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Nonunitary triplet superconductivity in the Z2 topological metal SrPd2As2
Authors:
Aarti,
Dibyendu Samanta,
Kartik Panda,
Devashibhai Adroja,
Daloo Ram,
Zakir Hossain,
Rhea Stewart,
Adrian Hillier,
Amitava Bhattacharyya,
Samar Layek,
Sudeep Kumar Ghosh,
Vivek Kumar Anand
Abstract:
In Z2 topological metals, nontrivial band topology and strong spin-orbit coupling (SOC) impose symmetry constraints that can stabilize unconventional superconducting states, even when thermodynamic probes indicate an isotropic gap. Here, we investigate the superconducting ground state of such a material, SrPd2As2, using muon spin rotation and relaxation (muSR), first-principles calculations, and G…
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In Z2 topological metals, nontrivial band topology and strong spin-orbit coupling (SOC) impose symmetry constraints that can stabilize unconventional superconducting states, even when thermodynamic probes indicate an isotropic gap. Here, we investigate the superconducting ground state of such a material, SrPd2As2, using muon spin rotation and relaxation (muSR), first-principles calculations, and Ginzburg-Landau analysis. Transverse-field muSR indicates a fully gapped superconducting state below Tc = 0.94 K, while zero-field muSR detects spontaneous internal magnetic fields below Tc, establishing time-reversal symmetry (TRS) breaking. Electronic structure calculations identify SrPd2As2 as a Z2 topological metal with surface states crossing the Fermi level. Standard anisotropic Migdal-Eliashberg calculations predict a nodal gap and overestimate Tc, indicating that a purely phonon-mediated pairing mechanism is insufficient. We resolve this apparent contradiction by showing that the interplay of SOC, tetragonal symmetry, and an open Fermi surface topology stabilizes a nonunitary triplet superconducting state whose symmetry-imposed nodes lie in momentum-space regions devoid of electronic states. This yields a fully gapped thermodynamic response while naturally breaking TRS. Our results establish SrPd2As2 as a clean platform for bulk nonunitary triplet pairing and a promising candidate for intrinsic topological superconductivity.
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Submitted 23 May, 2026;
originally announced May 2026.
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Modulation of quantum geometry and its coupling to pseudo-electric field by dynamic strain
Authors:
Surat Layek,
Mahesh A. Hingankar,
Ayshi Mukherjee,
Atasi Chakraborty,
Digambar A. Jangade,
Anil Kumar,
L. D. Varma Sangani,
Amit Basu,
R Bhuvaneswari,
Kenji Watanabe,
Takashi Taniguchi,
Amit Agarwal,
Umesh V. Waghmare,
Mandar M. Deshmukh
Abstract:
Two-dimensional materials are a fertile ground for exploring quantum geometric phenomena, with Berry curvature and its first moment, the Berry curvature dipole, playing a central role in their electronic response. These geometric properties influence electronic transport and result in the anomalous and nonlinear Hall effects, and are typically controlled using static electric fields or strain. How…
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Two-dimensional materials are a fertile ground for exploring quantum geometric phenomena, with Berry curvature and its first moment, the Berry curvature dipole, playing a central role in their electronic response. These geometric properties influence electronic transport and result in the anomalous and nonlinear Hall effects, and are typically controlled using static electric fields or strain. However, the possibility of modulating quantum geometric quantities in real-time remains unexplored. Here, we demonstrate the dynamic modulation of Berry curvature and its moments, as well as the generation of a pseudo-electric field using time-dependent strain. By placing heterostructures on a membrane, we introduce oscillatory strain together with an in-plane AC electric field and measure Hall signals that are modulated at linear combinations of the frequencies of strain and electric field. Our measurements reveal modulation of Berry curvature and its first moment. Notably, we provide direct experimental evidence of pseudo-electric field that results in an unusual dynamic strain-induced Hall response. This approach opens up a new pathway for controlling quantum geometry on demand, moving beyond conventional static perturbations. The pseudo-electric field provides a framework for external electric field-free anomalous Hall response and opens new avenues for probing the topological properties.
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Submitted 31 December, 2025;
originally announced December 2025.
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Rapid reduction of nitrophenols using reusable magnetic \textit{h}-BN/Ni--NiO nanocomposites
Authors:
Anjali Varshney,
Ritesh Dubey,
Sushil Kumar,
Tapas Goswami,
Samar Layek
Abstract:
The efficient and cost-effective conversion of nitro compounds to amines is crucial for industrial processes and environmental remediation, highlighting the growing demand for earth-abundant metal-based catalysts. In this study, magnetic Ni--NiO nanostructures and their composites with two-dimensional hexagonal boron nitride (\textit{h}-BN) were synthesized via a simple and scalable combustion met…
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The efficient and cost-effective conversion of nitro compounds to amines is crucial for industrial processes and environmental remediation, highlighting the growing demand for earth-abundant metal-based catalysts. In this study, magnetic Ni--NiO nanostructures and their composites with two-dimensional hexagonal boron nitride (\textit{h}-BN) were synthesized via a simple and scalable combustion method. The structural, morphological, and compositional properties of the synthesized materials were systematically investigated using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV--Vis spectroscopy. The catalytic activity of both Ni--NiO and \textit{h}-BN/Ni--NiO nanostructures was evaluated using nitrophenol reduction as a model reaction. The \textit{h}-BN/Ni--NiO nanocomposite exhibited significantly enhanced catalytic performance compared to pristine Ni--NiO, highlighting the synergistic interaction between \textit{h}-BN and Ni--NiO nanoparticles. Notably, the magnetic nature of the Ni--NiO core enabled facile recovery of the catalyst using an external magnetic field, and the composite demonstrated excellent stability and reusability for up to six catalytic cycles with minimal loss of activity. The combination of high catalytic efficiency, magnetic separability, and structural stability positions the \textit{h}-BN/Ni--NiO nanocomposite as a promising candidate for green and sustainable catalytic applications, particularly in environmental remediation.
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Submitted 22 December, 2025;
originally announced December 2025.
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High-Entropy Oxide Nanostructures for Rapid and Sustainable Nitrophenol Reduction
Authors:
Anjali Varshney,
Aishwery J. Verma,
Ritesh Dubey,
Sushil Kumar,
Tapas Goswami,
Samar Layek
Abstract:
High-entropy materials have emerged as a promising class of catalysts, driven by their high configurational entropy originating from structural disorder in single-phase multicomponent systems. Despite their potential, the catalytic performance of high-entropy oxides (HEOs) remains relatively underexplored. In this study, we present a simple solution-based combustion route to synthesize two low-cos…
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High-entropy materials have emerged as a promising class of catalysts, driven by their high configurational entropy originating from structural disorder in single-phase multicomponent systems. Despite their potential, the catalytic performance of high-entropy oxides (HEOs) remains relatively underexplored. In this study, we present a simple solution-based combustion route to synthesize two low-cost, transition metal-rich multicationic oxides positioned in the medium-entropy (HEO-4) and high-entropy (HEO-5) regimes. Rietveld refinement of powder X-ray diffraction data confirmed single-phase formation with a face-centered cubic (fcc) crystal structure for both nanostructures.
The morphology, particle size, and multicationic elemental distribution were investigated using scanning and transmission electron microscopy. The catalytic performance of the synthesized HEOs was evaluated in the hydrogenation of a series of nitrophenol derivatives. Notably, HEO-5 exhibited significantly enhanced catalytic activity ($k_{\mathrm{app}} \approx 0.5~\mathrm{min^{-1}}$, TOF $= 2.1 \times 10^{-3}~\mathrm{mol\,g^{-1}\,s^{-1}}$), achieving rapid conversion of \emph{p}-nitrophenol compared to the medium-entropy oxide nanostructures ($k_{\mathrm{app}} \approx 0.02~\mathrm{min^{-1}}$, TOF $= 7.2 \times 10^{-4}~\mathrm{mol\,g^{-1}\,s^{-1}}$). Furthermore, the kinetic and thermodynamic parameters of the reaction, including the activation energy ($E_a$), enthalpy of activation ($ΔH^{\ddagger}$), Gibbs free energy of activation ($ΔG^{\ddagger}$), and entropy of activation ($ΔS^{\ddagger}$), were determined to gain mechanistic insight into the reduction process. This study opens new avenues for the rational design and facile synthesis of high-entropy oxide catalysts, highlighting their potential for efficient and sustainable large-scale amine production.
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Submitted 19 December, 2025;
originally announced December 2025.
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MOF-derived Fe-doped $δ$-MnO$_2$ nanoflowers as oxidase mimics: Chromogenic sensing of Hg$^{2+}$ and hydroquinone in aqueous media
Authors:
Udisha Duhan,
Arnab Pan,
Ritesh Dubey,
Samar Layek,
Sushil Kumar,
Tapas Goswami
Abstract:
Structure and morphology play a crucial role in enhancing the biomimetic oxidase activity of nanozymes. In this study, a facile \emph{in situ} chemical oxidation strategy was employed to synthesize MOF-derived MnO$_x$, utilizing the structural features of the parent MOF to enhance oxidase-mimicking activity. We systematically investigated the effects of phase evolution, structural modulation, and…
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Structure and morphology play a crucial role in enhancing the biomimetic oxidase activity of nanozymes. In this study, a facile \emph{in situ} chemical oxidation strategy was employed to synthesize MOF-derived MnO$_x$, utilizing the structural features of the parent MOF to enhance oxidase-mimicking activity. We systematically investigated the effects of phase evolution, structural modulation, and morphology on the oxidase activity of MnO$_x$ with Fe substitution. The oxidase-like activity was evaluated using the chromogenic substrate 3,3$'$,5,5$'$-tetramethylbenzidine (TMB), which produced a blue-colored oxidized TMB (ox-TMB) with an absorption peak at 652~nm upon oxidation. While all Fe-doped MnO$_x$ nanostructures exhibited oxidase-like activity, the 10\% Fe-doped sample (10Fe-MnO$_x$) demonstrated the highest performance, likely due to a synergistic effect of structure, morphology, and the presence of oxygen vacancies. The underlying oxidase mechanism was investigated using steady-state kinetics and electron paramagnetic resonance (EPR) analysis. In addition, a colorimetric assay was developed for the detection of Hg$^{2+}$ and hydroquinone (HQ) in real water samples collected from industrial and natural sources. The calculated detection limits of the 10Fe-MnO$_x$ colorimetric probe for HQ (1.74~$μ$M) and Hg$^{2+}$ (0.47~$μ$M) outperformed those of conventional metal oxide-based nanozymes. These findings pave the way for the development of easily synthesizable, scalable, and highly sensitive oxidase-based MOF-derived metal oxide nanomaterials with significant potential in biological and environmental applications.
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Submitted 19 December, 2025;
originally announced December 2025.
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Design principles for metal-organic receptors targeting optical recognition of Pd(II) in environmental matrices
Authors:
Sudhanshu Naithani,
Pramod Kumar,
Ritesh Dubey,
Franck Thetiot,
Samar Layek,
Tapas Goswami,
Sushil Kumar
Abstract:
A precise detection of palladium (Pd) ions is a critical challenge with significant socio-economic implications across various industrial and chemical sectors. Due to its widespread use and poor biodegradability, Pd2+ accumulates in environmental ecosystems, posing severe risks to both the environment and living organisms. Consequently, there is a strong demand for selective, sensitive, and user-f…
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A precise detection of palladium (Pd) ions is a critical challenge with significant socio-economic implications across various industrial and chemical sectors. Due to its widespread use and poor biodegradability, Pd2+ accumulates in environmental ecosystems, posing severe risks to both the environment and living organisms. Consequently, there is a strong demand for selective, sensitive, and user-friendly detection methods. Among emerging strategies, optical detection techniques (both luminescent and colorimetric) using metal-based receptors have gained considerable attention. These sensors offer distinct advantages over traditional organic probes, including large Stokes shifts, long emission lifetimes, exceptional photostability, enhanced water solubility, recyclability, and remarkable chemical versatility. These attributes make them highly suitable for diverse applications in sensing and bioanalytical fields. This review provides a comprehensive overview of recent advancements in luminescent and colorimetric metal-based probes, including metal complexes and metal-organic frameworks (MOFs), for the selective detection of Pd2+. It discusses key design strategies, critical performance factors, and future prospects, offering valuable insights for researchers working on next-generation sensing platform.
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Submitted 19 December, 2025;
originally announced December 2025.
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A Pyridyl-Benzimidazole Based Ruthenium(II) Complex as Optical Sensor: Targeted Cyanide Detection and Live Cell Imaging Applications
Authors:
Sudhanshu Naithani,
Franck Thetiot,
Vikas Yadav,
Saakshi Saini,
Partha Roy,
Samar Layek,
Tapas Goswami,
Sushil Kumar
Abstract:
The extreme toxicity of cyanide (\ce{CN^-}) ions in diverse environmental media has garnered significant attention toward the design of well-organized molecular probes for their selective and sensitive detection. In this context, we present a monometallic Ru(II) complex (Ru-1), based on the 2-(pyridin-2-yl)-1H-benzo[d]imidazole moiety, acting as a highly selective luminescent probe for \ce{CN^-} r…
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The extreme toxicity of cyanide (\ce{CN^-}) ions in diverse environmental media has garnered significant attention toward the design of well-organized molecular probes for their selective and sensitive detection. In this context, we present a monometallic Ru(II) complex (Ru-1), based on the 2-(pyridin-2-yl)-1H-benzo[d]imidazole moiety, acting as a highly selective luminescent probe for \ce{CN^-} recognition in pure water. Additionally, Ru-1 also functions as an efficient sensor for \ce{F^-}, \ce{AcO^-}, and \ce{H2PO4^-} ions, along with \ce{CN^-}, when acetonitrile is used as the solvent system. The binding constant ($K_b$) and detection limit (LoD) for \ce{CN^-} were determined to be $3.05 \times 10^6$~M$^{-1}$ and 12.8~nM, respectively, in water. The close proximity of the N--H site to the Ru(II) center, along with its notable acidity, were identified as the primary factors responsible for the high selectivity of Ru-1 toward \ce{CN^-} in aqueous media. Job's plots and density functional theory (DFT) analyses were conducted to support the anion binding mechanism. Furthermore, time-resolved fluorescence (TRF) spectroscopy was employed to evaluate the \ce{CN^-}-induced emission lifetime change of Ru-1 in water. To explore practical applicability, the Ru-1 probe was developed into paper-based strips capable of detecting \ce{CN^-} ions in the millimolar range via the naked eye under 365~nm UV illumination. It was also effectively applied for the detection of \ce{CN^-} in human breast cancer MCF-7 cell lines and natural food sources, such as apple seeds and sprouting potatoes.
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Submitted 7 July, 2025;
originally announced July 2025.
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Nanoparticles and Quantum Dots as Emerging Optical Sensing Platforms for $\mathrm{Ni}^{2+}$ Detection: Recent Approaches and Perspectives
Authors:
Sudhanshu Naithani,
Heena,
Pooja Sharma,
Samar Layek,
Franck Thetiot,
Tapas Goswami,
Sushil Kumar
Abstract:
Over the preceding years, nickel (Ni) and its compounds have been increasingly employed in various aspects of human social life, metallurgical/industrial manufactures, healthcare, and chemical processes. Although Ni is considered an essential trace element in biological systems, excessive intake or metabolic deficiency of $\mathrm{Ni}^{2+}$ ions may cause detrimental health effects to living organ…
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Over the preceding years, nickel (Ni) and its compounds have been increasingly employed in various aspects of human social life, metallurgical/industrial manufactures, healthcare, and chemical processes. Although Ni is considered an essential trace element in biological systems, excessive intake or metabolic deficiency of $\mathrm{Ni}^{2+}$ ions may cause detrimental health effects to living organisms. Therefore, a facile and accurate detection of $\mathrm{Ni}^{2+}$, especially in environmental and biological settings, is of huge significance. As an efficient detection method, assaying $\mathrm{Ni}^{2+}$ using optical (colorimetric and/or fluorogenic) sensors has experienced quite a vigorous growth period, with a large number of excellent research contributions. Nanomaterial-based optical sensors, including metal nanoparticles (MNPs), quantum dots (QDs), and carbon dots (CDs), offer distinct advantages over conventional small-molecule organic and inorganic sensors. This study mainly provides an overview of the recent advancements and challenges related to the design strategies of various optical nanosensors to selectively detect the $\mathrm{Ni}^{2+}$ ion. Emphasis has also been placed on comparing the sensing performance of various nanosensors, along with exploring future perspectives.
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Submitted 7 July, 2025;
originally announced July 2025.
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Quantum geometric moment encodes stacking order of moiré matter
Authors:
Surat Layek,
Subhajit Sinha,
Atasi Chakraborty,
Ayshi Mukherjee,
Heena Agarwal,
Kenji Watanabe,
Takashi Taniguchi,
Amit Agarwal,
Mandar M. Deshmukh
Abstract:
Exploring the topological characteristics of electronic bands is essential in condensed matter physics. Moiré materials featuring flat bands provide a versatile platform for engineering band topology and correlation effects. In moiré materials that break either time-reversal symmetry or inversion symmetry or both, electronic bands exhibit Berry curvature hotspots. Different stacking orders in thes…
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Exploring the topological characteristics of electronic bands is essential in condensed matter physics. Moiré materials featuring flat bands provide a versatile platform for engineering band topology and correlation effects. In moiré materials that break either time-reversal symmetry or inversion symmetry or both, electronic bands exhibit Berry curvature hotspots. Different stacking orders in these materials result in varied Berry curvature distributions within the flat bands, even when the band dispersion remains similar. However, experimental studies probing the impact of stacking order on the quantum geometric quantities are lacking. 1.4$^\circ$ twisted double bilayer graphene (TDBG) facilitates two distinct stacking orders (AB-AB, AB-BA) and forms an inversion broken \moire superlattice with electrically tunable flat bands. The valley Chern numbers of the flat bands depend on the stacking order, and the nonlinear Hall (NLH) effect distinguishes the differences in Berry curvature dipole (BCD), the first moment of Berry curvature. The BCD exhibits antisymmetric behavior, flipping its sign with the polarity of the perpendicular electric field in AB-AB TDBG, while it displays a symmetric behavior, maintaining the same sign regardless of the electric field's polarity in AB-BA TDBG. This approach electronically detects stacking-induced quantum geometry, while opening a pathway to quantum geometry engineering and detection.
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Submitted 26 February, 2025;
originally announced February 2025.
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Superconducting magic-angle twisted trilayer graphene hosts competing magnetic order and moiré inhomogeneities
Authors:
Ayshi Mukherjee,
Surat Layek,
Subhajit Sinha,
Ritajit Kundu,
Alisha H. Marchawala,
Mahesh Hingankar,
Joydip Sarkar,
L. D. Varma Sangani,
Heena Agarwal,
Sanat Ghosh,
Aya Batoul Tazi,
Kenji Watanabe,
Takashi Taniguchi,
Abhay N. Pasupathy,
Arijit Kundu,
Mandar M. Deshmukh
Abstract:
The microscopic mechanism of superconductivity in the magic-angle twisted graphene family, including magic-angle twisted trilayer graphene (MATTG), is poorly understood. Properties of MATTG, like Pauli limit violation, suggest unconventional superconductivity. Theoretical studies propose proximal magnetic states in the phase diagram, but direct experimental evidence is lacking. We show direct evid…
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The microscopic mechanism of superconductivity in the magic-angle twisted graphene family, including magic-angle twisted trilayer graphene (MATTG), is poorly understood. Properties of MATTG, like Pauli limit violation, suggest unconventional superconductivity. Theoretical studies propose proximal magnetic states in the phase diagram, but direct experimental evidence is lacking. We show direct evidence for an in-plane magnetic order proximal to the superconducting state using two complementary electrical transport measurements. First, we probe the superconducting phase by using statistically significant switching events from superconducting to the dissipative state of MATTG. The system behaves like a network of Josephson junctions due to lattice relaxation-induced moiré inhomogeneity in the system. We observe non-monotonic and hysteretic responses in the switching distributions as a function of temperature and in-plane magnetic field. Second, in normal regions doped slightly away from the superconducting regime, we observe hysteresis in magnetoresistance with an in-plane magnetic field; showing evidence for in-plane magnetic order that vanishes $\sim$900 mK. Additionally, we show a broadened Berezinskii-Kosterlitz-Thouless transition due to relaxation-induced moiré inhomogeneity. We find superfluid stiffness $J_{\mathrm{s}}$$\sim$0.15 K with strong temperature dependence. Theoretically, the magnetic and superconducting order arising from the magnetic order's fluctuations have been proposed - we show direct evidence for both. Our observation that the hysteretic magnetoresistance is sensitive to the in-plane field may constrain possible intervalley-coherent magnetic orders and the resulting superconductivity that arises from its fluctuations.
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Submitted 4 June, 2024;
originally announced June 2024.
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Magnetic and structural properties of the iron silicide superconductor LaFeSiH
Authors:
M. F. Hansen,
S. Layek,
J. -B. Vaney,
L. Chaix,
M. R. Suchomel,
M. Mikolasek,
G. Garbarino,
A. Chumakov,
R. Rüffer,
V. Nassif,
T. Hansen,
E. Elkaim,
T. Pelletier,
H. Mayaffre,
F. Bernardini,
A. Sulpice,
M. Núñez-Regueiro,
P. Rodière,
A. Cano,
S. Tencé,
P. Toulemonde,
M. -H. Julien,
M. d'Astuto
Abstract:
The magnetic and structural properties of the recently discovered pnictogen/chalcogen-free superconductor LaFeSiH ($T_c\simeq10$~K) have been investigated by $^{57}$Fe synchrotron M{ö}ssbauer source (SMS) spectroscopy, x-ray and neutron powder diffraction and $^{29}$Si nuclear magnetic resonance spectroscopy (NMR). No sign of long range magnetic order or local moments has been detected in any of t…
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The magnetic and structural properties of the recently discovered pnictogen/chalcogen-free superconductor LaFeSiH ($T_c\simeq10$~K) have been investigated by $^{57}$Fe synchrotron M{ö}ssbauer source (SMS) spectroscopy, x-ray and neutron powder diffraction and $^{29}$Si nuclear magnetic resonance spectroscopy (NMR). No sign of long range magnetic order or local moments has been detected in any of the measurements and LaFeSiH remains tetragonal down to 2 K. The activated temperature dependence of both the NMR Knight shift and the relaxation rate $1/T_1$ is analogous to that observed in strongly overdoped Fe-based superconductors. These results, together with the temperature-independent NMR linewidth, show that LaFeSiH is an homogeneous metal, far from any magnetic or nematic instability, and with similar Fermi surface properties as strongly overdoped iron pnictides. This raises the prospect of enhancing the $T_c$ of LaFeSiH by reducing its carrier concentration through appropriate chemical substitutions. Additional SMS spectroscopy measurements under hydrostatic pressure up to 18.8~GPa found no measurable hyperfine field.
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Submitted 5 September, 2023;
originally announced September 2023.
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Lattice dynamics in the FeSi-based family of superconductors
Authors:
Samar Layek,
Mads Fonager Hansen,
Jean-Baptiste Vaney,
Pierre Toulemonde,
Sophie Tencé,
Philippe Boullay,
Andres Cano,
Marie-Aude Méasson
Abstract:
The lattice dynamics of the superconducting materials LaFeSiH and LaFeSiO as well as their intermetallic precursor LaFeSi are investigated by polarized Raman spectroscopy and first-principles calculations, together with X-ray and advanced electron diffraction techniques for their structural analysis. We find that the Fe-dominated Raman-active modes reflect the chemical peculiarities of these silic…
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The lattice dynamics of the superconducting materials LaFeSiH and LaFeSiO as well as their intermetallic precursor LaFeSi are investigated by polarized Raman spectroscopy and first-principles calculations, together with X-ray and advanced electron diffraction techniques for their structural analysis. We find that the Fe-dominated Raman-active modes reflect the chemical peculiarities of these silicides compared to their pnictide counterparts, with enhanced structural couplings between the FeSi layer and the spacer that can be related to the ionic vs covalent character of the latter. In addition, we find signatures of enhanced electron-phonon coupling for some of the Raman-active modes. Beyond that, our study reveals intriguing Fe-based Raman features as well as structural subtleties in LaFeSiH suggesting that this superconductor may formally be non-centrosymmetric.
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Submitted 6 March, 2024; v1 submitted 24 July, 2023;
originally announced July 2023.
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Perpendicular electric field drives Chern transitions and layer polarization changes in Hofstadter bands
Authors:
Pratap Chandra Adak,
Subhajit Sinha,
Debasmita Giri,
Dibya Kanti Mukherjee,
Chandan,
L. D. Varma Sangani,
Surat Layek,
Ayshi Mukherjee,
Kenji Watanabe,
Takashi Taniguchi,
H. A. Fertig,
Arijit Kundu,
Mandar M. Deshmukh
Abstract:
Moiré superlattices engineer band properties and enable observation of fractal energy spectra of Hofstadter butterfly. Recently, correlated-electron physics hosted by flat bands in small-angle moiré systems has been at the foreground. However, the implications of moiré band topology within the single-particle framework are little explored experimentally. An outstanding problem is understanding the…
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Moiré superlattices engineer band properties and enable observation of fractal energy spectra of Hofstadter butterfly. Recently, correlated-electron physics hosted by flat bands in small-angle moiré systems has been at the foreground. However, the implications of moiré band topology within the single-particle framework are little explored experimentally. An outstanding problem is understanding the effect of band topology on Hofstadter physics, which does not require electron correlations. Our work experimentally studies Chern state switching in the Hofstadter regime using twisted double bilayer graphene (TDBG), which offers electric field tunable topological bands, unlike twisted bilayer graphene. Here we show that the nontrivial topology reflects in the Hofstadter spectra, in particular, by displaying a cascade of Hofstadter gaps that switch their Chern numbers sequentially while varying the perpendicular electric field. Our experiments together with theoretical calculations suggest a crucial role of charge polarization changing concomitantly with topological transitions in this system. Layer polarization is likely to play an important role in the topological states in few-layer twisted systems. Moreover, our work establishes TDBG as a novel Hofstadter platform with nontrivial magnetoelectric coupling.
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Submitted 19 December, 2022;
originally announced December 2022.
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Hidden Granular Superconductivity Above 500K in off-the-shelf graphite materials
Authors:
Raphael Rousset-Zenou,
Samar Layek,
Miguel Monteverde,
Frédéric Gay,
Didier Dufeu,
Manuel Núñez-Regueiro
Abstract:
It has been reported that graphite hosts room temperature superconductivity. Here we provide new results that confirm these claims on different samples of highly oriented pyrolytic graphite (HOPG) and commercial flexible graphite gaskets (FGG). After subtraction of the intrinsic graphite diamagnetism, magnetization measurements show convoluted ferromagnetism and superconducting-like hysteresis loo…
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It has been reported that graphite hosts room temperature superconductivity. Here we provide new results that confirm these claims on different samples of highly oriented pyrolytic graphite (HOPG) and commercial flexible graphite gaskets (FGG). After subtraction of the intrinsic graphite diamagnetism, magnetization measurements show convoluted ferromagnetism and superconducting-like hysteresis loops. The ferromagnetism is deconvoluted by fitting with a sigmoidal function and subtracting it from the data. The obtained superconducting-like hysteresis loops are followed to the highest available temperature, 400K. The extrapolation of the decrease of its moment width with temperature indicates a transition temperature T$_{c}$$\sim$ 550K$\pm$50K for all samples. Electrical resistance measurements confirm the existence at these temperatures of a transition in HOPG samples, albeit without percolation. Besides, the FGG show transitions at temperatures (70K, 270K) near to those reported previously on intercalated-deintercalated graphite, confirming the general character of these superconducting transitions. These results are the first steps in the unveiling of the above room temperature superconductivity of graphite.
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Submitted 19 July, 2022;
originally announced July 2022.
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Possible high temperature superconducting transitions in disordered graphite obtained from room temperature deintercalated KC$_8$
Authors:
Samar Layek,
Miguel Monteverde,
Gastón Garbarino,
Marie-Aude Méasson,
André Sulpice,
Nedjma Bendiab,
Pierre Rodière,
Romain Cazali,
Abdellali Hadj-Azzem,
Vivian Nassif,
Daniel Bourgault,
Frédéric Gay,
Didier Dufeu,
Sébastien Pairis,
Jean-Louis Hodeau,
Manuel Núñez-Regueiro
Abstract:
Although progress with twisted graphene nano-devices is boosting the superconductivity that is the consequence of their Moiré flat electronic bands, the immense choice for future development is an obstacle for their optimisation. We report here that soft-chemistry deintercalation of KC$_8$ breaks down graphite stacking generating a strong disorder that includes stacking twists and variable local d…
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Although progress with twisted graphene nano-devices is boosting the superconductivity that is the consequence of their Moiré flat electronic bands, the immense choice for future development is an obstacle for their optimisation. We report here that soft-chemistry deintercalation of KC$_8$ breaks down graphite stacking generating a strong disorder that includes stacking twists and variable local doping. We obtain a bulk graphite whose individual crystallites have different stackings with arbitrary twists and doping, scanning in the same sample a huge number of stacking configurations. We perform magnetisation measurements on batches with different synthesis conditions. The disorder weakens the huge diamagnetism of graphite, revealing several phase transitions. A "ferromagnetic-like" magnetisation appears with Curie temperatures T$_0$$\sim$450K, that has to be subtracted from the measured magnetisation. Depending on sample synthesis, anomalies towards diamagnetic states appear at T$_c$$\sim$110K (3 samples), $\sim$240K (4 samples), $\sim$320K (2 samples). Electrical resistivity measurements yield anomalies for the T$_c\sim$240K transition, with one sample showing a 90% drop. We discuss the possibility that these (diamagnetic and resistitive) anomalies could be due to superconductivity.
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Submitted 19 May, 2022;
originally announced May 2022.
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Interplay between structural and magnetic-electronic responses of FeAl$_{2}$O$_{4}$ to a megabar: site inversion and spin crossover
Authors:
W. M. Xu,
G. R. Hearne,
S. Layek,
D. Levy,
M. P. Pasternak,
G. Kh. Rozenberg,
E. Greenberg
Abstract:
X-ray diffraction pressure studies at room temperature demonstrate that the spinel FeAl$_{2}$O$_{4}$ transforms to a tetragonal phase at ~18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to ~45 GPa, after which a transformation to a Cmcm post-spinel phase is onset. This is attributable to pressure driven Fe-Al site inversion at room temperature, c…
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X-ray diffraction pressure studies at room temperature demonstrate that the spinel FeAl$_{2}$O$_{4}$ transforms to a tetragonal phase at ~18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to ~45 GPa, after which a transformation to a Cmcm post-spinel phase is onset. This is attributable to pressure driven Fe-Al site inversion at room temperature, corroborated by signatures in the 57Fe Mössbauer spectroscopy pressure data. At the tetragonal to post-spinel transition, onset in the range 45-50 GPa, there is a concurrent emergence of a non-magnetic spectral component in the Mössbauer data at variable cryogenic temperatures. This is interpreted as spin crossover at sixfold coordinated Fe locations emanated from site inversion. Spin crossover commences at the end of the pressure range of the tetragonal phase and progresses in the post-spinel structure. There is also a much steeper volume change dV/V ~ 10% in the range 45-50 GPa compared to the preceding pressure regime, from the combined effects of the structural transition and spin crossover electronic change. At the highest pressure attained, ~106 GPa, the Mössbauer data evidences a diamagnetic Fe low-spin abundance of ~50%. The rest of the high-spin Fe in eightfold coordinated sites continue to experience a relatively small internal magnetic field of ~33 T. This is indicative of a magnetic ground state associated with strong covalency, as well as substantive disorder from site inversion and the mixed spin-state configuration. Intriguingly magnetism survives in such a spin-diluted post-spinel lattice at high densities. The R(300 K) data decreases by only two orders of magnitude from ambient pressure to the vicinity of ~100 GPa. Despite a ~26% unit-cell volume densification from the lattice compressibility, structural transitions and spin crossover.
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Submitted 25 September, 2018;
originally announced September 2018.
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Site-specific spin crossover in Fe$_{2}$TiO$_{4}$ post-spinel under high pressures to near a megabar
Authors:
W. M. Xu,
G. R. Hearne,
S. Layek,
D. Levy,
J-P. Itié,
M. P. Pasternak,
G. Kh. Rozenberg,
E. Greenberg
Abstract:
X-ray diffraction studies to ~90 GPa at room temperature show that Fe$_{2}$TiO$_{4}$ ferrous inverse spinel undergoes the following sequence of structural transitions : cubic (Fd3m) to tetragonal (I41/amd)to orthorhombic(Cmcm) to orthorhombic(Pmma),at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahe…
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X-ray diffraction studies to ~90 GPa at room temperature show that Fe$_{2}$TiO$_{4}$ ferrous inverse spinel undergoes the following sequence of structural transitions : cubic (Fd3m) to tetragonal (I41/amd)to orthorhombic(Cmcm) to orthorhombic(Pmma),at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahedral sites constituting ~25% of all Fe locations. This is manifest as a steeper volume decrease of dV/V0 ~ 3.5% beyond ~40 GPa and an emergent diamagnetic component discerned in 57Fe Mössbauer spectroscopy at variable cryogenic temperatures. A subsequent Cmcm to Pmma Fe/Ti disorder-order reconfiguration is facilitated at 6-fold coordinated (octahedral) sites. The rest of the high-spin Fe in 6-fold and 8-fold coordinated sites (~75% abundance) in the Pmma phase exhibit average saturation internal magnetic fields of H$_{hf}$ ~ 42 T to ~90 GPa, typical of spin-only (orbitally quenched) Fermi-contact values. By contrast average H$_{hf}$ ~ 20 T values, signifying unquenched orbital moments, occur below the 40-45 GPa spin-crossover initiation regime in the Cmcm phase. Therefore site-specific spin crossover invokes a cooperative lattice response and polyhedral distortions at the rest of the high-spin Fe sites, translating to 3d level (sub-band) changes and consequential orbital moment quenching. Near ~90 GPa Fe$_{2}$TiO$_{4}$ is a partially spin-converted chemically ordered Pmma post-spinel structure having a persistent charge gap of ~100 meV. Despite structural symmetry changes, partial spin crossover and lattice compressibility resulting in a ~33% total reduction in unit-cell volume and corresponding 3d bandwidth broadening, strong electron correlations persist at high densification.
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Submitted 24 September, 2018;
originally announced September 2018.
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Pressure-driven collapse of the relativistic electronic ground state in a honeycomb iridate
Authors:
J. P. Clancy,
H. Gretarsson,
J. A. Sears,
Yogesh Singh,
S. Desgreniers,
Kavita Mehlawat,
Samar Layek,
Gregory Kh. Rozenberg,
Yang Ding,
M. H. Upton,
D. Casa,
N. Chen,
Junhyuck Im,
Yongjae Lee,
R. Yadav,
L. Hozoi,
D. Efremov,
J. van den Brink,
Young-June Kim
Abstract:
The electronic ground state in many iridate materials is described by a complex wave-function in which spin and orbital angular momenta are entangled due to relativistic spin-orbit coupling (SOC). Such a localized electronic state carries an effective total angular momentum of $J_{eff}=1/2$. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and…
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The electronic ground state in many iridate materials is described by a complex wave-function in which spin and orbital angular momenta are entangled due to relativistic spin-orbit coupling (SOC). Such a localized electronic state carries an effective total angular momentum of $J_{eff}=1/2$. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and Na2IrO3, these $J_{eff}=1/2$ moments are expected to be coupled through a special bond-dependent magnetic interaction, which is a necessary condition for the realization of a Kitaev quantum spin liquid. However, this relativistic electron picture is challenged by an alternate description, in which itinerant electrons are confined to a benzene-like hexagon, keeping the system insulating despite the delocalized nature of the electrons. In this quasi-molecular orbital (QMO) picture, the honeycomb iridates are an unlikely choice for a Kitaev spin liquid. Here we show that the honeycomb iridate Li2IrO3 is best described by a $J_{eff}=1/2$ state at ambient pressure, but crosses over into a QMO state under the application of small (~ 0.1 GPa) hydrostatic pressure. This result illustrates that the physics of iridates is extremely rich due to a delicate balance between electronic bandwidth, spin-orbit coupling, crystal field, and electron correlation.
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Submitted 11 March, 2018;
originally announced March 2018.
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Pressure induced spin crossover in disordered α-LiFeO2
Authors:
Samar Layek,
Eran Greenberg,
Weiming Xu,
Gregory Kh. Rozenberg,
Moshe P. Pasternak,
Jean-Paul Itié,
Dániel G. Merkel
Abstract:
Structural, magnetic and electrical-transport properties of α-LiFeO2, crystallizing in the rock salt structure with random distribution of Li and Fe ions, have been studied by synchrotron X-ray diffraction, 57Fe Mössbauer spectroscopy and electrical resistance measurements at pressures up to 100 GPa using diamond anvil cells. It was found that the crystal structure is stable at least to 82 GPa, th…
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Structural, magnetic and electrical-transport properties of α-LiFeO2, crystallizing in the rock salt structure with random distribution of Li and Fe ions, have been studied by synchrotron X-ray diffraction, 57Fe Mössbauer spectroscopy and electrical resistance measurements at pressures up to 100 GPa using diamond anvil cells. It was found that the crystal structure is stable at least to 82 GPa, though a significant change in compressibility has been observed above 50 GPa. The changes in the structural properties are found to be on a par with a sluggish Fe3+ high- to low-spin (HS-LS) transition (S=5/2 to S=1/2) starting at 50 GPa and not completed even at ~100 GPa. The HS-LS transition is accompanied by an appreciable resistance decrease remaining a semiconductor up to 115 GPa and is not expected to be metallic even at about 200 GPa. The observed feature of the pressure-induced HS-LS transition is not an ordinary behavior of ferric oxides at high pressures. The effect of Fe3+ nearest and next nearest neighbors on the features of the spin crossover is discussed.
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Submitted 27 September, 2017;
originally announced September 2017.
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Superconductor-insulator transition in fcc-GeSb2Te4 at elevated pressures
Authors:
Bar Hen,
Samar Layek,
Moshe Goldstein,
Victor Shelukhin,
Mark Shulman,
Michael Karpovski,
Eran Greenberg,
Eran Sterer,
Yoram Dagan,
Gregory Kh. Rozenberg,
Alexander Palevski
Abstract:
We show that polycrystalline GeSb2Te4 in the fcc phase (f-GST), which is an insulator at low temperature at ambient pressure, becomes a superconductor at elevated pressures. Our study of the superconductor to insulator transition versus pressure at low temperatures reveals a second order quantum phase transition with linear scaling (critical exponent close to unity) of the transition temperature w…
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We show that polycrystalline GeSb2Te4 in the fcc phase (f-GST), which is an insulator at low temperature at ambient pressure, becomes a superconductor at elevated pressures. Our study of the superconductor to insulator transition versus pressure at low temperatures reveals a second order quantum phase transition with linear scaling (critical exponent close to unity) of the transition temperature with the pressure above the critical zero-temperature pressure. In addition, we demonstrate that at higher pressures the f-GST goes through a structural phase transition via amorphization to bcc GST (b-GST), which also become superconducting. We also find that the pressure regime where an inhomogeneous mixture of amorphous and b-GST exists, there is an anomalous peak in magnetoresistance, and suggest an explanation for this anomaly.
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Submitted 6 September, 2017;
originally announced September 2017.
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Theoretical and experimental evidence of a site-selective Mott transition in Fe2O3 under pressure
Authors:
E. Greenberg,
I. Leonov,
S. Layek,
Z. Konopkova,
M. P. Pasternak,
L. Dubrovinsky,
R. Jeanloz,
I. A. Abrikosov,
G. Kh. Rozenberg
Abstract:
We provide experimental and theoretical evidence for a novel type of pressure-induced insulator-metal transition characterized by site-selective delocalization of the electrons. Mössbauer spectroscopy, X-ray diffraction and electrical transport measurements on Fe$_2$O$_3$ to 100 GPa, along with dynamical mean-field theory (DFT+DMFT) calculations, reveal this site-selective Mott transition between…
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We provide experimental and theoretical evidence for a novel type of pressure-induced insulator-metal transition characterized by site-selective delocalization of the electrons. Mössbauer spectroscopy, X-ray diffraction and electrical transport measurements on Fe$_2$O$_3$ to 100 GPa, along with dynamical mean-field theory (DFT+DMFT) calculations, reveal this site-selective Mott transition between 50 and 68 GPa, such that the metallization can be described by ($^\rm{VI}$Fe$^{3+\rm{HS}}$)$_2$O$_3$ [$R\bar{3}c$ structure] $\overrightarrow{\tiny\rm{50~GPa}}$ ($^\rm{VIII}$Fe$^{3+\rm{HS~VI}}$Fe$^\rm{M}$)O$_3$ [$P2_1/n$ structure] $\overrightarrow{\tiny\rm{68~GPa}}$ ($^\rm{VI}$Fe$^\rm{M}$)$_2$O$_3$ [$Aba2$ structure]. Within the $P2_1/n$ crystal structure, characterized by two distinct coordination sites (VI and VIII), we observe equal abundances of ferric ions (Fe$^{3+}$) and ions having delocalized electrons (Fe$^\rm{M}$), and only at higher pressures is a fully metallic $Aba2$ structure obtained, all at room temperature. The transition is characterized by delocalization/metallization of the $3d$ electrons on half the Fe sites, with a site-dependent collapse of local moments. Above $\sim$50 GPa, Fe$_2$O$_3$ is a strongly correlated metal with reduced electron mobility (large band renormalizations) of m*/m$\sim$4 and 6 near the Fermi level. Upon decompression, we observe a site-selective (metallic) to conventional Mott insulator phase transition ($^\rm{VIII}$Fe$^{3+\rm{HS~VI}}$Fe$^\rm{M}$)O$_3$ $\overrightarrow{\tiny\rm{50~GPa}}$ ($^\rm{VIII}$Fe$^{3+\rm{HS~VI}}$Fe$^{3+ \rm{HS}}$)O$_3$ within the same $P2_1/n$ structure, indicating a decoupling of the electronic and lattice degrees of freedom, characteristic of a true Mott transition. Our results show that the interplay of electronic correlations and lattice may result in rather complex behavior of the electronic structure and magnetic state.
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Submitted 8 June, 2017;
originally announced June 2017.
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Superconductivity in multiple phases of compressed GeSb2Te4
Authors:
E. Greenberg,
B. Hen,
Samar Layek,
I. Pozin,
R. Friedman,
V. Shelukhin,
Y. Rosenberg,
M. Karpovski,
M. P. Pasternak,
E. Sterer,
Y. Dagan,
G. Kh. Rozenberg,
A. Palevski
Abstract:
Here we report the discovery of superconductivity in multiple phases of the compressed GeSb2Te4 (GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous (a-GST) and for the crystalline (c-GST)…
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Here we report the discovery of superconductivity in multiple phases of the compressed GeSb2Te4 (GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous (a-GST) and for the crystalline (c-GST) phases. The superconducting critical temperature, TC, continuously increases with the applied pressure reaching a maximum Tc =6K at P=20 GPa for a-GST, whereas the critical temperature of the cubic phase reaches a maximum Tc =8 K at 30 GPa. This new material system, exhibiting a superconductor-insulator quantum phase transition (SIT) has an advantage over disordered metals since it has a continuous control of the crystal structure and the electronic properties using pressure as an external stimulus, which was lacking in SIT studies until today.
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Submitted 5 May, 2016;
originally announced May 2016.
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Mössbauer and Magnetic Studies of Surfactant Mediated Ca-Mg Doped Ferrihydrite Nanoparticles
Authors:
Samar Layek,
M. Mohapatra,
S. Anand,
H. C. Verma
Abstract:
Ultrafine (2-5 nm) particles of amorphous Ca-Mg co-doped ferrihydrite have been synthesized by surfactant mediated co-precipitation method. The evolution of the amorphous ferrihydrite by Ca-Mg co-doping is quite different from our earlier investigations on individual Ca and Mg doping studies. Amorphous phase of ferrihydrite for the present study has been confirmed by X-ray diffraction (XRD) and Mö…
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Ultrafine (2-5 nm) particles of amorphous Ca-Mg co-doped ferrihydrite have been synthesized by surfactant mediated co-precipitation method. The evolution of the amorphous ferrihydrite by Ca-Mg co-doping is quite different from our earlier investigations on individual Ca and Mg doping studies. Amorphous phase of ferrihydrite for the present study has been confirmed by X-ray diffraction (XRD) and Mössbauer spectroscopy at room temperature and low temperatures (40 K and 20 K). Hematite nanoparticles with crystallite size about 8, 38 and 70 nm were obtained after annealing the as-prepared samples at 400, 600 and 800$^0$C respectively in air atmosphere. Superparamagnetism has been found in 8 nm sized hematite nanoparticles which has been confirmed from the magnetic hysteresis loop with zero remanent magnetization and coercive field and also from the superparamagnetic doublet of its room temperature Mössbauer spectrum. The magnetic properties of the 38 and 70 nm sized particles have been studied by room temperature magnetic hysteresis loop measurements and Mössbauer spectroscopy. The coercive field in these hematite nanoparticles increases with increasing particle size. Small amount of spinel MgFe$_{2}$O$_{4}$ phase has been detected in the 800$^0$C annealed sample.
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Submitted 26 February, 2015;
originally announced February 2015.
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Preparation, structural and magnetic studies on BiFe1-xCrxO3 (x=0.0, 0.05 and 0.1) multiferroic nanoparticles
Authors:
Samar Layek,
Santanu Saha,
H. C. Verma
Abstract:
BiFe1-xCrxO3 (x=0.0, 0.05 and 0.1) nanoparticles are prepared by a combustion method without using any solvent. All the synthesized nanoparticles are single phase in nature, nearly spherical in shape and crystallize in distorted perovskite structure and space group R3c with an average crystallite size of the order of 40 nm. The room temperature magnetization observed in BiFeO3 nanoparticles is lar…
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BiFe1-xCrxO3 (x=0.0, 0.05 and 0.1) nanoparticles are prepared by a combustion method without using any solvent. All the synthesized nanoparticles are single phase in nature, nearly spherical in shape and crystallize in distorted perovskite structure and space group R3c with an average crystallite size of the order of 40 nm. The room temperature magnetization observed in BiFeO3 nanoparticles is larger than that in the bulk. Increasing Cr doping leads to increase in the magnetization and coercivity. Strong superexchange interaction between Fe3+ and Cr3+ atoms is likely to give rise to such increase in magnetization with Cr-doping. Mössbauer data of these nanoparticles show ordered magnetic state in which Fe atoms are in 3+ oxidation states.
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Submitted 20 February, 2015;
originally announced February 2015.
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Magnetic and Dielectric Properties of Multiferroic BiFeO3 Nanoparticles Synthesized by a Novel Citrate Combustion Method
Authors:
Samar Layek,
H. C. Verma
Abstract:
Single phase BiFeO3 nanoparticles have been successfully synthesized for the first time by a novel citrate combustion method without using any solvent. Well mixed metal nitrates along with citric acid which is used as fuel combust to give BiFeO3 nanoparticles after annealing. These particles are single phase in nature and crystallize in the rhombohedral distorted perovskite structure (space group-…
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Single phase BiFeO3 nanoparticles have been successfully synthesized for the first time by a novel citrate combustion method without using any solvent. Well mixed metal nitrates along with citric acid which is used as fuel combust to give BiFeO3 nanoparticles after annealing. These particles are single phase in nature and crystallize in the rhombohedral distorted perovskite structure (space group-R3c) which has been confirmed by the Rietveld refinement of the room temperature powder x-ray diffraction data. Nearly spherical particles of average particle size 47 nm have been seen from transmission electron micrograph. Room temperature magnetic hysteresis measurement shows weak ferromagnetism though the magnetization does not saturate upto 1.75 T applied field. The coercive field value is calculated to be 180 Oe which is 3 times higher than that prepared by solvent free combustion method using Glycine. 57Fe Mössbauer spectrum can be fitted with a sextet corresponding to single magnetic state of hyperfine field about 49.5 T corresponding to Fe3+ state of the iron atom. The dielectric relaxation and ac conductivity as a function of frequency have been discussed. High dielectric permittivity has not been found in these nanoparticles like other reported BiFeO3 ceramics.
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Submitted 20 February, 2015;
originally announced February 2015.
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Valence fluctuation in Ce$_{2}$Co$_{3}$Ge$_{5}$ and crystal field effect in Pr$_{2}$Co$_{3}$Ge$_{5}$
Authors:
Samar Layek,
V. K. Anand,
Z. Hossain
Abstract:
Polycrystalline samples of ternary rare earth germanides R$_{2}$Co$_{3}$Ge$_{5}$(R = La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U$_{2}$Co$_{3}$Si$_{5}$ structure (space group \textit{Ibam}). No evidence of magnetic or superc…
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Polycrystalline samples of ternary rare earth germanides R$_{2}$Co$_{3}$Ge$_{5}$(R = La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U$_{2}$Co$_{3}$Si$_{5}$ structure (space group \textit{Ibam}). No evidence of magnetic or superconducting transition is observed in any of these compounds down to 2 K. The unit cell volume of Ce$_{2}$Co$_{3}$Ge$_{5}$ deviates from the expected lanthanide contraction, indicating non trivalent state of Ce ions in this compound. The reduced value of effective moment ($μ_{eff}$ $\approx$ 0.95 $μ_{B}$) compared to that expected for trivalent Ce ions further supports valence fluctuating nature of Ce in Ce$_{2}$Co$_{3}$Ge$_{5}$. The observed temperature dependence of magnetic susceptibility is consistent with ionic interconfiguration fluctuation (ICF) model. Although no sharp anomaly due to a phase transition is seen, a broad Schottky-type anomaly is observed in the magnetic part of specific heat of Pr$_{2}$Co$_{3}$Ge$_{5}$. An analysis of $C_{mag}$ data suggests a singlet ground state in Pr$_{2}$Co$_{3}$Ge$_{5}$ separated from the singlet first excited state by 22 K and a doublet second excited state at 73 K.
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Submitted 26 August, 2009;
originally announced August 2009.