimire, Makariy A. Tanatar, Amlan Datta, Jin-Su Oh, Lin Zhou, Cameron J. Kopas, Jayss Marshall, Josh Y. Mutus, Julie Slaughter, Matthew J. Kramer, James A. Sauls, Ruslan Prozorov

Abstract: Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. We study niobium films to provide an example of a comprehensive analytical set that may benefit superconducting circui… ▽ More Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. We study niobium films to provide an example of a comprehensive analytical set that may benefit superconducting circuits such as those used in quantum computers. The films show outstanding superconducting transition temperature of $T_{c}=9.35$ K and a fairly clean superconducting gap, along with superfluid density enhanced at intermediate temperatures. These observations are consistent with the recent theory of anisotropic strong-coupling superconductivity in Nb. However, the response to the magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat conductance leading to thermo-magnetic instabilities. These may present an issue for further improvement of transmon quantum coherence. Possible mitigation strategies are discussed. △ Less

Submitted 23 July, 2022; originally announced July 2022.

Journal ref: Phys. Rev. Applied 20, 024031 (2023)

Distinguishing parity-switching mechanisms in a superconducting qubit

Authors: Spencer Diamond, Valla Fatemi, Max Hays, Heekun Nho, Pavel D. Kurilovich, Thomas Connolly, Vidul R. Joshi, Kyle Serniak, Luigi Frunzio, Leonid I. Glazman, Michel H. Devoret

Abstract: Single-charge tunneling is a decoherence mechanism affecting superconducting qubits, yet the origin of excess quasiparticle excitations (QPs) responsible for this tunneling in superconducting devices is not fully understood. We measure the flux dependence of charge-parity (or simply, ``parity'') switching in an offset-charge-sensitive transmon qubit to identify the contributions of photon-assisted… ▽ More Single-charge tunneling is a decoherence mechanism affecting superconducting qubits, yet the origin of excess quasiparticle excitations (QPs) responsible for this tunneling in superconducting devices is not fully understood. We measure the flux dependence of charge-parity (or simply, ``parity'') switching in an offset-charge-sensitive transmon qubit to identify the contributions of photon-assisted parity switching and QP generation to the overall parity-switching rate. The parity-switching rate exhibits a qubit-state-dependent peak in the flux dependence, indicating a cold distribution of excess QPs which are predominantly trapped in the low-gap film of the device. Moreover, we find that the photon-assisted process contributes significantly to both parity switching and the generation of excess QPs by fitting to a model that self-consistently incorporates photon-assisted parity switching as well as inter-film QP dynamics. △ Less

Submitted 15 April, 2022; originally announced April 2022.

Mass transport via in-plane nanopores in graphene oxide membranes

Authors: Tobias Foller, Lukas Madauss, Dali Ji, Xiaojun Ren, K. Kanishka H. De Silva, Tiziana Musso Masamichi Yoshimura, Henning Lebius, Abdenacer Benyagoub, Priyank Kumar, Marika Schleberger, Rakesh Joshi

Abstract: Angstrom confined solvents in two-dimensional laminates travel through interlayer spacings, gaps between adjacent sheets, and via in plane pores. Among these, experimental access to investigate the mass transport through in plane pores is lacking. Here, we create these nanopores in graphene oxide membranes via ion irradiation with precise control over functional groups, pore size and pore density.… ▽ More Angstrom confined solvents in two-dimensional laminates travel through interlayer spacings, gaps between adjacent sheets, and via in plane pores. Among these, experimental access to investigate the mass transport through in plane pores is lacking. Here, we create these nanopores in graphene oxide membranes via ion irradiation with precise control over functional groups, pore size and pore density. Low ion induced pore densities result in mild reduction and increased water permeation for the membranes. Higher pore densities lead to pronounced reduction and complete blockage of pure water however allows permeation of ethanol water mixture due to weakening of hydrogen network. We confirm with simulations, that the attraction of the solvents towards the pores with functional groups and disruption of the angstrom confined hydrogen network is crucial to allow in plane pore transport. △ Less

Submitted 27 January, 2022; originally announced January 2022.

Frequency-tunable Kerr-free three-wave mixing with a gradiometric SNAIL

Authors: A. Miano, G. Liu, V. V. Sivak, N. E. Frattini, V. R. Joshi, W. Dai, L. Frunzio, M. H. Devoret

Abstract: Three-wave mixing is a key process in superconducting quantum information processing, being involved in quantum-limited amplification and parametric coupling between superconducting cavities. These operations can be implemented by SNAIL-based devices that present a Kerr-free flux-bias point where unwanted parasitic effects such as Stark shift are suppressed. However, with a single flux-bias parame… ▽ More Three-wave mixing is a key process in superconducting quantum information processing, being involved in quantum-limited amplification and parametric coupling between superconducting cavities. These operations can be implemented by SNAIL-based devices that present a Kerr-free flux-bias point where unwanted parasitic effects such as Stark shift are suppressed. However, with a single flux-bias parameter, these circuits can only host one Kerr-free point, limiting the range of their applications. In this Letter, we demonstrate how to overcome this constraint with a gradiometric SNAIL, a doubly-flux biased superconducting circuit for which both effective inductance and Kerr coefficient can be independently tuned. Experimental data show the capability of the gradiometric SNAIL to suppress Kerr effect in a three-wave mixing parametric amplifier over a continuum of flux bias points corresponding to a 1.7 GHz range of operating frequencies. △ Less

Submitted 12 May, 2022; v1 submitted 17 December, 2021; originally announced December 2021.

Journal ref: Appl. Phys. Lett. 120, 184002 (2022)

Effect of Topological Non-hexagonal Rings and Stone Wale Defects on the Vibrational Response of Single and Multi-Layer Ion Irradiated Graphene

Authors: Ashis K. Manna, Simeon J. Gilbert, Shalik R. Joshi, Takashi Komesu, Shikha Varma

Abstract: Present study explores the observation of topological non-hexagonal rings (NHR) and Stone Wale (SW) defects by Raman experiments in both single (SLG) and multi-layer graphene (MLG) after they are irradiated with 100- 300 eV Ar ions. Although predicted by theoretical studies, here it is experimentally shown for the first time that graphene SW/NHR defects have a signature in Raman. Broad bandwidth o… ▽ More Present study explores the observation of topological non-hexagonal rings (NHR) and Stone Wale (SW) defects by Raman experiments in both single (SLG) and multi-layer graphene (MLG) after they are irradiated with 100- 300 eV Ar ions. Although predicted by theoretical studies, here it is experimentally shown for the first time that graphene SW/NHR defects have a signature in Raman. Broad bandwidth of the pertinent Raman features suggests the presence of more than one SW/NHR defect mode, in agreement with the DFT studies. Variations in the SW/NHR related Raman mode intensities demonstrate the annihilation of these topological defects at higher energies. Behavior of Raman allowed G and 2D excitations, as well as the disorder-activated D, D' and G* lines, has also been investigated in SLG and MLG. These indicate an evolution of defects in graphene with ion irradiation, as well as presence of a transition state beyond which the Raman modes are dominated by a rise in sp3 content. Correlation of these aspects with the SW/NHR Raman provide significant insight into ion induced evolution of graphene. The direct observation of SW/NHR defects by Raman spectroscopy could be important in promoting exploration of rich topological aspects of Graphene in various fields. △ Less

Submitted 12 December, 2021; originally announced December 2021.

Comments: 15 pages, 7 figures

Possible unconventional pairing in $(\text{Ca,Sr})_{3}(\text{Ir,Rh})_{4}\text{Sn}_{13}$ superconductors revealed by controlling disorder

Authors: E. H. Krenkel, M. A. Tanatar, M. Konczykowski, R. Grasset, E. I. Timmons, S. Ghimire, K. R. Joshi, Y. Lee, Liqin Ke, S. Chen, C. Petrovic, P. P. Orth, M. S. Scheurer, R. Prozorov

Abstract: We study the evolution of temperature-dependent resistivity with controlled point-like disorder induced by 2.5 MeV electron irradiation in stoichiometric compositions of the "3-4-13" stannides, $(\text{Ca,Sr})_{3}(\text{Ir,Rh})_{4}\text{Sn}_{13}$.Three of these cubic compounds exhibit a microscopic coexistence of charge-density wave (CDW) order and superconductivity (SC), while… ▽ More We study the evolution of temperature-dependent resistivity with controlled point-like disorder induced by 2.5 MeV electron irradiation in stoichiometric compositions of the "3-4-13" stannides, $(\text{Ca,Sr})_{3}(\text{Ir,Rh})_{4}\text{Sn}_{13}$.Three of these cubic compounds exhibit a microscopic coexistence of charge-density wave (CDW) order and superconductivity (SC), while $\text{Ca}_{3}\text{Rh}_{4}\text{Sn}_{13}$ does not develop CDW order. As expected, the CDW transition temperature, $T_{\text{CDW}}$, is universally suppressed by irradiation in all three compositions. The superconducting transition temperature, $T_{c}$, behaves in a more complex manner. In $\text{Sr}_{3}\text{Rh}_{4}\text{Sn}_{13}$, it increases initially in a way consistent with a direct competition of CDW and SC, but quickly saturates at higher irradiation doses. In the other three compounds, $T_{c}$ is monotonically suppressed by irradiation. The strongest suppression is found in $\text{Ca}_{3}\text{Rh}_{4}\text{Sn}_{13}$, which does not have CDW order. We further examine this composition by measuring the London penetration depth, $λ(T)$, from which we derive the superfluid density. The result unambiguously points to a weak-coupling, full single gap, isotropic superconducting state. Therefore, we must explain two seemingly incompatible experimental observations: a single isotropic superconducting gap and a significant suppression of $T_{c}$ by non-magnetic disorder. We conduct a quantitative theoretical analysis based on a generalized Anderson theorem which points to an unconventional multiband $s^{+-}$-pairing state where the sign of the order parameter is different on one (or a small subset) of the smaller Fermi surface sheets, but remains overall fully-gapped. △ Less

Submitted 26 February, 2022; v1 submitted 5 October, 2021; originally announced October 2021.

Journal ref: Phys. Rev. B 105, 094521 (2022)

Enhanced graphitic domains of unreduced graphene oxide and the interplay of hydration behaviour and catalytic activity

Authors: Tobias Foller, Rahman Daiyan, Xiaoheng Jin, Joshua Leverett, Hangyel Kim, Richard Webster, Jeaniffer E. Yap, Xinyue Wen, Aditya Rawal, K. Kanishka H. DeSilva, Masamichi Yoshimura, Heriberto Bustamante, Shery L. Y. Chang, Priyank Kumar, Yi You, Gwan Hyoung Lee, Rose Amal, Rakesh Joshi

Abstract: Previous studies indicate that the properties of graphene oxide (GO) can be significantly improved by enhancing its graphitic domain size through thermal diffusion and clustering of functional groups. Remarkably, this transition takes place below the decomposition temperature of the functional groups and thus allows fine-tuning of graphitic domains without compromising with the functionality of GO… ▽ More Previous studies indicate that the properties of graphene oxide (GO) can be significantly improved by enhancing its graphitic domain size through thermal diffusion and clustering of functional groups. Remarkably, this transition takes place below the decomposition temperature of the functional groups and thus allows fine-tuning of graphitic domains without compromising with the functionality of GO. By studying the transformation of GO under mild thermal treatment, we directly observe this size enhancement of graphitic domains from originally 40 nm2 to 200 nm2 through an extensive transmission electron microscopy (TEM) study. Additionally, we confirm the integrity of the functional groups during this process by comprehensive chemical analysis. A closer look into the process confirms the theoretically predicted relevance for the room temperature stability of GO. We further investigate the influence of enlarged graphitic domains on the hydration behaviour of GO and catalytic performance of single-atom catalysts supported by GO. △ Less

Submitted 21 May, 2021; v1 submitted 1 July, 2020; originally announced July 2020.

Analysis of the London penetration depth in Ni-doped CaKFe4As4

Authors: D. Torsello, K. Cho, K. R. Joshi, S. Ghimire, G. A. Ummarino, N. M. Nusran, M. A. Tanatar, W. R. Meier, M. Xu, S. L. Budko, P. C. Canfield, G. Ghigo, R. Prozorov

Abstract: We report combined experimental and theoretical analysis of superconductivity in CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ (CaK1144) for $x=$0, 0.017 and 0.034. To obtain the superfluid density, $ρ=\left(1+Δλ_L(T)/λ_L(0) \right)^{-2}$, the temperature dependence of the London penetration depth, $Δλ_L (T)$, was measured by using tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar… ▽ More We report combined experimental and theoretical analysis of superconductivity in CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ (CaK1144) for $x=$0, 0.017 and 0.034. To obtain the superfluid density, $ρ=\left(1+Δλ_L(T)/λ_L(0) \right)^{-2}$, the temperature dependence of the London penetration depth, $Δλ_L (T)$, was measured by using tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar resonator (MWR) with the small differences accounted for by considering a three orders of magnitude higher frequency of MWR. The absolute value of $λ_L (T \ll T_c) \approx λ_L(0)$ was measured by using MWR, $λ_L (\mathrm{5~K}) \approx 170 \pm 20$ nm, which agreed well with the NV-centers in diamond optical magnetometry that gave $λ_L (\mathrm{5~K}) \approx 196 \pm 12$ nm. The experimental results are analyzed within the Eliashberg theory, showing that the superconductivity of CaK1144 is well described by the nodeless s$_{\pm}$ order parameter and that upon Ni doping the interband interaction increases. △ Less

Submitted 10 September, 2019; originally announced September 2019.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. B 100, 094513 (2019)

Machine Learning the Voltage of Electrode Materials in Metal-ion Batteries

Authors: Rajendra P. Joshi, Jesse Eickholt, Liling Li, Marco Fornari, Veronica Barone, Juan E. Peralta

Abstract: Machine learning (ML) techniques have rapidly found applications in many domains of materials chemistry and physics where large data sets are available. Aiming to accelerate the discovery of materials for battery applications, in this work, we develop a tool (http://se.cmich.edu/batteries) based on ML models to predict voltages of electrode materials for metal-ion batteries. To this end, we use de… ▽ More Machine learning (ML) techniques have rapidly found applications in many domains of materials chemistry and physics where large data sets are available. Aiming to accelerate the discovery of materials for battery applications, in this work, we develop a tool (http://se.cmich.edu/batteries) based on ML models to predict voltages of electrode materials for metal-ion batteries. To this end, we use deep neural network, support vector machine, and kernel ridge regression as ML algorithms in combination with data taken from the Materials Project Database, as well as feature vectors from properties of chemical compounds and elemental properties of their constituents. We show that our ML models have predictive capabilities for different reference test sets and, as an example, we utilize them to generate a voltage profile diagram and compare it to density functional theory calculations. In addition, using our models, we propose nearly 5,000 candidate electrode materials for Na- and K-ion batteries. We also make available a web-accessible tool that, within a minute, can be used to estimate the voltage of any bulk electrode material for a number of metal-ions. These results show that ML is a promising alternative for computationally demanding calculations as a first screening tool of novel materials for battery applications. △ Less

Submitted 8 May, 2019; v1 submitted 15 March, 2019; originally announced March 2019.

Comments: 28 pages

Journal ref: ACS Appl. Mater. Interfaces, 11, 20, 18494-18503, 2019

Quantum phase transition inside the superconducting dome of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ probed by optical magneto-sensing using NV-centers in diamond

Authors: K. R. Joshi, N. M. Nusran, M. A. Tanatar, K. Cho, S. L. Bud'ko, P. C. Canfield, R. M. Fernandes, A. Levchenko, R. Prozorov

Abstract: While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration depth in th… ▽ More While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration depth in the electron-doped iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic-magnetic transition above, and even somewhat below, $T_c$, the former is a significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transition above $T_c$. Thus our observation that such distinct systems display remarkably similar penetration depth peaks, combined with the theoretical result that a QPT alone does not ensure the existence of a peak, unveils a puzzling and seemingly universal manifestation of quantum fluctuations in the iron pnictides. △ Less

Submitted 28 August, 2019; v1 submitted 28 February, 2019; originally announced March 2019.

Journal ref: New Journal of Physics, (2020)

Kerr-free three-wave mixing in superconducting quantum circuits

Authors: V. V. Sivak, N. E. Frattini, V. R. Joshi, A. Lingenfelter, S. Shankar, M. H. Devoret

Abstract: Quantum-limited Josephson parametric amplifiers are crucial components in circuit QED readout chains. The dynamic range of state-of-the-art parametric amplifiers is limited by signal-induced Stark shifts that detune the amplifier from its operating point. Using a Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL) as an active component, we show the ability to in situ tune the device fl… ▽ More Quantum-limited Josephson parametric amplifiers are crucial components in circuit QED readout chains. The dynamic range of state-of-the-art parametric amplifiers is limited by signal-induced Stark shifts that detune the amplifier from its operating point. Using a Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL) as an active component, we show the ability to in situ tune the device flux and pump to a dressed Kerr-free operating point, which provides a 10-fold increase in the number of photons that can be processed by our amplifier, compared to the nominal working point. Our proposed and experimentally verified methodology of Kerr-free three-wave mixing can be extended to improve the dynamic range of other pumped operations in quantum superconducting circuits. △ Less

Submitted 27 May, 2019; v1 submitted 27 February, 2019; originally announced February 2019.

Comments: 16 pages, 8 figures

Journal ref: Phys. Rev. Applied 11, 054060 (2019)

Measurements of the lower critical field of superconductors using NV centers in diamond optical magnetometry

Authors: K. R. Joshi, N. M. Nusran, K. Cho, M. A. Tanatar, W. R. Meier, S. L. Bud'ko, P. C. Canfield, R. Prozorov

Abstract: The lower critical magnetic field, $H_{c1}$, of superconductors is measured by using ensembles of NV-centers-in-diamond optical magnetometry. The technique is minimally invasive, and has sub-gauss field sensitivity and sub-$μ$m spatial resolution, which allow for accurate detection of the vector field at which the vortices start penetrating the sample from the corners. Aided by the revised calcula… ▽ More The lower critical magnetic field, $H_{c1}$, of superconductors is measured by using ensembles of NV-centers-in-diamond optical magnetometry. The technique is minimally invasive, and has sub-gauss field sensitivity and sub-$μ$m spatial resolution, which allow for accurate detection of the vector field at which the vortices start penetrating the sample from the corners. Aided by the revised calculations of the effective demagnetization factors of actual cuboid - shaped samples, $H_{c1}$ and the London penetration depth, $λ$, derived from $H_{c1}$ can be obtained. We apply this method to three well-studied superconductors: optimally doped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$, stoichiometric CaKFe$_4$As$_4$, and high-$T_c$ cuprate YBa$_2$Cu$_3$O$_{7-δ}$. Our results are well compared with the values of $λ$ obtained using other techniques, thus adding another non-destructive and sensitive method to measure these important parameters of superconductors. △ Less

Submitted 27 June, 2018; originally announced June 2018.

Journal ref: Phys. Rev. Applied 11, 014035 (2019)

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

Authors: Padmanabhan Balasubramanian, Shalik Ram Joshi, Ruchika Yadav, Frank M. F. de Groot, Amit Kumar Singh, Avijeet Ray, Mukul Gupta, Ankita Singh, Suja Elizabeth, Shikha Varma, Tulika Maitra, Vivek Malik

Abstract: The electronic structure of double perovskite Pr2MnNiO6 is studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 2+ and 4+ states respectively. Using charge transfer multiplet analysis of Ni and Mn 2p XPS spectra, we find charge transfer energies Δ of 3.5 and 2.5 eV for Ni and Mn respectively. The ground st… ▽ More The electronic structure of double perovskite Pr2MnNiO6 is studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 2+ and 4+ states respectively. Using charge transfer multiplet analysis of Ni and Mn 2p XPS spectra, we find charge transfer energies Δ of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the atomic values of 8.00 and 3.00 respectively thereby indicating the covalent nature of the system. The O 1s edge absorption spectra reveal a band gap of 0.9 eV which is comparable to the value obtained from first principle calculations for U-J >= 2 eV. The density of states clearly reveal a strong p-d type charge transfer character of the system, with band gap proportional to average charge transfer energy of Ni2+ and Mn4+ ions. △ Less

Submitted 24 October, 2017; originally announced October 2017.

Comments: 18 pages, 9 figures

Spatially - resolved study of the Meissner effect in superconductors using NV-centers-in-diamond optical magnetometry

Authors: N. M. Nusran, K. R. Joshi, K. Cho, M. A. Tanatar, W. R. Meier, S. L. Bud'ko, P. C. Canfield, Y. Liu, T. A. Lograsso, R. Prozorov

Abstract: Non-invasive magnetic field sensing using optically - detected magnetic resonance of nitrogen-vacancy (NV) centers in diamond was used to study spatial distribution of the magnetic induction upon penetration and expulsion of weak magnetic fields in several representative superconductors. Vector magnetic fields were measured on the surface of conventional, Pb and Nb, and unconventional, LuNi$_2$B… ▽ More Non-invasive magnetic field sensing using optically - detected magnetic resonance of nitrogen-vacancy (NV) centers in diamond was used to study spatial distribution of the magnetic induction upon penetration and expulsion of weak magnetic fields in several representative superconductors. Vector magnetic fields were measured on the surface of conventional, Pb and Nb, and unconventional, LuNi$_2$B$_2$C, Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$, and CaKFe$_4$As$_4$, superconductors, with diffraction - limited spatial resolution using variable - temperature confocal system. Magnetic induction profiles across the crystal edges were measured in zero-field-cooled (ZFC) and field-cooled (FC) conditions. While all superconductors show nearly perfect screening of magnetic fields applied after cooling to temperatures well below the superconducting transition, $T_c$, a range of very different behaviors was observed for Meissner expulsion upon cooling in static magnetic field from above $T_c$. Substantial conventional Meissner expulsion is found in LuNi$_2$B$_2$C, paramagnetic Meissner effect (PME) is found in Nb, and virtually no expulsion is observed in iron-based superconductors. In all cases, good correlation with macroscopic measurements of total magnetic moment is found. Our measurements of the spatial distribution of magnetic induction provide insight into microscopic physics of the Meissner effect. △ Less

Submitted 8 September, 2017; originally announced September 2017.

Extraordinary water adsorption characteristics of graphene oxide

Authors: B. Lian, S. De Luca, Y. You, S. Alwarappan, M. Yoshimura, V. Sahajwalla, S. C. Smith, G. Leslie, R. K. Joshi

Abstract: The laminated structure of graphene oxide (GO) confers unique interactions with water molecules which may be utilised in a range of applications that require materials with tuneable hygroscopic properties. Precise roles of the expandable interlayer spacing and functional groups in GO laminates are not fully understood till date. Herein, we report experimental and theoretical study on the adsorptio… ▽ More The laminated structure of graphene oxide (GO) confers unique interactions with water molecules which may be utilised in a range of applications that require materials with tuneable hygroscopic properties. Precise roles of the expandable interlayer spacing and functional groups in GO laminates are not fully understood till date. Herein, we report experimental and theoretical study on the adsorption and desorption behaviour of water in GO laminates as a function of relative pressure. We have observed that GO imparts excellent water uptake capacity of up to 0.58 gram of water per gram of GO (g g-1), which is much higher than silica gel a conventional desiccant material. More interestingly, the adsorption and desorption kinetics of GO is one order of magnitude higher than silica gel. The observed extraordinary adsorption/desorption rate can be attributed to the high capillary pressure in GO laminates as well as micro meter sized tunnel like wrinkles located at the surface. △ Less

Submitted 29 July, 2017; originally announced July 2017.

Anisotropic super-paramagnetism in cobalt implanted rutile-TiO2 single crystals

Authors: Shalik Ram Joshi, B. Padmanabhan, Anupama Chanda, N. Shukla, Vivek Malik, D. Kanjilal, Shikha Varma

Abstract: We study the magnetic properties of single crystals of rutile TiO2 implanted with cobalt for various fluences. The temperature variation of zero field cooled(ZFC) and field cooled (FC) magnetization shows a much higher blocking temperature (TB) along [1-10]. Similarly the scaling of magnetization isotherms above TB is seen only when the field is parallel to [1-10] direction. With field along this… ▽ More We study the magnetic properties of single crystals of rutile TiO2 implanted with cobalt for various fluences. The temperature variation of zero field cooled(ZFC) and field cooled (FC) magnetization shows a much higher blocking temperature (TB) along [1-10]. Similarly the scaling of magnetization isotherms above TB is seen only when the field is parallel to [1-10] direction. With field along this direction, the magnetization shows near saturation at a much smaller field compared to that of[001] direction. The Co nanoclusters possess an "easy" and "hard axis" of magnetization coupled by the magneto crystalline anisotropy of secondary phases of cobalt with TiO2. In addition, at T=2 K we observe a crossover in the magnetization vs field isotherms between the two field directions in the samples which has been attributed to the anisotropic paramagnetism arising from cobalt present in 2+ ionic state with S = 3/2. △ Less

Submitted 15 March, 2017; originally announced March 2017.

Comments: 20 Pages, 7 Figures