arXiv subjects

Rajib Mondal

Publications and source records attributed to Rajib Mondal.

14 recordsLinked to original sources

arXiv2026

Room temperature intrinsic anomalous Hall effect in disordered half-metallic ferromagnetic quaternary Heusler alloy CoRuFeSi

Quaternary Heusler alloys offer a versatile platform for engineering magnetic and topological transport phenomena through chemical flexibility and tunable disorder. Here, we report a comprehensive experimental and theoretical investigation of the magnetic, magnetotransport, and anomalous Hall properties of the quaternary Heusler alloy CoRuFeSi. The compound crystallizes in the LiMgPdSn-type structure with significant Co--Ru antisite disorder and exhibits soft ferromagnetism with a saturation magnetization of $4.21~μ_{\mathrm{B}}/\mathrm{f.u.}$ at low temperature and a Curie temperature well above room temperature. Hall measurements reveal a robust anomalous Hall effect persisting up to 300~K, with an anomalous Hall conductivity of $\sim 74$~S/cm that is nearly temperature independent. Scaling analysis demonstrates that the anomalous Hall response is dominated by the intrinsic Berry-curvature mechanism. First-principles calculations identify CoRuFeSi as a topologically nontrivial nodal-line semimetal in its ordered phase. Incorporation of experimentally relevant Co--Ru antisite disorder redistributes the Berry curvature and quantitatively reproduces the experimentally observed anomalous Hall conductivity, while preserving half-metallicity. These results establish CoRuFeSi as a disorder-tolerant half-metallic ferromagnet with a sizable intrinsic anomalous Hall effect at room temperature, highlighting its potential for spintronic and Hall-based device applications.

cond-mat.mtrl-sci↗

arXiv2024

Probing the superconducting gap structure of ScRuSi via $μ$SR and first-principles calculations

In this study, we present a thorough investigation into the superconducting state of the ruthenium-based ternary equiatomic compound ScRuSi. Our analysis combines experimental techniques, including muon spin rotation/relaxation ($μ$SR) and low-temperature resistivity measurements, with theoretical insights derived from first-principles calculations. The low-temperature resistivity measurements reveal a distinct superconducting phase transition in the orthorhombic structure of ScRuSi at a critical temperature ($T_\text{C}$) of $2.5$ K. Further, the TF-$μ$SR analysis yields a gap-to-critical-temperature ratio of $2Δ/k_\mathrm{B}T_\mathrm{C} = 2.71$, a value consistent with results obtained from previous heat capacity measurements. The temperature dependence of the superconducting normalized depolarization rate is fully described by the isotropic $s$-wave gap model. Additionally, zero-field $μ$SR measurements indicate that the relaxation rate remains nearly identical below and above $T_\text{C}$. This observation strongly suggests the preservation of time-reversal symmetry within the superconducting state. By employing the McMillan-Allen-Dynes equation, we calculate a $T_\text{C}$ of $2.11$ K from first-principles calculations within the density functional theory framework. This calculated value aligns closely with the experimentally determined critical temperature. The coupling between the low-frequency phonon modes and the transition metal d-orbital states play an important role in governing the superconducting pairing in ScRuSi. The combination of experimental and theoretical approaches provides a comprehensive microscopic understanding of the superconducting nature of ScRuSi, offering insights into its critical temperature, pairing symmetry, and the underlying electron-phonon coupling mechanism.

cond-mat.supr-con↗

arXiv2024

Dimension-Dependent Critical Scaling Analysis and Emergent Competing Interaction Scales in a 2D Van der Waals magnet Cr$_{2}$Ge$_{2}$Te$_{6}$

We investigate thickness-dependent transformation from a paramagnetic to ferromagnetic phase in Cr$_{2}$Ge$_{2}$Te$_{6}$ (CGT) in bulk and few-layer flake forms. 2D Ising-like critical transition in bulk CGT occurs at $T_{c}$ = 67 K with out-of-plane magnetic anisotropy. Few-layer CGT on hBN/SiO$_{2}$/Si substrate displays the same $T_{c}$ but also exhibits a new critical transition at $T^{\prime}_c$ = 14.2 K. Here, critical scaling analysis reveals the critical exponents differ significantly from those in bulk and do not align with the known universality classes. Our Density Functional Theory (DFT) and classical calculations indicate competition between magnetocrystalline and dipolar anisotropy emerges with reduced dimensions. The observed behavior is due to minor structural distortions in low dimensional CGT, which modify the balance between spin-orbit coupling, exchange interactions and dipolar anisotropy. This triggers a critical crossover at $T^{\prime}_c$. Our study shows the emergence of a complex interplay of short- and long-range interactions below $T^{\prime}_c$ as CGT approaches the 2D limit.

cond-mat.mtrl-sci↗

arXiv2023

Emergence of a Non-van der Waals Magnetic Phase in a van der Waals Ferromagnet

Manipulation of long-range order in two-dimensional (2D) van der Waals (vdW) magnetic materials (e.g., CrI$_3$, CrSiTe$_3$ etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets which, in turn, affects the nanoelectronic/spintronic device performance. Counterintuitively, our current study reveals that exposure to the air at ambient atmosphere results in advent of a stable nonlayered secondary ferromagnetic phase in the form of Cr$_2$Te$_3$ (T$_{C2}$ ~ 160 K) in the parent vdW magnetic semiconductor Cr$_2$Ge$_2$Te$_6$ (T$_{C1}$ ~ 69 K). In addition, the magnetic anisotropy energy (MAE) enhances in the hybrid by an order from the weakly anisotropic pristine Cr$_2$Ge$_2$Te$_6$ crystal, increasing the stability of the FM ground state with time. Comparing with the freshly prepared Cr$_2$Ge$_2$Te$_6$, the coexistence of the two ferromagnetic phases in the time elapsed bulk crystal is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, and magnetotransport measurement. To capture the concurrence of the two ferromagnetic phases in a single material, Ginzburg-Landau theory with two independent order parameters (as magnetization) with a coupling term can be introduced. In contrast to rather common poor environmental stability of the vdW magnets, our results open possibilities of finding air-stable novel materials having multiple magnetic phases.

cond-mat.mtrl-sci↗

arXiv2022

Behavior of gapped and ungapped Dirac cones in an antiferromagnetic topological metal, SmBi

We studied the behavior of nontrivial Dirac fermion states in an antiferromagnetic metal SmBi using angle-resolved photoemission spectroscopy (ARPES). The experimental results exhibit multiple Fermi pockets around $\overlineΓ$ and $\overline{M}$ points along with a band inversion in the spectrum along the $\overlineΓ$-$\overline{M}$ line consistent with the density functional theory results. In addition, ARPES data reveal Dirac cones at $\overlineΓ$ and $\overline{M}$ points within the energy gap of the bulk bands. The Dirac cone at $\overline{M}$ exhibit a distinct Dirac point and is intense in the high photon energy data while the Dirac cone at $\overlineΓ$ is intense at low photon energies. Employing ultra-high-resolution ARPES, we discover destruction of a Fermi surface constituted by the surface states across the Neél temperature of 9 K. Interestingly, the Dirac cone at $\overlineΓ$ is found to be gapped at 15 K and the behavior remains similar across the magnetic transition. These results reveal complex momentum dependent gap formation and fermi surface destruction across magnetic transition in an exotic correlated topological material; the interplay between magnetism and topology in this system calls for ideas beyond existing theoretical models.

cond-mat.str-el↗

arXiv2022

Shubnikov-de Haas and de Haas-van Alphen oscillation in Czochralski grown CoSi single crystal

Anisotropic transport, Shubnikov-de Haas (SdH), and de Haas-van Alphen (dHvA) quantum oscillations studies are reported on a high-quality CoSi single crystal grown by the Czochralski method. Temperature-dependent resistivities indicate the dominating electron-electron scattering. Magnetoresistance (MR) at 2 K reaches 610% for I||[111] and B||[01-1], whereas it is 500% for I||[01-1] and B||[111]. A negative slope in field-dependent Hall resistivity suggests electrons are the majority carriers. The carrier concentration extracted from Hall conductivity indicates no electron-hole compensation. In 3D CoSi, the electron transport lifetime is found to be approximately in the same order as quantum lifetime, whereas in 2D electron gas the long-range scattering drives the transport life much larger than the quantum lifetime. From linear and Hall SdH oscillations the effective masses and Dingle temperatures have been calculated. The dHvA oscillation reveals three frequencies at 18 ($γ$), 558 ($α$) and 663 T ($β$)), whereas, SdH oscillation results in only two frequencies $α$ and $β$. The $γ$ frequency observed in dHvA oscillation is a tiny hole pocket at the $Γ$ point.

cond-mat.mtrl-sci↗

arXiv2021

Weak antilocalization and Shubnikov-de Haas oscillations in CaCuSb single crystal

Quantum oscillations in both linear and Hall resistivities and weak antilocalization (WAL) are barely observed in bulk single crystals. Here we report the transport properties of a CaCuSb single crystal that crystallizes in the hexagonal crystal structure. The magnetotransport studies reveal WAL and Shubnikov-de Haas (SdH) quantum oscillations with a unique frequency at 314 T. A cusp-like behavior in the low field regime of magnetotransport for J // (ab)-plane and B // [0001] confirms the WAL in CaCuSb. Angular-dependent normalized magnetoconductance and SdH oscillations studies reveal that the observed phenomena originate from the 2D transport channels. The high magnetic field (up to 45 T) experiments demonstrate plateau-like features in the Hall measurements. The first-principles calculations unfold that CaCuSb is a non-topological semimetal with dominant hole carries at the Fermi level. Our study reveals that CaCuSb is a promising candidate to explore the quasi-2D quantum transport phenomenon in the transition metal pnictide materials.

cond-mat.str-el↗

arXiv2021

Nanoscale devices with superconducting electrodes to locally channel current in 3D Weyl semimetals

We report on the fabrication of nano-devices on the \hkl[-1 0 1] surface of a Weyl semimetal, a macroscopic crystal of TaAs, and low-temperature transport measurements. We can implement electron beam lithography by peeling off and transferring the resist for nanofabrication onto the irregular crystal. We fabricate the device electrodes with superconducting Niobium nitride (NbN) to control the current flow through the intended active area of the devices. Our device structure enables the reduction of the current jetting effect, and we demonstrate the negative magnetoresistance measurement as a function of angle. The high field magnetotransport show three distinct oscillation frequencies corresponding to the three bands at the Fermi level. Resistance measured in the low magnetic field shows the usual weak anti-localization dip near the zero-field -- a signature of a Weyl material. Our method of fabricating devices with superconducting electrodes provides a way to probe the electrical properties of macroscopic single crystals at the nanoscale. As we use conventional lithographic techniques for patterning, this method can be extended to a wide gamut of electrode materials and a large class of 3D quantum materials.

cond-mat.mes-hall↗

arXiv2021

Emergence of well screened states in a superconducting material of the CaFe$_2$As$_2$ family

Coupling among conduction electrons (e.g. Zhang-Rice singlet) are often manifested in the core level spectra of exotic materials such as cuprate superconductors, manganites, etc. These states are believed to play key roles in the ground state properties and appear as low binding energy features. To explore such possibilities in the Fe-based systems, we study the core level spectra of a superconductor, CaFe$_{1.9}$Co$_{0.1}$As$_2$ (CaCo122) in the CaFe$_2$As$_2$ (Ca122) family employing high-resolution hard $x$-ray photoemission spectroscopy. While As core levels show almost no change with doping and cooling, Ca 2$p$ peak of CaCo122 show reduced surface contribution relative to Ca122 and a gradual shift of the peak position towards lower binding energies with cooling. In addition, we discover emergence of a feature at lower binding energy side of the well screened Fe 2$p$ signal in CaCo122. The intensity of this feature grows with cooling and indicate additional channels to screen the core holes. The evolution of this feature in the superconducting composition and it's absence in the parent compound suggests relevance of the underlying interactions in the ground state properties of this class of materials. These results reveal a new dimension in the studies of Fe-based superconductors and the importance of such states in the unconventional superconductivity in general.

cond-mat.supr-con↗

arXiv2020

Structural anomaly in superconductivity of CaFe2As2 class of materials

Quantum transitions in Fe-based systems are believed to involve spin, charge and nematic fluctuations. Complex structural phase diagram in these materials often emphasizes importance of covalency in their exotic properties, which is directly linked to the local structural network and barely understood. In order to address this outstanding issue, we investigate the evolution of structural parameters and their implication in unconventional superconductivity of 122 class of materials employing extended x-ray absorption fine structure studies. The spectral functions near the Fe K- and As K-absorption edge of CaFe2As2 and its superconducting composition, CaFe1.9Co0.1As2 (Tc = 12 K) exhibit evidence of enhancement of Fe contribution with Co-substitution near the Fermi level. As-Fe and Fe-Fe bondlengths derived from the experimental data exhibit interesting changes with temperature across the magneto-structural transition. Curiously, the evolution in Co-doped composition is similar to its parent compound despite absence of magneto-structural transition. In addition, we discover anomalous change of Ca-X (X = Fe, As) bondlengths with temperature in the vicinity of magneto-structural transition and disorder appears to be less important presumably due to screening by the charge reservoir layer. These results reveal evidence of doping induced evolution to the proximity to critical behavior presumably leading to superconductivity in the system.

cond-mat.supr-con↗

arXiv2020

Anisotropic Physical Properties of the Kondo Semimetal CeCu$_{1.11}$As$_2$

The recently proposed novel materials class called Weyl-Kondo semimetal (WKSM) is a time reversal invariant but inversion symmetry broken Kondo semimetal in which Weyl nodes are pushed to the Fermi level by the Kondo interaction. Here we explore whether CeCu$_{1+x}$As$_2$ may be a new WKSM candidate. We report on its single-crystal growth, structure determination and physical properties investigation. Previously published studies on polycrystalline samples suggest that it is indeed a Kondo semimetal, which is confirmed by our investigations on single crystals. X-ray diffraction reveals that CeCu$_{1+x}$As$_2$ crystallizes in a tetragonal centrosymmetric structure, although the inversion symmetry could still be broken locally due to partially occupied Cu sites. Chemical analysis results in an average occupation $x$ = 0.11(1). The electrical resistivity increases logarithmically with decreasing temperature, and saturates below 10 K. A Kondo temperature $T_{\mathrm{K}}$ $\approx$ 4 K is extracted from entropy, estimated from the specific heat measurements. From Hall effect experiments, a charge carrier density of $8.8 \times 10^{20}$ cm$^{-3}$ is extracted, a value characteristic of a semimetal. The magnetization shows pronounced anisotropy, with no evidence of magnetic ordering down to 0.4 K. We thus classify CeCu$_{1.11}$As$_2$ as a tetragonal Kondo semimetal with anisotropic magnetic properties, with a possibly broken inversion symmetry, thus fulfilling the necessary conditions for a WKSM state.

cond-mat.str-el↗

arXiv2019

Weak antilocalization in a noncentrosymmetric CaAgBi single crystal

We report on the single crystal growth and transport properties of a topological semimetal CaAgBi which crystallises in the hexagonal $ABC-$type structure with the non-centrosymmetric space group $\mathit{P6_3mc}$ (No. 186). The transverse magnetoresistance measurements with current in the basal plane of the hexagonal crystal structure reveal a value of about 30 % for I // [10-10] direction and about 50 % for I // [1-210] direction at 10 K in an applied magnetic field of 14 T. The magnetoresistance shows a cusp-like behavior in the low magnetic-field region, suggesting the presence of weak antilocalization effect for temperatures less than 100 K. The Hall measurements reveal that predominant charge carriers are $p$ type exhibiting a linear behavior for fields up to 14 T and can be explained based on the single band model. The magnetoconductance of CaAgBi is analysed based on the modified Hikami-Larkin-Nagaoka (HLN) model. Our first-principles calculations within a density-functional theory framework reveal that CaAgBi supports a topological Dirac semimetal state with Dirac points located on the rotational axis slightly above the Fermi level and are protected by $C_{6v}$ point-group symmetry. The Fermi surface consists of both the electron and hole pockets. However, the size of hole pockets is much larger than electron pockets suggesting the dominant $p$ type carriers in accord with our experimental results.

cond-mat.mtrl-sci↗

arXiv2019

Elemental substitution tuned magneto elastoviscous behavior of nanoscale ferrite MFe2O4 M = Mn, Fe, Co, Ni based complex fluids

The present article reports the governing influence of substituting the M2 site in nanoscale MFe2O4 spinel ferrites by different magnetic metals Fe,Mn,Co,Ni on magnetorheological and magneto elastoviscous behaviors of the corresponding magnetorheological fluids MRFs. Different doped MFe2O4 nanoparticles have been synthesized using the polyol assisted hydrothermal method. Detailed steady and oscillatory shear rheology have been performed on the MRFs to determine the magneto-viscoelastic responses. The MRFs exhibit shear thinning behavior and augmented yield characteristics under influence of magnetic field. The steady state magnetoviscous behaviors are scaled against the governing Mason number and self similar response from all the MRFs have been noted. The MRFs conform to an extended Bingham plastic model under field effect. Transient magnetoviscous responses show distinct hysteresis behaviors when the MRFs are exposed to time varying magnetic fields. Oscillatory shear studies using frequency and strain amplitude sweeps exhibit predominant solid like behaviors under field environment. However, the relaxation behaviors and strain amplitude sweep tests of the MRFs reveal that while the fluids show solid like behaviors under field effect, they cannot be termed as typical elastic fluids. Comparisons show that the MnFe2O4 MRFs have superior yield performance among all. However, in case of dynamic and oscillatory systems, CoFe2O4 MRFs show the best performance. The viscoelastic responses of the MRFs are noted to correspond to a three element viscoelastic model. The study may find importance in design and development strategies of nano MRFs for different applications.

cond-mat.soft↗

arXiv2018

Magnetocrystalline anisotropy in the Kondo lattice compound CeAgAs$_2$

We report on the single crystal growth and anisotropic physical properties of CeAgAs$_2$. The compound crystallizes as on ordered variant of the HfCuSi$_2$-type crystal structure and adopts the orthorhombic space group $Pmca$~(\#57) with two symmetry inequivalent cerium atomic positions in the unit cell. The orthorhombic crystal structure of our single crystal was confirmed from the powder x-ray diffraction and from electron diffraction patterns obtained from the transmission electron microscope. The anisotropic physical properties have been investigated on a good quality single crystal by measuring the magnetic susceptibility, isothermal magnetization, electrical transport and heat capacity. The magnetic susceptibility and magnetization measurements revealed that this compound orders antiferromagnetically with two closely spaced magnetic transitions at $T_{\rm N1} = 6$~K and $T_{\rm N2} = 4.9$~K. Magnetization studies have revealed a large magnetocrystalline anisotropy due to the crystalline electric field (CEF) with an easy axis of magnetization along the [010] direction. The magnetic susceptibility measured along the [001] direction exhibited a broad hump in the temperature range 50 to 250~K, while typical Curie-Weiss behaviour was observed along the other two orthogonal directions. The electrical resistivity and the heat capacity measurements revealed that CeAgAs$_2$ is a Kondo lattice system with a magnetic ground state.

cond-mat.str-el↗