The three-dimensional $S = 1/2$ system Y$_{3}$Cu$_{2}$Sb$_{3}$O$_{14}$ consists of two inequivalent Cu$^{2+}$ ions, each forming edge shared triangular lattices. Our magnetic susceptibility $\chi(T)=M/H$, specific heat $C_p(T)$, $^{89}$Y nuclear magnetic resonance (NMR), muon spin relaxation ($\mu$SR), and electron spin resonance (ESR) measurements on this system confirm the absence of any long-range magnetic ordering and the persistence of spin dynamics down to 0.077 K. From $^{89}$Y NMR we find evidence of a transition at about 120 K which we suggest to arise from a fraction of the spins condensing into a singlet (a valence bond solid VBS or a quantum spin liquid QSL) state. A plateau in the muon relaxation rate is observed between 60 K and 10 K (signifying the VBS/QSL state from a fraction of the spins) followed by an increase and another plateau below about 1 K (presumably signifying the VBS/QSL state from all the spins). Our density functional theory calculations find a dominant antiferromagnetic interaction along the body diagonal with inequivalent Cu(1) and Cu(2) ions alternately occupying the corners of the cube. All other near neighbour interactions between the Cu ions are also found to be antiferromagnetic and are thought to drive the frustration.

Double perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. Herein, we report the electronic structure and magnetic properties of an ordered double perovskite material Ho2CoMnO6. In the double perovskite with general class A2BB'O6, the octahedral B and B'-site has a distinct crystallographic site. The Rietveld refinement of XRD data reveals that Ho2CoMnO6 crystallizes in the monoclinic P21/n space group. The X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibit a long-range ferromagnetic ordering at Tc ~ 76 K owing to the presence of super exchange interaction between Co2+ and Mn4+ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms ferromagnetic behavior of this double perovskite. We observed a re-entrant glassy state in the intermediate temperature regime, which is attributed to inherent anti-site disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. The temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above Tc in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infers the compound to be a narrow band gap insulator with the gap arising between the lower and upper Hubbard Co-d subbands. Our results demonstrate that anti-site disorder and complex 3d-4f exchange interactions in the spin-lattice account for the observed electronic and magnetic properties in this promising double perovskite material.

First-order phase transitions, characterized by a discontinuous change in the order parameter, are intriguing phenomena in condensed matter physics. However, the underlying, material-specific, microscopic mechanisms often remain unclear. Here, we unveil a high-temperature incommensurate charge-order precursor with the wave vector $\mathbf{q}^* = (0, \frac{1}{4}+\delta, \frac{1}{2})$ in the 1T' phase of TaTe$_2$, which competes with fluctuating high-temperature Ta trimer bonding states at $\mathbf{q}_\mathrm{CO} =(0, \frac{1}{3}, 0)$. The precursor state follows the temperature dependence of the hidden incommensurability of the $\textit{quasi}$-1D nested Fermi surface. In contrast, the low-temperature commensurate charge order at $\mathbf{q}_\mathrm{CO}$, characterized by a charge disproportionation of the inequivalent Ta sites, appears to be driven by local chemical bonding. Dynamical lattice calculations identify an imaginary optical mode at $\mathbf{q}^*$, involving an in-plane vibration of the Ta atoms forming a chain-like structure that renormalizes below $T_\mathrm{CO}$. Our experimental and theoretical observations suggest that the controversial first-order phase transition, as captured by phenomenological Ginzburg-Landau theory, results from the competition between two order parameters: one involving Fermi surface nesting and the other involving local chemical bonding.

For disordered Heisenberg systems with small single ion anisotropy, two spin glass transitions below the long range ordered phase transition temperature has been predicted theoretically for compositions close to the percolation threshold. Experimental verification of these predictions is still controversial for conventional spin glasses. We show that multiferroic spin glass systems can provide a unique platform for verifying these theoretical predictions via a study of change in magnetoelastic and magnetoelectric couplings, obtained from an analysis of diffraction data, at the spin glass transition temperatures. Results of macroscopic and microscopic (x-ray and neutron scattering) measurements are presented on disordered BiFeO3, a canonical Heisenberg system with small single ion anisotropy, which reveal appearance of two spin glass phases SG1 and SG2 in coexistence with the LRO phase below the A-T and G-T lines. It is shown that the temperature dependence of the integrated intensity of the antiferromagnetic peak shows dips with respect to the Brillouin function behaviour around the SG1 and SG2 transition temperatures. The ferroelectric polarisation changes significantly at the two spin glass transition temperatures. These results, obtained using microscopic techniques, clearly demonstrate that the SG1 and SG2 transitions occur on the same magnetic sublattice and are intrinsic to the system. We also construct a phase diagram showing all the magnetic phases in BF-xBT system. While our results on the two spin glass transitions support the theoretical predictions, it also raises several open questions which need to be addressed by revisiting the existing theories of spin glass transitions by taking into account the effect of magnetoelastic and magnetoelectric couplings as well as electromagnons.

In this work, the effect of anti-site disorder on magnetic, electrical resistivity, transverse magnetoresistance MR, and anomalous Hall resistivity of off-stoichiometric CFTS Heusler alloy thin films, with a particular focus on martensitic phase transformation and spin gapless semiconductor SGS-like behavior, is investigated. These thin films were grown on Si (100) substrate at different substrate temperatures, TS, ranging from 200 C to 550 C using magnetron sputtering, enabling control over the degree of anti-site atomic ordering from disordered A2 to ordered L21. All the films, irrespective of their disorder, exhibit a distinct thermal hysteresis and significant drop in resistivity, crossover from asymmetric to symmetric magnetoresistance, and a sharp increase in MR around 300 K, confirming the occurrence of a thermo-elastic martensitic phase transformation. Detailed analysis of resistivity data indicates that for TS200 and TS350 films, a SGS based two channel model describes the conductivity in the martensite phase, whereas TS450, TS500, and TS550 films, exhibit a usual metallic behavior with a resistivity minimum at low temperatures. All the CFTS films show soft ferromagnetic nature and follow the spin-wave equation up to 390 K. The saturation magnetization and Hall conductivity increase with increasing crystalline order. The scaling relation between the longitudinal resistivity and the anomalous Hall resistivity in the martensite phase revels that skew scattering is the dominating contribution in disordered films, and a change in charge carrier type from hole to electron around the martensitic transformation temperature. The asymmetric MR, persistent up to room temperature, highlights the potential of these films for spintronic applications such as spin valves.

Spontaneous formation of charge density wave (CDW) superstructures in monolayers (MLs) of a two-dimensional (2D) crystal lattice is fundamental in understanding its complex quantum states. We report a successful top-down liquid phase exfoliation and stamp transfer process (LPESTP) to create ML VS\textsubscript{2}, undergoing a CDW transition at room temperature. Using high-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED), we observed the coexistence of 1T and 2H polymorphic phases in VS\textsubscript{2} at room temperature, and only the 1T phase undergoes CDW transition. We discovered a novel incommensurate CDW superstructure ($\sqrt{7} \times \sqrt{7}$) R19.1\textsuperscript{o} in ML 1T-VS\textsubscript{2}. With an increase in the number of layers, the CDW order changes to a commensurate ($2 \times 2$) superstructure. Using angle-dependent photoelectron spectroscopy, we have shown that vanadium atoms self-intercalate as V\textsuperscript{3+} ions in multilayer VS\textsubscript{2} and are responsible for the evolution of the CDW superstructure from the incommensurate $\sqrt{7} \times \sqrt{7}$) R 19.1\textsuperscript{o} to the commensurate ($2\times2$) order. We also report the observation of novel Moiré superlattices in twisted bilayer 1T-VS\textsubscript{2} flakes with trapped CDW superstructure of the monolayer. Our findings provide an important platform for understanding the evolution of CDW superstructures in 1T-VS\textsubscript{2} with thickness and V self-intercalation.

The growth and characterization of high quality superconducting thin films is essential for fundamental understanding and also for the use of these films in technological applications. In the present study, Ti40V60 alloy thin films have been deposited using DC magnetron co sputtering of Ti and V at ambient temperatures. The effect of deposition pressure on the film morphology, superconducting and normal state properties has been studied. Measurement of electrical resistance as a function of temperature indicates that up to a certain deposition pressure, the 20 nm thick Ti40V60 films exhibit metallic behavior in the normal state and superconductivity at low temperatures. Beyond a threshold pressure, the films show a negative temperature coefficient of resistance with a residual resistance ratio less than one. Electrical transport measurements in the presence of magnetic field were performed to find the current voltage characteristics of the thin films. Analysis of the I V curves indicates that the Ti40V60 alloy thin films have a large transport critical current density (JC) e.g. 1.475E10 A per m2 in zero magnetic field and 2.657E09 A per m2 in 4 T (both at 4 K). Analysis of the field dependence of flux line pinning force density indicates a combined effect of core delta k surface and core delta k point pinning mechanisms (where k is the Ginzburg Landau parameter). Additionally, spatial variations in the superconducting critical temperature (TC ) across the sample contribute to delta TC pinning. In higher magnetic fields, a contribution from delta l pinning (where l is the electron mean free path) also becomes significant. The findings indicate the potential of Ti40V60 alloy thin film for superconducting device applications like cryogenic radiation detectors.

Disorder can be utilized as an effective parameter to probe the interplay between two long range orders such as superconductivity and charge density wave. In the present work, we report on the experimental evidence for filamentary superconductivity in polycrystalline TiSe1.2S0.8 with superconducting transition Tc ~ 7K. This is validated from magnetization and magneto-transport measurements. Strain induced dislocations, substitutional defects, and randomly distributed Ti ions (with local moments) are considered as possible sources of disorder. A detailed analysis of the temperature dependent resistivity evaluates the degree of disorder and the consequent localization effects. The findings are in striking contrast to the fact that superconductivity has not been reported in single crystals of TiSe2-xSx system. It is established that disorder serves as a stabilizing factor for the superconducting phase due to in-commensuration of the charge density wave.

(Bi1-xSbx)2Te3 (x=0.60, 0.65, 0.68, 0.70, 0.75 and 0.80) mixed crystals have been synthesized by solid state reaction. In depth structural, thermal, transport and electronic properties are reported. Defect and disorder play a crucial role in structural and transport behaviour. Disorder induced iso-structural phase transition is observed at x=0.70, which is supported by the structural and transport properties data. Debye temperature has been estimated from the powder diffraction data. Differential scanning calorimetry (DSC) data confirms the glass transition in the material. Low temperature resistivity data shows Variable range hopping mechanism whereas high temperature data follows activated behaviour. Activation energy is calculated from the semiconducting region of resistivity data. Both Hall measurement and temperature dependent thermopower data (S(T)) confirms that samples are p-type in nature. Density of state effective mass has been estimated from Pisarenko relation and corroborated with resistivity data. Thermal conductivity (k) is estimated using experimentally obtained data. Figure of Merit (ZT) of the synthesized samples are calculated using resistivity, S(T) and k. Structural and transport properties are correlated, confirms the transition from disorder to order state. Defect and disorder are corroborated with structural and Thermoelectric properties of the synthesized samples.

First-order phase transitions, characterized by a discontinuous change in the order parameter, are intriguing phenomena in condensed matter physics. However, the underlying, material-specific, microscopic mechanisms often remain unclear. Here, we unveil a high-temperature incommensurate charge-order precursor with the wave vector $\mathbf{q}^* = (0, \frac{1}{4}+\delta, \frac{1}{2})$ in the 1T' phase of TaTe$_2$, which competes with fluctuating high-temperature Ta trimer bonding states at $\mathbf{q}_\mathrm{CO} =(0, \frac{1}{3}, 0)$. The precursor state follows the temperature dependence of the hidden incommensurability of the $\textit{quasi}$-1D nested Fermi surface. In contrast, the low-temperature commensurate charge order at $\mathbf{q}_\mathrm{CO}$, characterized by a charge disproportionation of the inequivalent Ta sites, appears to be driven by local chemical bonding. Dynamical lattice calculations identify an imaginary optical mode at $\mathbf{q}^*$, involving an in-plane vibration of the Ta atoms forming a chain-like structure that renormalizes below $T_\mathrm{CO}$. Our experimental and theoretical observations suggest that the controversial first-order phase transition, as captured by phenomenological Ginzburg-Landau theory, results from the competition between two order parameters: one involving Fermi surface nesting and the other involving local chemical bonding.

Spontaneous formation of charge density wave (CDW) superstructures in monolayers (MLs) of a two-dimensional (2D) crystal lattice is fundamental in understanding its complex quantum states. We report a successful top-down liquid phase exfoliation and stamp transfer process (LPESTP) to create ML VS\textsubscript{2}, undergoing a CDW transition at room temperature. Using high-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED), we observed the coexistence of 1T and 2H polymorphic phases in VS\textsubscript{2} at room temperature, and only the 1T phase undergoes CDW transition. We discovered a novel incommensurate CDW superstructure ($\sqrt{7} \times \sqrt{7}$) R19.1\textsuperscript{o} in ML 1T-VS\textsubscript{2}. With an increase in the number of layers, the CDW order changes to a commensurate ($2 \times 2$) superstructure. Using angle-dependent photoelectron spectroscopy, we have shown that vanadium atoms self-intercalate as V\textsuperscript{3+} ions in multilayer VS\textsubscript{2} and are responsible for the evolution of the CDW superstructure from the incommensurate $\sqrt{7} \times \sqrt{7}$) R 19.1\textsuperscript{o} to the commensurate ($2\times2$) order. We also report the observation of novel Moiré superlattices in twisted bilayer 1T-VS\textsubscript{2} flakes with trapped CDW superstructure of the monolayer. Our findings provide an important platform for understanding the evolution of CDW superstructures in 1T-VS\textsubscript{2} with thickness and V self-intercalation.

The magnetic structure of the Eu2+ moments in the superconducting EuFe2(As1-xPx)2 sample with x = 0.15 has been determined using element specific x-ray resonant magnetic scattering. Combining magnetic, thermodynamic and scattering measurements, we conclude that the long range ferromagnetic order of the Eu2+ moments aligned primarily along the c axis coexists with the bulk superconductivity at zero field. At an applied magnetic field >= 0.6 T, superconductivity still coexists with the ferromagnetic Eu2+ moments which are polarized along the field direction. We propose a spontaneous vortex state for the coexistence of superconductivity and ferromagnetism in EuFe2(As0.85P0.15)2.

The three-dimensional $S = 1/2$ system Y$_{3}$Cu$_{2}$Sb$_{3}$O$_{14}$ consists of two inequivalent Cu$^{2+}$ ions, each forming edge shared triangular lattices. Our magnetic susceptibility $\chi(T)=M/H$, specific heat $C_p(T)$, $^{89}$Y nuclear magnetic resonance (NMR), muon spin relaxation ($\mu$SR), and electron spin resonance (ESR) measurements on this system confirm the absence of any long-range magnetic ordering and the persistence of spin dynamics down to 0.077 K. From $^{89}$Y NMR we find evidence of a transition at about 120 K which we suggest to arise from a fraction of the spins condensing into a singlet (a valence bond solid VBS or a quantum spin liquid QSL) state. A plateau in the muon relaxation rate is observed between 60 K and 10 K (signifying the VBS/QSL state from a fraction of the spins) followed by an increase and another plateau below about 1 K (presumably signifying the VBS/QSL state from all the spins). Our density functional theory calculations find a dominant antiferromagnetic interaction along the body diagonal with inequivalent Cu(1) and Cu(2) ions alternately occupying the corners of the cube. All other near neighbour interactions between the Cu ions are also found to be antiferromagnetic and are thought to drive the frustration.

Debye temperature decrease, residual resistivity increase and electron-phonon coupling constant increase as crystallite size decreases have been found from electrical resistivity in temperature range 5 K to 300 K of well-characterized Ag nanoparticles synthesized with oleylamine, trioctylphosphine and/ polyvinylpyrrolidone with Scherrer sizes ranging from 15.1 nm to 33.4 nm. Notably, about 36 % reduction in Debye temperature in 15.1 nm compared to bulk Ag is found. Remarkably, usual phonon drag peak found in Seebeck coefficient for bulk Ag turned into dips or phonon drag minima in these NPs that gradually gets suppressed and shifted towards lower temperature with decrease in crystallite size in oleylamine and trioctylphosphine-induced NPs. Contrastingly, it appears at higher temperature in trioctylphosphine-induced nanoparticles. A broad hump between 125 K to 215 K, a slope change near 270 K in resistivity and an additional dip-like feature near 172 K in Seebeck coefficient are seen in oleylamine-polyvinylpyrrolidone-induced nanoparticles with different shapes. They are attributed to spatial confinement of electrons and phonons, varying barrier heights, different charge-transfer mechanisms among metal nanoparticles and surfactant/s, enhanced disorders (grain boundaries, increase in fraction of surface atoms, surfactant matrix and other defects), leading to modifications in their overall electron and phonon interactions. Finally, their thermoelectric power factor has also been assessed.

We investigated the crystal structure, magnetic behavior, and optical properties of the layered honeycomb compound Na2Cu2TeO6. X-ray and neutron diffraction confirmed a monoclinic structure, with Cu ions arranged in dimerized chains. Magnetic susceptibility measurements yielded a Curie-Weiss temperature significantly lower than the expected spin-only value, indicating the presence of strong antiferromagnetic interactions and enhanced quantum fluctuations. A broad maximum near 160 K in the susceptibility data is consistent with short-range one-dimensional antiferromagnetic correlations. Magnetization measurements showed negligible coercivity, and specific heat data revealed no anomalies down to 3 K. Temperature-dependent neutron diffraction showed no evidence of long-range magnetic order. Optical absorption studies using UV-Visible spectroscopy displayed a sharp absorption edge in the UV region. Tauc analysis estimated a direct optical band gap of approximately 2.10 eV, with no clear indication of an indirect transition. These observations provide insight into the interplay between structural distortions, low-dimensional magnetism, and optical behavior in Na2Cu2TeO6

BaTiO3 is a classical ferroelectric studied for last one century for its ferroelectric properties. Lattice dynamics of BaTiO3 is crucial as the utility of devices is governed by phonons. In this work, we show that traditional characterization of the polar phonon modes is ambiguous and often misinterpreted. By combining Raman, Neutron and X-ray diffraction, dielectric spectroscopic observations with first principle calculations, we have re-examined the character of the normal modes of phonons of BaTiO3. We obtained Eigen displacements of vibrational modes through DFT calculations and reclassified the polar modes being Slater (Ti-O), Last (Ba-TiO3) and Axe (BO6) vibrations by correlating experimental and theoretical calculations. The study thus provides correct nomenclature of the polar modes along with the evidence of presence of short range polar distortions along (111) directions in all the phases shown by BaTiO3. The Burns temperature and absence of second order contributions have been witnessed in the temperature dependent Raman study.

We have analyzed spectral weight changes in the conduction and the valence band across insulator to metal transition (IMT) in the VO2 thin film using X-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (PES). Through temperature dependent XAS and resonant PES measurements we unveil that spectral changes in the d$_{\|}$ states (V 3$\it{d_{x^2-y^2}}$ orbitals) are directly associated with temperature dependent electrical conductivity. Due to presence of the strong electron-electron correlations among the d$_{\|}$ states, across IMT, these states are found to exhibit significant intensity variation compared to insignificant changes in the $\pi^{\ast}$ and the $\sigma^{\ast}$ states (which are O 2$\it{p}$ hybridized V 3$\it{d}$ $e_g^{\pi}$ and $e_g^{\sigma}$ states) in the conduction band. Experimentally obtained values of the correlation parameter (U$_{dd}$ $\sim$ 5.1 eV, intra-atomic V 3$\it{d}$ correlations) and crystal field splitting (10 Dq $\sim$ 2.5 eV) values are used to simulate the V $\it{L_{2,3}}$ edge XAS spectra and an agreement between simulated and experimental spectra also manifests strong correlations. These results unravel that the IMT observed in the VO2 thin film is the Mott-Hubbard insulator-metal transition.

We explore the influence of demagnetization interaction on magnetic memory effect by varying organization geometry of anisotropic ZnFe$_2$O$_4$ nanoparticles in an ensemble. The static and dynamic behaviour of two differently organized ensembles, compact ensemble (CE) and hollow core ensemble (HCE), are extensively studied by both dc and ac susceptibility, magnetic memory effect and spin relaxation. The frequency-dependence peak shifting of freezing temperature in both the systems is analyzed properly with the help of two dynamic scaling models: Vogel-Fulcher law and power law. Presence of cluster spin-glass phase is reflected from Vogel-Fulcher temperature $T_0$ $\simeq$ 142.58 K for CE, $\simeq$ 97 K for HCE and characteristic time constant $\tau_0$ $\simeq$ $8.85\times10^{-9}$ s for CE, $\simeq$ $3.8\times10^{-10}$ s for HCE; along with $\delta$T$_{Th}$ $\sim$ 0.1 for CE and 0.2 for HCE. The power law fitting with dynamic exponent $zv'$ = 6.2 $\pm$ 1.1 for CE, 6.3 $\pm$ 0.5 for HCE and single spin flip $\tau^*$ $\simeq$ $7.7\times10^{-11}$ s for CE, $\simeq$ $1.3\times10^{-10}$ s for HCE provide firm confirmation of cluster spin-glass phase. The progressive spin freezing across multiple metastable states with prominent memory effects is reflected in both the systems via nonequilibrium dynamics study. The hollow core geometry with anisotropic nanoparticles on surface with closer proximity leads to complex anisotropy energy landscape with enhanced demagnetizing field resulting highly frustrated surface spins. As a consequence, more prominent magnetic memory effect is observed in HCE with higher activation energy, reduced blocking temperature and enhanced coercivity than that of CE.

We report large magnetoresistance (MR) and Shubnikov-de Haas (SdH) oscillations in single crystals of magnetically (M= Ni and Fe) doped M$_x$Bi$_{0.97-x}$Sb$_{0.03}$ ($x=$ 0, 0.02) topological insulators. The R$\bar{3}$m symmetry and phase have been confirmed by the Rietveld refinement of x-ray diffraction data. Interestingly, a magnetic field induced phase transition from semi-metallic to semi-conducting type is found with the energy gap around 80 meV at 15 Tesla in the $x=$ 0 sample. Moreover, we observe linear behavior of MR up to 15 Tesla in transverse mode and SdH oscillations in longitudinal mode where the field direction is with respect to the current and crystal plane. For the parent sample, we found the coherence length L$_\phi=$ 12.7 nm through the fitting of MR data in transverse mode with modified H-L-N equation. The extracted frequencies of SdH oscillations using the fast Fourier transform method and Landau level (LL) fan diagram are found to be consistent for the parent and Ni doped samples. The determined Fermi surface area is found to be slightly larger in Ni doped as compared to the parent sample possibly due to change in the Fermi energy. The Kohler's plot indicates a single scattering mechanism below 100 K. More importantly, the analysis with the help of LL fan diagram reveals the non-zero Berry phase $\phi_{\rm B}= -$(1$\pm$0.1)$\pi$, which demonstrates the non-trivial topological states near the Dirac point in the parent and Ni doped samples.

BaFe$_2$S$_3$ and BaFe$_2$Se$_3$ are the only two quasi-one-dimensional iron-based compounds that become superconductors under pressure. Interestingly, these two compounds exhibit different symmetries and properties. While more detailed and recent studies on BaFe$_2$Se$_3$ using single crystals have advanced the filed towards a more universal description of this family, such a study is still lacking for the compound BaFe$_2$S$_3$. Here, we present a detailed study of the crystalline and magnetic structure performed on single crystals using X-ray and neutron diffraction. We demonstrate a polar structure at room temperature within the $Cm2m$ space group, followed by a structural transition at 130 K to the polar $Pb2_1m$ space group. This space group remains unchanged across the magnetic transition at $T_N =95$ K, revealing multiferroic characteristics with a weak magnetoelastic coupling. The determined magnetic structure is monoclinic ($P_am$), with non-collinear magnetic moments, displaying a significant angle of 18$^\circ$ relative to the $a$-axis in the $(a, c)$ plane. This reexamination of the temperature-dependent properties of BaFe$_2$S$_3$ provides new insights into the physics of this system from multiple key perspectives.