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Ytterbium charge state and stabilization in the Ba(Ca)F$_2$ host by electron paramagnetic resonance and infrared photoluminescence
Authors:
David John,
Shelja Sharma,
Marius Stef,
Gabriel Buse,
Zdeněk Remeš,
Anna Artemenko,
Sergii Chertopalov,
Vineet Sikarwar,
Alan Mašláni,
Jafar Fathi,
Jakub Pilař,
Tomáš Hostinský,
Jan Zich,
Tomáš Mates,
Brenda Natalia Lopez Nino,
Michal Hlína,
Karol Bartosiewicz,
Marina Konuhova,
Anatoli Popov,
Ján Lančok,
Maksym Buryi
Abstract:
Lanthanide-doped fluorides are promising materials for advanced photonic and quantum applications due to their wide bandgap, low phonon energy, and chemical stability. In this work, we present a systematic comparative study of ytterbium incorporation at low doping levels (0.05--0.2 mol\%) in BaF$_2$ and CaF$_2$ single crystals, focusing on the interplay between host lattice properties, charge-stat…
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Lanthanide-doped fluorides are promising materials for advanced photonic and quantum applications due to their wide bandgap, low phonon energy, and chemical stability. In this work, we present a systematic comparative study of ytterbium incorporation at low doping levels (0.05--0.2 mol\%) in BaF$_2$ and CaF$_2$ single crystals, focusing on the interplay between host lattice properties, charge-state stabilization, and defect formation mechanisms. Using a combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), transmittance, and infrared photoluminescence (IR PL), we explore how host lattice properties affect the stabilization of Yb$^{3+}$ and Yb$^{2+}$ ions. XRD confirmed cubic phase purity and lattice parameter stability in both hosts, while XPS revealed surface chemical composition variations associated with charge-compensating defects and trace impurities. EPR spectra indicated that BaF$_2$ favored perturbed Yb$^{3+}$ environments with increasing dopant levels, while CaF$_2$ maintained predominantly unperturbed sites, suggesting a more favorable ionic match for Yb$^{2+}$. Photothermal deflection spectroscopy (PDS) and IR PL results showed host-specific optical responses, with CaF$_2$ exhibiting crystal-field splitting and broader local field effects. These results reveal a clear decoupling between long-range structural stability and local lattice perturbations, and demonstrate that host cation identity governs the balance between Yb$^{2+}$ and Yb$^{3+}$ stabilization as well as defect-driven optical behavior. This offers valuable insights for optimizing rare-earth-doped fluoride crystals in laser, scintillator, and quantum device applications.
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Submitted 11 May, 2026;
originally announced May 2026.
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On-chip time-domain terahertz spectroscopy of superconducting films below the diffraction limit
Authors:
Alex Potts,
Abhay Nayak,
Michael Nagel,
Kelson Kaj,
Biljana Stamenic,
Demis D. John,
Richard D. Averitt,
Andrea F. Young
Abstract:
Free-space time domain THz spectroscopy accesses electrodynamic responses in a frequency regime ideally matched to interacting condensed matter systems. However, THz spectroscopy is challenging when samples are physically smaller than the diffraction limit of ~0.5 mm, as is typical, for example, in van der Waals materials and heterostructures. Here, we present an on-chip, time-domain THz spectrome…
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Free-space time domain THz spectroscopy accesses electrodynamic responses in a frequency regime ideally matched to interacting condensed matter systems. However, THz spectroscopy is challenging when samples are physically smaller than the diffraction limit of ~0.5 mm, as is typical, for example, in van der Waals materials and heterostructures. Here, we present an on-chip, time-domain THz spectrometer based on semiconducting photoconductive switches with a bandwidth of 200 GHz to 750 GHz. We measure the optical conductivity of a 7.5-$μ$m wide NbN film across the superconducting transition, demonstrating spectroscopic signatures of the superconducting gap in a sample smaller than 2% of the Rayleigh diffraction limit. Our spectrometer features an interchangeable sample architecture, making it ideal for probing superconductivity, magnetism, and charge order in strongly correlated van der Waals materials.
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Submitted 10 February, 2023;
originally announced February 2023.
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The influence of magnetic vortices motion on the inverse ac Josephson effect in asymmetric arrays
Authors:
Boris Chesca,
Marat Gaifullin,
Daniel John,
Jonathan Cox,
Sergey Savelev,
Christopher Mellor
Abstract:
We report on the influence a preferential magnetic vortices motion has on the magnitude of the inverse ac Josephson effect (the appearance of dc current Shapiro steps) and the coherent operation of asymmetrical parallel arrays of YBaCuO Josephson junctions (JJ) irradiated with microwave (MW) radiation in the presence of an applied magnetic field B. The preferential direction of motion of the Josep…
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We report on the influence a preferential magnetic vortices motion has on the magnitude of the inverse ac Josephson effect (the appearance of dc current Shapiro steps) and the coherent operation of asymmetrical parallel arrays of YBaCuO Josephson junctions (JJ) irradiated with microwave (MW) radiation in the presence of an applied magnetic field B. The preferential direction of motion of the Josephson vortices is due to the asymmetry-induced ratchet effect and has a dramatic impact: for a particular positive dc bias current I when the flux-flow is robust multiple pronounced Shapiro-steps are observed consistent with a coherent operation of the array. This suggests an efficient emission/detection of MW in related applications. In contrast, when we reverse the direction of I, the flux-flow is reduced and the Shapiro-steps are strongly suppressed due to a highly incoherent operation that suggests an inefficient emission/detection of MW. Remarkably, by changing B slightly, the situation is reversed: Shapiro steps are now suppressed for a positive I, while well pronounced for a reverse current -I. Our results suggest that a preferential vortex-flow has a very significant impact on the coherent MW operation of superconducting devices consisting of either multiple JJs or a single long JJ asymmetrically biased. This is particular relevant in the case of flux-flow oscillators for sub-terahertz integrated-receivers, flux-driven Josephson (travelling-wave) parametric amplifiers, or on-chip superconducting MW generators which usually operate at bias currents in the Shapiro step region.
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Submitted 4 November, 2022; v1 submitted 19 September, 2022;
originally announced September 2022.
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Parametric amplification of electromagnetic waves produced by a flux-flow-oscillator made of YBaCuO Josephson junction arrays
Authors:
Boris Chesca,
Daniel John,
Marat Gaifullin
Abstract:
We observe parametric amplification of electromagnetic (EM) waves produced by a flux-flow oscillator made of YBa2Cu3O7 Josephson junctions arrays coupled to the resonant modes of a millimeter wave Fabry-Perot resonator at a pump frequency fP=45 GHz. For temperatures in the range (30-45) K the frequency fS of the EM signal to be amplified could be tuned continuously in the range (1-25) GHz by an ap…
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We observe parametric amplification of electromagnetic (EM) waves produced by a flux-flow oscillator made of YBa2Cu3O7 Josephson junctions arrays coupled to the resonant modes of a millimeter wave Fabry-Perot resonator at a pump frequency fP=45 GHz. For temperatures in the range (30-45) K the frequency fS of the EM signal to be amplified could be tuned continuously in the range (1-25) GHz by an applied B-field induced flux with a one-flux-quantum periodicity. Consequently, we measured a significant parametric gain that is almost frequency independent, with a maximum of (8-10.4) dB reached at 40K. For temperatures in the range (14-30) K the magnetic field tunability of fS is gradually suppressed to a minimum of (1-5) GHz range where a parametric gain between (5-6) dB was measured. With an appropriate adjustment of design/fabrication parameters our results suggests that the development of tunable MW generators/detectors, as well as parametric amplifiers made of high transition temperature superconductors and operating in a wide range of temperatures (10 mK-77K) is a reasonable and appealing possibility.
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Submitted 4 November, 2022; v1 submitted 13 September, 2022;
originally announced September 2022.
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A reentrant superspin glass state and magnetization steps in the oxyborate Co2AlBO5
Authors:
Jitender Kumar,
Soumendra Nath Panja,
Deepak John,
Arpan Bhattacharyya,
A. K. Nigam,
Sunil Nair
Abstract:
An oxyborate Co2AlBO5 belonging to the ludwigite family is investigated using structural, thermodynamic, dielectric and magnetic measurements. Magnetic measurements indicate that this system is seen to exhibit long range magnetic ordering at T{_N} = 42 K, signatures of which are also seen in the specific heat, dielectric susceptibility, and the lattice parameters. The absence of a structural phase…
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An oxyborate Co2AlBO5 belonging to the ludwigite family is investigated using structural, thermodynamic, dielectric and magnetic measurements. Magnetic measurements indicate that this system is seen to exhibit long range magnetic ordering at T{_N} = 42 K, signatures of which are also seen in the specific heat, dielectric susceptibility, and the lattice parameters. The absence of a structural phase transition down to the lowest measured temperatures, distinguishes it from the more extensively investigated Fe-based ludwigites. At low temperatures, the system is seen to stabilize in a reentrant superspin glass phase at T{_G} = 10.6 K from within the magnetically ordered state. This ground state is also characterized by magnetic field induced metamagnetic transitions, which at the lowest measured temperatures exhibit a number of sharp magnetization steps, reminiscent of that observed in the mixed valent manganites.
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Submitted 15 December, 2016;
originally announced December 2016.
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Amplification of electromagnetic waves excited by a chain of propagating magnetic vortices in YBaCuO Josephson-junction arrays at 77K and above
Authors:
Boris Chesca,
Daniel John,
Christopher J. Mellor
Abstract:
When a soliton propagates in a discrete lattice it excites small-amplitude linear waves in its wake. In a dc current-biased Josephson-junction (JJ) array these manifest as electromagnetic (EM) waves excited by a (magnetic field induced) chain of propagating magnetic vortices. When the vortex velocity and the phase velocity of one of the excited EM waves match, phase-locking occurs. This produces r…
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When a soliton propagates in a discrete lattice it excites small-amplitude linear waves in its wake. In a dc current-biased Josephson-junction (JJ) array these manifest as electromagnetic (EM) waves excited by a (magnetic field induced) chain of propagating magnetic vortices. When the vortex velocity and the phase velocity of one of the excited EM waves match, phase-locking occurs. This produces resonant steps in the current-voltage characteristics where amplification of EM radiation occurs. We report the first observation of phase-locking-induced amplification of EM radiation at 77K and above in JJ arrays made of high temperature superconductors.
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Submitted 23 January, 2014;
originally announced January 2014.