Om Prakash

@vttresearch.com

Research Scientist, Quantum computing hardware, VTT Technical Research Centre of Finland Ltd
VTT Technical Research Centre of Finland Ltd

Om Prakash


                     Download Compact PDF  
https://researchid.co/omultphy

RESEARCH INTERESTS

Superconducting materials and devices, condensed matter physics and low temperature physics
14

Scopus Publications

356

Scholar Citations

7

Scholar h-index

6

Scholar i10-index

Scopus Publications

  • Native-oxide-passivated trilayer junctions for superconducting qubits
    Pankaj Sethi, Om Prakash, Jukka-Pekka Kaikkonen, Mikael Kervinen, Elsa T. Mannila, Mário Ribeiro, Debopam Datta, Christopher W. Förbom, Jorden Senior, Renan P. Loreto, Joel Hätinen, Klaara Viisanen, Jukka I. Väyrynen, Alberto Ronzani, Antti Kemppinen, Visa Vesterinen, Mika Prunnila, Joonas Govenius
    Physical Review Applied, 2025
  • Efficient electronic cooling by niobium-based superconducting tunnel junctions
    J. Hätinen, A. Ronzani, R.P. Loreto, E. Mykkänen, A. Kemppinen, K. Viisanen, T. Rantanen, J. Geisor, J.S. Lehtinen, M. Ribeiro, J-P. Kaikkonen, O. Prakash, V. Vesterinen, C. Förbom, E.T. Mannila, M. Kervinen, J. Govenius, M. Prunnila
    Physical Review Applied, 2024
    Replacing the bulky cryoliquid-based cooling stages of cryoenabled instruments by chip-scale refrigeration is envisioned to disruptively reduce the system size similar to microprocessors did for computers. Electronic refrigerators based on superconducting tunnel junctions have been anticipated to provide a solution, but reaching the necessary above the 1-K operation temperature range has remained a goal out of reach for several decades. We show efficient electronic refrigeration by $\mathrm{Al}$-${\mathrm{Al}\mathrm{O}}_{x}$-$\mathrm{Nb}$ superconducting tunnel junctions starting from bath temperatures above 2 K. The junctions can deliver electronic cooling power up to approximately $\mathrm{mW}/{\mathrm{mm}}^{2}$, which enables us to demonstrate tunnel-current-driven electron temperature reduction from 2.4 K to below 1.6 K (34% relative cooling) against the phonon bath. Our work shows that the key material of integrated superconducting circuits---niobium---enables powerful cryogenic refrigerator technology. This result is a prerequisite for practical cryogenic chip-scale refrigerators and, at the same time, it introduces a new electrothermal tool for quantum heat-transport experiments.
  • Broken time-reversal symmetry in the cubic skutterudite-like superconductor Y3 Ru4 Ge13
    A. Kataria, J. A. T. Verezhak, O. Prakash, R. K. Kushwaha, A. Thamizhavel, S. Ramakrishnan, M. S. Scheurer, A. D. Hillier, R. P. Singh
    Physical Review B, 2023
    The microscopic properties of the superconducting cubic skutterudite-like material ${\mathrm{Y}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Ge}}_{13}$ are investigated using muon spin relaxation and rotation ($\ensuremath{\mu}\mathrm{SR}$) measurements. Zero-field $\ensuremath{\mu}\mathrm{SR}$ measurements reveal the presence of a spontaneous internal field with a magnitude of $\ensuremath{\simeq}0.18$ mT below the superconducting transition temperature, indicating broken time-reversal symmetry in the ground state. In line with previous experiments, transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements are consistent with a fully developed superconductivity gap in ${\mathrm{Y}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Ge}}_{13}$. Our observations point towards the relevance of electronic correlations beyond electron-phonon coupling as the superconducting origin and indicate that spin-orbit coupling is likely not the key driving force behind the spontaneous breaking of time-reversal symmetry in this system.
  • Fragile electronic superconductivity in a Bi single crystal
    Anil Kumar, Om Prakash, Rajendra Loke, Arindam Pramanik, Rajdeep Sensarma, Sitaram Ramakrishnan, Biplab Bag, Arumugam Thamizhavel, Srinivasan Ramakrishnan
    Physical Review B, 2023
    It was presumed that semimetallic bismuth (Bi) would not show superconductivity (SC) even at ultralow temperatures $(<10 \mathrm{mK})$ due to its very low carrier density $(\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$. Recently, we have established bulk superconductivity in an ultrapure (99.9999%) Bi single crystal at ${T}_{c}=0.53 \mathrm{mK}$ with an extrapolated upper critical field ${H}_{c}(0)=5.2\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{T}$ measured along the [0001] (trigonal) crystallographic direction [O. Prakash et al., Science 355, 52 (2017)]. At very low concentrations of the charge carriers, we are dealing with fragile Cooper pairs with an estimated large coherence length ${\ensuremath{\xi}}_{\text{GL}}(0)\ensuremath{\approx}96\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{m}$. We also stated that one needs to go beyond the conventional electron-phonon coupling (BCS-like) mechanism to understand the SC state in Bi. Bi is a compensated semimetal with electrons and holes as charge carriers. In order to find the charge carriers responsible for the SC, we report the temperature dependence of the anisotropic critical field along the $H\ensuremath{\parallel}[01\overline{1}0]$ (bisectrix) crystallographic direction and compare it with the earlier data from measurements along the $H\ensuremath{\parallel}[0001]$ (trigonal) axis. Our theoretical analysis of the critical field anisotropy suggests that the light electrons in the three pockets of the Bi Fermi surface are responsible for the SC, which indicates that Bi is an extremely weak type-II (close to type-I) superconductor. Finally, we present a brief review of the current theories proposed to explain the SC in Bi.
  • Time-reversal symmetry breaking in the superconducting low carrier density quasiskutterudite Lu3 Os4 Ge13
    A. Kataria, J. A. T. Verezhak, O. Prakash, R. K. Kushwaha, A. Thamizhavel, S. Ramakrishnan, M. S. Scheurer, A. D. Hillier, R. P. Singh
    Physical Review B, 2023
    The complex structure of the Remeika phases, the intriguing quantum states they display, and their low carrier concentrations are strong motivations to study the nature of their superconducting phases. In this Letter, the microscopic properties of the superconducting phase of single-crystalline ${\mathrm{Lu}}_{3}{\mathrm{Os}}_{4}{\mathrm{Ge}}_{13}$ are investigated by muon-spin relaxation and rotation ($\ensuremath{\mu}\mathrm{SR}$) measurements. The zero-field $\ensuremath{\mu}\mathrm{SR}$ data reveal the presence of spontaneous static or quasistatic magnetic fields in the superconducting state, breaking time-reversal symmetry; the associated internal magnetic-field scale is found to be exceptionally large ($\ensuremath{\simeq}$ 0.11 mT). Furthermore, transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements in the vortex state of ${\mathrm{Lu}}_{3}{\mathrm{Os}}_{4}{\mathrm{Ge}}_{13}$ imply a complex gap function with significantly different strengths on different parts of the Fermi surface. Although our measurements do not completely determine the order parameter, they strongly indicate that electron-electron interactions are essential to stabilizing pairing in the system, thus, demonstrating its unconventional nature.
  • Nodeless superconductivity and the peak effect in the quasiskutterudites Lu3Os4Ge13 and Y3Ru4Ge13
    Z. F. Weng, M. Smidman, G. M. Pang, O. Prakash, Y. Chen, Y. J. Zhang, S. Ramakrishnan, H. Q. Yuan
    Physical Review B, 2017
    We report an investigation of the superconducting states of $\\mathrm{Lu}_3\\mathrm{Os}_4\\mathrm{Ge}_{13}$ and $\\mathrm{Y}_3\\mathrm{Ru}_4\\mathrm{Ge}_{13}$ single crystals by measurements of the electrical resistivity, ac susceptibility and London penetration depth. The analysis of the penetration depth and the derived superfluid density indicates the presence of nodeless superconductivity and suggest that there are multiple superconducting gaps in both materials. Furthermore, ac susceptibility measurements of both compounds display the peak effect in the low temperature region of the $H-T$ phase diagram. This anomalous increase of the critical current with field gives an indication of a change of the arrangement of flux lines in the mixed state, as found in some of the isostructural stannide materials.
  • Evidence for bulk superconductivity in pure bismuth single crystals at ambient pressure
    Om Prakash, Anil Kumar, A. Thamizhavel, S. Ramakrishnan
    Science, 2017
    Going cold with Bismuth Many elemental metals, such as lead and aluminum, become superconducting at low temperatures. Bismuth, a semimetal with very low carrier density, stays nonsuperconducting down to 10 mK. Prakash et al. performed tricky magnetization measurements to show that pure bulk bismuth does undergo the superconducting transition at a tiny temperature of about 0.5 mK (see the Perspective by Behnia). Because bismuth does not fit neatly into the standard picture of superconductivity, further theoretical work is necessary to explain the findings. Science , this issue p. 52 ; see also p. 26
  • Ferromagnetic ordering of minority Ce3+ spins in a quasi-skutterudite Ce3Os4Ge13 single crystal
    Om Prakash, A. Thamizhavel, S. Ramakrishnan
    Physical Review B, 2016
    We report site disorder driven ferromagnetic ordering of nearly 8$\\%$ Ce atoms in single crystalline Ce${_3}$Os${_4}$Ge$_{13}$ below 0.5~K. Ce${_3}$Os${_4}$Ge$_{13}$ crystallizes in a cubic structure with space group $\\it{Pm\\bar{3}n}$. The structural analysis shows the presence of site-disorder in the system, where 2(a) Ge site is partially occupied by the Ce atoms. Due to the small Ce $4f$-ligand hybridization, these Ce atoms are in the Ce$^{3+}$ (magnetic) state while rest of the Ce atoms in the unit cell are in Ce$^{4-\\delta}$ (non-magnetic) state. The heat capacity shows a peak below 0.5~K corresponding to the ferromagnetic ordering of the Ce$^{3+}$ moments. The ferromagnetic ordering below 0.5~K is also seen in the ac-susceptibility data. At low temperatures (1.8~K$\\leq T\\leq 6$~K), the magnetization shows $log(T)$ dependence, whereas the resistivity shows power law ($T^n$) temperature dependence indicating the non-Fermi liquid behavior of the quasiparticles.
  • Multiband superconductivity in Lu3Os4Ge13
    Om Prakash, A Thamizhavel, S Ramakrishnan
    Superconductor Science and Technology, 2015
    Intermetallic R3T4X13 series consist of a cage-like structure and have attracted attention due to their unconventional electronic ground states. In this work, we report the normal and superconducting state properties of a high-quality single crystal of Lu3Os4Ge13. Lu3Os4Ge13 belongs to the above-mentioned series and crystallizes in a cubic crystal structure with the space group Pm 3 ¯ n . Using electrical transport, magnetization and heat capacity measurements, we show that Lu3Os4Ge13 is a type-II multi-band superconductor (Tc = 3.1 K) with unusual superconducting properties. The analysis of the low-temperature heat capacity data suggests that Lu3Os4Ge13 is a moderately coupled multi-band BCS superconductor with two gaps ( 2 Δ / k B T c = 3.68 ± 0.04 & 0.34 ± 0.02 ) in the superconducting state. The dc-magnetization (M−H) shows a large reversible region in the superconducting state similar to the vortex liquid phase observed in high-Tc superconductors. The value of the Ginzburg number Gi suggests that the thermal fluctuations, though small compared to those in high-Tc cuprates, may play an important role in the unpinning of the vortices in this compound. The electronic band structure calculations show that three bands cross the Fermi level and constitute a complex Fermi surface in Lu3Os4Ge13.
  • Superconductivity in Y3Ru4Ge13 and Lu3Os4Ge13: A comparative study
    Om Prakash, A Thamizhavel, S Ramakrishnan
    Journal of Physics Conference Series, 2014
    A variety of unconventional superconductors have low carrier density as a common factor. However, the underlying mechanism of superconductivity in such low carrier density systems is not well understood. Besides, small carrier density is an unfavourable component for conventional superconductivity as described by the Bardeen-Cooper-Schrieffer (BCS) theory. Therefore, studying low carrier density systems can lead to a better understanding in such systems. In this paper, we report superconductivity property studies in low carrier density systems, Y3Ru4Ge13 and Lu3Os4Ge13, using various experimental techniques. Single crystals of Y3Ru4Ge13 and Lu3Os4Ge13 have been grown using the Czochralski crystal pulling method in a tetra-arc furnace. The x-ray diffraction experiment reveals that both compounds crystallize in cubic structure (space group Pm3n, no. 223). The transport, magnetization and heat capacity measurements show that Y3Ru4Ge13 single crystal undergoes a superconducting transition at 2.85 K, whereas, Lu3Os4Ge13 becomes superconductor at 3.1 K.
  • Development of a nano-tesla magnetic field shielded chamber and highly precise AC-susceptibility measurement coil at μk temperatures
    Anil Kumar, Om Prakash, S. Ramakrishanan
    Aip Conference Proceedings, 2014
  • Superconductivity in Lu3Os4Ge13
    Om Prakash, A Thamizhavel, S Ramakrishnan
    Journal of Physics Conference Series, 2014
  • Superconductivity in a low carrier density system: A single crystal study of cubic Y3Ru4Ge13
    Om Prakash, A. Thamizhavel, A. K. Nigam, S. Ramakrishnan
    Aip Conference Proceedings, 2014
  • Superconductivity in a low carrier density system: A single crystal study of cubic Y3Ru4Ge13
    Om Prakash, A. Thamizhavel, A.K. Nigam, S. Ramakrishnan
    Physica C Superconductivity and Its Applications, 2013

RECENT SCHOLAR PUBLICATIONS

  • Native-oxide-passivated trilayer junctions for superconducting qubits
    P Sethi, O Prakash, JP Kaikkonen, M Kervinen, ET Mannila, M Ribeiro, ...
    Phys. Rev. Applied 24 (064056) , 2025
    2025
    Citations: 3
  • Efficient electronic cooling by niobium-based superconducting tunnel junctions
    J Hätinen, A Ronzani, RP Loreto, E Mykkänen, A Kemppinen, K Viisanen, ...
    Physical Review Applied 22 (6), 064048 , 2024
    2024
    Citations: 10
  • Broken time-reversal symmetry in the cubic skutterudite-like superconductor
    A Kataria, JAT Verezhak, O Prakash, RK Kushwaha, A Thamizhavel, ...
    Physical Review B 108 (21), 214512 , 2023
    2023
    Citations: 4
  • Fragile electronic superconductivity in a Bi single crystal
    A Kumar, O Prakash, R Loke, A Pramanik, R Sensarma, S Ramakrishnan, ...
    Physical Review B 108 (22), 224512 , 2023
    2023
    Citations: 2
  • Time-reversal symmetry breaking in the superconducting low carrier density quasiskutterudite
    A Kataria, JAT Verezhak, O Prakash, RK Kushwaha, A Thamizhavel, ...
    Physical Review B 107 (10), L100506 , 2023
    2023
    Citations: 8
  • Superconductivity in Pt-doped ScPdBi single crystals
    O Prakash, M Klemm, M Aronson
    APS March Meeting Abstracts 2018, F31. 004 , 2018
    2018
  • Discovery of superconductivity in a low carrier density system: Bismuth
    S Ramakrishnan, O Prakash, A Thamizhavel, A Kumar
    APS March Meeting Abstracts 2018, V05. 005 , 2018
    2018
  • Nodeless superconductivity and the peak effect in the quasiskutterudites and
    ZF Weng, M Smidman, GM Pang, O Prakash, Y Chen, YJ Zhang, ...
    Physical Review B 95 (18), 184501 , 2017
    2017
    Citations: 13
  • Evidence for bulk superconductivity in pure bismuth single crystals at ambient pressure
    O Prakash, A Kumar, A Thamizhavel, S Ramakrishnan
    Science 355 (6320), 52-55 , 2017
    2017
    Citations: 229
  • Ferromagnetic ordering of minority spins in a quasi-skutterudite single crystal
    O Prakash, A Thamizhavel, S Ramakrishnan
    Physical Review B 93 (6), 064427 , 2016
    2016
    Citations: 39
  • Multiband superconductivity in Lu 3 Os 4 Ge 13
    O Prakash, A Thamizhavel, S Ramakrishnan
    Superconductor Science and Technology 28 (11), 115012 , 2015
    2015
    Citations: 23
  • Superconductivity in Lu 3 Os 4 Ge 13
    O Prakash, A Thamizhavel, S Ramakrishnan
    Journal of Physics: Conference Series 592 (1), 012065 , 2015
    2015
    Citations: 6
  • Superconductivity in Y 3 Ru 4 Ge 13 and Lu 3 Os 4 Ge 13 : A comparative study
    O Prakash, A Thamizhavel, S Ramakrishnan
    Journal of Physics: Conference Series 568 (2), 022039 , 2014
    2014
    Citations: 3
  • Superconductivity in a low carrier density system: A single crystal study of cubic
    O Prakash, A Thamizhavel, AK Nigam, S Ramakrishnan
    AIP Conference Proceedings 1591 (1), 1572-1574 , 2014
    2014
    Citations: 1
  • Development of a nano-tesla magnetic field shielded chamber and highly precise AC-susceptibility measurement coil at μK temperatures
    A Kumar, O Prakash, S Ramakrishanan
    Solid State Physics 1591 (1), 644-645 , 2014
    2014
  • Superconductivity in a low carrier density system: A single crystal study of cubic Y3Ru4Ge13
    O Prakash, A Thamizhavel, AK Nigam, S Ramakrishnan
    Physica C: Superconductivity 492, 90-95 , 2013
    2013
    Citations: 15

MOST CITED SCHOLAR PUBLICATIONS

  • Evidence for bulk superconductivity in pure bismuth single crystals at ambient pressure
    O Prakash, A Kumar, A Thamizhavel, S Ramakrishnan
    Science 355 (6320), 52-55 , 2017
    2017
    Citations: 229
  • Ferromagnetic ordering of minority spins in a quasi-skutterudite single crystal
    O Prakash, A Thamizhavel, S Ramakrishnan
    Physical Review B 93 (6), 064427 , 2016
    2016
    Citations: 39
  • Multiband superconductivity in Lu 3 Os 4 Ge 13
    O Prakash, A Thamizhavel, S Ramakrishnan
    Superconductor Science and Technology 28 (11), 115012 , 2015
    2015
    Citations: 23
  • Superconductivity in a low carrier density system: A single crystal study of cubic Y3Ru4Ge13
    O Prakash, A Thamizhavel, AK Nigam, S Ramakrishnan
    Physica C: Superconductivity 492, 90-95 , 2013
    2013
    Citations: 15
  • Nodeless superconductivity and the peak effect in the quasiskutterudites and
    ZF Weng, M Smidman, GM Pang, O Prakash, Y Chen, YJ Zhang, ...
    Physical Review B 95 (18), 184501 , 2017
    2017
    Citations: 13
  • Efficient electronic cooling by niobium-based superconducting tunnel junctions
    J Hätinen, A Ronzani, RP Loreto, E Mykkänen, A Kemppinen, K Viisanen, ...
    Physical Review Applied 22 (6), 064048 , 2024
    2024
    Citations: 10
  • Time-reversal symmetry breaking in the superconducting low carrier density quasiskutterudite
    A Kataria, JAT Verezhak, O Prakash, RK Kushwaha, A Thamizhavel, ...
    Physical Review B 107 (10), L100506 , 2023
    2023
    Citations: 8
  • Superconductivity in Lu 3 Os 4 Ge 13
    O Prakash, A Thamizhavel, S Ramakrishnan
    Journal of Physics: Conference Series 592 (1), 012065 , 2015
    2015
    Citations: 6
  • Broken time-reversal symmetry in the cubic skutterudite-like superconductor
    A Kataria, JAT Verezhak, O Prakash, RK Kushwaha, A Thamizhavel, ...
    Physical Review B 108 (21), 214512 , 2023
    2023
    Citations: 4
  • Native-oxide-passivated trilayer junctions for superconducting qubits
    P Sethi, O Prakash, JP Kaikkonen, M Kervinen, ET Mannila, M Ribeiro, ...
    Phys. Rev. Applied 24 (064056) , 2025
    2025
    Citations: 3
  • Superconductivity in Y 3 Ru 4 Ge 13 and Lu 3 Os 4 Ge 13 : A comparative study
    O Prakash, A Thamizhavel, S Ramakrishnan
    Journal of Physics: Conference Series 568 (2), 022039 , 2014
    2014
    Citations: 3
  • Fragile electronic superconductivity in a Bi single crystal
    A Kumar, O Prakash, R Loke, A Pramanik, R Sensarma, S Ramakrishnan, ...
    Physical Review B 108 (22), 224512 , 2023
    2023
    Citations: 2
  • Superconductivity in a low carrier density system: A single crystal study of cubic
    O Prakash, A Thamizhavel, AK Nigam, S Ramakrishnan
    AIP Conference Proceedings 1591 (1), 1572-1574 , 2014
    2014
    Citations: 1
  • Superconductivity in Pt-doped ScPdBi single crystals
    O Prakash, M Klemm, M Aronson
    APS March Meeting Abstracts 2018, F31. 004 , 2018
    2018
  • Discovery of superconductivity in a low carrier density system: Bismuth
    S Ramakrishnan, O Prakash, A Thamizhavel, A Kumar
    APS March Meeting Abstracts 2018, V05. 005 , 2018
    2018
  • Development of a nano-tesla magnetic field shielded chamber and highly precise AC-susceptibility measurement coil at μK temperatures
    A Kumar, O Prakash, S Ramakrishanan
    Solid State Physics 1591 (1), 644-645 , 2014
    2014