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In rare-earth-based frustrated magnets, the synergistic interplay between spin correlations, spin-orbit coupling and competing exchange interactions provide a promising route to realize exotic quantum states with nontrivial excitations. Here, through thermodynamic and local-probe measurements down to 16 mK, we demonstrate the exotic magnetism and spin dynamics in the nearly perfect emergent honeycomb lattice antiferromagnet TbBO3. The latter embodies a frustrated lattice with a superimposed triangular lattice, constituted by additional Tb3+ ions at the center of each hexagon. Thermodynamic experiments reveal the presence of dominant antiferromagnetic interactions with no indications of either long-range order or spin freezing down to 50 mK. Despite sizable antiferromagnetic exchange interactions between the Tb3+ moments, muon-spin relaxation does not detect any signatures of long-range magnetic order or spin-freezing down to 16 mK. This suggests that the spin-orbit-driven anisotropic exchange interaction engenders a strong frustration, crucial to induce persistent spin dynamics. The specific-heat data exhibit a T^2.2 power-law behavior at low temperatures, suggesting gapless excitations consistent with theoretical predictions. The scaling of muon relaxation rate as a function of the characteristic energy scale for several spin-liquid candidates, including TbBO3, demonstrates a thermally activated behavior. This is consistent with NMR results on TbBO3 and reminiscent of a universal QSL behavior, here attributed to short-range spin correlations. Our experimental results are supported by density functional theory + Hubbard U and crystal electric-field calculations, which propose TbBO3 as a promising platform to realize the theoretically proposed quantum disorder state in an anisotropy-driven frustrated honeycomb lattice.

Exploring the physics of low-dimensional spin systems and their pressure-driven electronic and magnetic transitions are thriving research field in modern condensed matter physics. In this context, recently antiferromagnetic Cr-based compounds such as CrI3, CrBr3, CrGeTe3 have been investigated experimentally and theoretically for their possible spintronics applications. Motivated by the fundamental and industrial importance of these materials, we theoretically studied the electronic and magnetic properties of a relatively less explored Cr-based chalcogenide, namely LaCrS3 where 2D layers of magnetic Cr3+ ions form a rectangular lattice. We employed density functional theory + Hubbard U approach in conjunction with constrained random-phase approximation (cRPA) where the later was used to estimate the strength of U. Our findings at ambient pressure show that the system exhibits semiconducting antiferromagnetic ground state with a gap of 0.5 eV and large Cr moments that corresponds to nominal S=3/2 spin-state. The 1st nearest neighbor (NN) interatomic exchange coupling (J1) is found to be strongly antiferromagnetic (AFM), while 2nd NN couplings are relatively weaker ferromagnetic (FM), making this system a candidate for 1D non-frustrated antiferromagnetic spin-chain family of materials. Based on orbital resolved interactions, we demonstrated the reason behind two different types of interactions among 1st and 2nd NN despite their very similar bond lengths. We observe a significant spin-orbit coupling effect, giving rise to a finite magneto crystalline anisotropy, and Dzyaloshinskii-Moriya (DM) interaction. Further, we found that by applying uniaxial tensile strain along crystallographic a and b-axis, LaCrS3 exhibits a magnetic transition to a semi-conducting FM ground state, while compression gives rise to the realization of novel gapless semiconducting antiferromagnetic ground state., Comment: Accepted in Phys. Rev. B

Let $\mathcal{H}$ be a connected $k$-uniform hypergraph on $n$ vertices and $m$ hyperedges. In [A.~Banerjee, On the spectrum of hypergraph, Linear Algebra and its Application, 614(2021), 82--110], Anirban Banerjee introduced a new adjacency matrix for hypergraphs. In this article we consider the corresponding signless Laplacian matrix $Q(\mathcal{H})$ and discuss about its spectrum.

In the present study, we have discussed the up-turn behavior in the resistivity pattern of the topological nodal line semimetal InBi. We argued that such nature could be generalized with a mathematical model, that can be applied to any compounds exhibiting similar behavior. The extremely high magnetoresistance (XMR) has also been explained by the carrier compensation in the compound, estimated from the Hall conductivity. Moreover, from the study of Subhnikov-de Haas (SdH) oscillation and density functional theory (DFT), we obtained the complete three-dimensional (3D) Fermi surface topology of the compound InBi. A detailed understanding of carriers' behavior has been discussed using those studies. We have also unfurled the topology of each electron and hole pocket and its possible modulation with electron and hole doping.

Dimerized quantum magnets provide a unique possibility to investigate Bose-Einstein condensation of magnetic excitations in crystalline systems at low temperature. Here, we model the low-temperature magnetic properties of the recently synthesized spin $S=1$ dimer system K${}_2$Ni(MoO${}_4$)$_2$ and propose it as a new candidate material for triplon and quintuplon condensation. Based on a first principles analysis of its electronic structure, we derive an effective spin-dimer model that we first solve within a mean-field approximation to refine its parameters in comparison to experiment. Finally, the model is solved by employing a numerically exact quantum Monte Carlo technique which leads to magnetic properties in good agreement with experimental magnetization and thermodynamic results. We discuss the emergent spin model of K${}_2$Ni(MoO${}_4$)$_2$ in view of condensation of magnetic excitations in a broad parameter regime. Finally, we comment on a geometrical peculiarity of the proposed model and discuss how it could host a supersolid phase upon structural distortions., Comment: 7 pages, 4 figures + supplemental material of 6 pages, 6 figures. Comments are welcome

Competing magnetic interactions in low-dimensional quantum magnets can lead to the exotic ground state with fractionalized excitations. Herein, we present our results on an S = 5/2 quasi-one-dimensional spin system Bi3FeMo2O12. The structure of Bi3FeMo2O12 consists of very well separated, infinite zig-zag S = 5/2 spin chains. The observation of a broad maximum around 10 K in the magnetic susceptibility suggests the presence of short-range spin correlations. Magnetic susceptibility data do not fit to S=5/2 uniform spin chain model due to the presence of 2nd nearest-neighbor coupling (J2) along with the 1st nearest-neighbor coupling J1 of the zig-zag chain. The electronic structure calculations infer that the value of J1 is comparable with J2 (J2/J1~1.1) with a negligible inter-chain interaction (J'/J ~ 0.01), implying that Bi3FeMo2O12 is a highly frustrated triangular chain system. The absence of magnetic long-range ordering down to 0.2 K is seen in the heat capacity data, despite a relatively large antiferromagnetic Curie-Weiss temperature of -40 K. The magnetic heat capacity follows nearly a linear behavior at low temperatures indicating that the S = 5/2 anisotropic triangular chain exhibits the gapless excitations., Comment: 7 pages, 6 figures, 1 table

The quantum spin liquid (QSL) is an exotic phase of magnetic materials where the spins continue to fluctuate without any symmetry breaking down to zero temperature. Among the handful reports of QSL with spin $S\ge$1, examples with magnetic ions on a three-dimensional magnetic lattice are extremely rare since both larger spin and higher dimension tend to suppress quantum fluctuations. In this work, we offer a new strategy to achieve 3-D QSL with high spin by utilizing two types of transition metal ions, both are magnetically active but located at crystallographically inequivalent positions. We design a 3-D magnetic system Ba$_3$NiIr$_2$O$_9$ consisting of interconnected corner shared NiO$_6$ octahedra and face shared Ir$_2$O$_9$ dimer, both having triangular arrangements in \textit{a-b} plane. X-ray absorption spectroscopy measurements confirm the presence of Ni$^{2+}$ ($S$=1). Our detailed thermodynamic and magnetic measurements reveal that this compound is a realization of gapless QSL state down to at least 100 mK. Ab-initio calculations find a strong magnetic exchange between Ir and Ni sublattices and in-plane antiferromagnetic coupling between the dimers, resulting in dynamically fluctuating magnetic moments on both the Ir and Ni sublattice., Comment: 4 Figures

We report the synthesis and characterization of a new quantum magnet [2-[Bis(2-hydroxybenzyl)aminomethyl]pyridine]Ni(II)-trimer (BHAP-Ni3) in single-crystalline form. Our combined experimental and theoretical investigations reveal an exotic spin state that stabilizes a robust 2/3 magnetization plateau between 7 and 20 T in an external magnetic field. AC-susceptibility measurements show the absence of any magnetic order/glassy state down to 60 mK. The magnetic ground state is disordered and specific-heat measurements reveal the gapped nature of the spin excitations. Most interestingly, our theoretical modeling suggests that the 2/3 magnetization plateau emerges due to the interplay between antiferromagnetic Heisenberg and biquadratic exchange interactions within nearly isolated spin $S$ = 1 triangles.

The magnetic correlations in the austenite phase and the consequent martensitic transition in inverse magnetocaloric alloys, Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$, have been a matter of debate for decades. We conclusively establish using {\it ab initio} phonon calculations that the spin alignment of excess Mn at the Sn site (Mn$_{Sn}$) with the existing Mn in the unit cell in the high temperature cubic phase of Ni-Mn-Sn alloy is ferromagnetic (FM), and not ferrimagnetic (FI), resolving a long lasting controversy. Using first principles density functional perturbation theory (DFPT), we observe an instability of the TA$_{2}$ mode along the $\Gamma$-M direction in the FM phase, very similar to that observed in the prototypical ferromagnetic shape memory alloy (FSMA) Ni$_{2}$MnGa. This specific instability is not observed in the FI phase. Further finite temperature first principles lattice dynamics calculations reveal that at 300 K the FM phase becomes mechanically stable, while the FI phase continue to remain unstable providing credence to the fact that the high-temperature phase has FM order. These results will be primordial to understand the magneto-structural properties of this class of compounds., Comment: Supplemental materials are uploaded as a separate .pdf file

Tailoring transport properties of strongly correlated electron systems in a controlled fashion counts among the dreams of materials scientists. In copper oxides, varying the carrier concentration is a tool to obtain high-temperature superconducting phases. In manganites, doping results in exotic physics such as insulator-metal transitions (IMT), colossal magnetoresistance (CMR), orbital- or charge-ordered (CO) or charge-disproportionate (CD) states. In most oxides, antiferromagnetic order and charge-disproportionation are asssociated with insulating behavior. Here we report the realization of a unique physical state that can be induced by Mo doping in LaFeO$_3$: the resulting metallic state is a site-selective Mott insulator where itinerant electrons evolving in low-energy Mo states coexist with localized carriers on the Fe sites. In addition, a local breathing-type lattice distortion induces charge disproportionation on the latter, without destroying the antiferromagnetic order. A state, combining antiferromangetism, metallicity and CD phenomena is rather rare in oxides and may be of utmost significance for future antiferromagnetic memory devices., Comment: 8 pages, 4 figures

We present a comprehensive study of the magnetic properties of Sr$_{3-x}$Y$_{x}$(Fe$_{1.25}$Ni$_{0.75}$)O$_{7-\delta}$ ($0 \leq x \leq 0.75$). Experimentally, the magnetic properties are investigated using superconducting quantum interference device (SQUID) magnetometry and neutron powder diffraction (NPD). This is complemented by the theoretical study based on density functional theory as well as the Heisenberg exchange parameters. Experimental results show an increase in the N\'eel temperature ($T_N$) with the increase of Y concentrations and O occupancy. The NPD data reveals all samples are antiferromagnetically ordered at low temperatures, which has been confirmed by our theoretical simulations for the selected samples. Our first-principles calculations suggest that the 3D magnetic order is stabilized due to finite inter-layer exchange couplings. The latter give rise to a finite inter-layer spin correlations which disappear above the $T_N$.

We have studied the structural, magnetic properties, and electronic structure of the compound InCuPO5 synthesized by solid state reaction method. The structure of InCuPO5 comprises of S = 1/2 uniform spin chains formed by corner-shared CuO4 units. Magnetic susceptibility chi(T) data shows a broad maximum at about 65 K, a characteristic feature of one-dimensional (1D) magnetism. The chi(T) data is fitted to the coupled, S = 1/2 Heisenberg antiferromagnetic (HAFM) uniform chain model that gives the intra-chain coupling (J/kB) between nearest neighbour Cu2+ ions as -100 K and the ratio of inter-chain to intra-chain coupling (J'/J) as about 0.07. The exchange couplings estimated from the magnetic data analysis are in good agreement with the computed values from the electronic structure calculations based on density functional theory + Hubbard U (DFT+U) approach. The combination of theoretical and experimental analysis confirms that InCuPO5 is a candidate material for weakly coupled S = 1/2 uniform chains. A detailed theoretical analysis of the electronic structure further reveals that the system is insulating with a gap of 2.4 eV and a local moment of 0.70 muB /Cu., Comment: 14 pages, 6 figures, submitted Material Research Express

In this work, we report the pressure dependence of the effective Coulomb interaction parameters (Hubbard U) in paramagnetic NiO within the constrained random phase approximation (cRPA). We consider five different low energy models starting from the most expensive one that treats both Ni-d and O-p states as correlated orbitals (dp-dp model) to the smallest possible two-orbital model comprising the eg states only (eg-eg model). We find that in all the considered models, the bare interactions are not very sensitive to the compression. However the partially screened interaction parameters show an almost linear increment as a function of compression, resulting from the substantial weakening of screening effects upon compression. This counterintuitive trend is explained from the specific characteristic changes of the basic electronic structure of this system. We further calculate the nearest neighbor inter-site d-d interaction terms which also show substantial enhancement due to compression. Our results for both the experimental and highly compressed structures reveal that the frequency dependence of the partially screened interactions can not be ignored in a realistic modeling of NiO. We also find that the computed interaction parameters for the antiferromagnetic NiO are almost identical to their paramagnetic counter parts.

We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab-initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO.We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide., Comment: 16 pages, 5 figures

The electronic structure and magnetic properties of a single Fe adatom on a CuN surface have been studied using density functional theory in the local spin density approximation (LSDA), the LSDA+U approach and the local density approximation plus dynamical mean-field theory (LDA+DMFT). The impurity problem in LDA+DMFT is solved through exact diagonalization and in the Hubbard-I approximation. The comparison of the one-particle spectral functions obtained from LSDA, LSDA+U and LDA+DMFT show the importance of dynamical correlations for the electronic structure of this system. Most importantly, we focused on the magnetic anisotropy and found that neither LSDA, nor LSDA+U can explain the measured, high values of the axial and transverse anisotropy parameters. Instead, the spin excitation energies obtained from our LDA+DMFT approach with exact diagonalization agree significantly better with experimental data. This affirms the importance of treating fluctuating magnetic moments through a realistic many-body treatment when describing this class of nano-magnetic systems. Moreover, it facilitates insight to the role of the hybridization with surrounding orbitals., Comment: 17 pages, 4 figures

We have carried out a detailed first-principles study of a d$^{4.5}$ quaternary iridate Ba$_3$YIr$_2$O$_9$ both in its 6H-perovskite-type ambient pressure (AP) phase and also for the high pressure (HP) cubic phase. Our analysis reveals that the AP phase belongs to the intermediate spin-orbit coupling (SOC) regime. This is further supported by the identification of the spin moment as the primary order parameter (POP) obtained from a magnetic multipolar analysis. The large $t_{2g}$ band width renormalizes the strength of SOC and the Ir intersite exchange interaction dominates resulting in long range magnetic order in the AP phase. In addition to SOC and Hubbard $U$, strong intradimer coupling is found to be crucial for the realization of the insulating state. At high pressure (HP) the system undergoes a structural transformation to the disordered cubic phase. In sharp contrast to the AP phase, the calculated exchange interactions in the HP phase are found to be much weaker and SOC dominates leading to a quantum spin orbital liquid (SOL) state.

We investigate magnetic, thermal, and dielectric properties of SrCuTe2O6, which is isostructural to PbCuTe2O6, a recently found, Cu-based 3D frustrated magnet with a corner sharing triangular spin network having dominant first and second nearest neighbor (nn) couplings [B. Koteswararao, et al. Phys. Rev. B 90, 035141 (2014)]. Although SrCuTe2O6 has a structurally similar spin network, but the magnetic data exhibit the characteristic features of a typical quasi -one-dimensional magnet, which mainly resulted from the magnetically dominant third nn coupling, uniform chains. The magnetic properties of this system are studied via magnetization (M), heat capacity (Cp), dielectric constant, measurements along with ab-initio band structure calculations. Magnetic susceptibility chi(T) data show a broad maximum at 32 K and the system orders at low temperatures TN1=5.5 K and TN2=4.5 K, respectively. The analysis of chi(T) data gives an intra-chain coupling, J3/kB, to be about - 42 K with non-negligible frustrated inter-chain couplings (J1/kB and J2/kB). The hopping parameters obtained from LDA band structure calculations also suggest the presence of coupled uniform chains. The observation of simultaneous anomalies in dielectric constant at TN1 and TN2 suggests the presence of magneto-dielectric effect in SrCuTe2O6. A magnetic phase diagram is also built based on M, C p, and dielectric constant results., Comment: 16 pages, 9 figures

We report magnetic susceptibility (chi) and heat capacity Cp measurements along with ab-initio electronic structure calculations on PbCuTe2O6, a compound made up of a three dimensional 3D network of corner-shared triangular units. The presence of antiferromagnetic interactions is inferred from a Curie-Weiss temperature (theta_CW) of about -22 K from the chi(T) data. The magnetic heat capacity (Cm) data show a broad maximum at T^max ~ 1.15 K (i.e. T^max/theta_CW ~ 0.05), which is analogous to the the observed broad maximum in the Cm/T data of a hyper-Kagome system, Na4Ir3O8. In addition, Cm data exhibit a weak kink at T^* ~ 0.87 K. While the T^max is nearly unchanged, the T^* is systematically suppressed in an increasing magnetic field (H) up to 80 kOe. For H > 80 kOe, the Cm data at low temperatures exhibit a characteristic power-law (T^{\alpha}) behavior with an exponent {\alpha} slightly less than 2. Hopping integrals obtained from the electronic structure calculations show the presence of strongly frustrated 3D spin interactions along with non-negligible unfrustrated couplings. Our results suggest that PbCuTe2O6 is a candidate material for realizing a 3D quantum spin liquid state at high magnetic fields., Comment: 12 pages, 7 figures, Published in Phys. Rev. B

We have studied in detail the electronic structure of IrO$_2$ including spin-orbit coupling (SOC) and electron-electron interaction, both within the GGA+U and GGA+DMFT approximations. Our calculations reveal that the Ir t$_{2g}$ states at the Fermi level largely retain the J$_{\rm eff}$ = $\frac{1}{2}$ character, suggesting that this complex spin-orbit entangled state may be robust even in metallic IrO$_2$. We have calculated the phase diagram for the ground state of IrO$_2$ as a function of $U$ and find a metal insulator transition that coincides with a magnetic phase change, where the effect of SOC is only to reduce the critical values of $U$ necessary for the transition. We also find that dynamic correlations, as given by the GGA+DMFT calculations, tend to suppress the spin-splitting, yielding a Pauli paramagnetic metal for moderate values of the Hubbard $U$. Our calculated optical spectra and photoemission spectra including SOC are in good agreement with experiment demonstrating the importance of SOC in IrO$_2$.

NiS, exhibiting a text-book example of a first-order transition with many unusual properties at low temperatures, has been variously described in terms of conflicting descriptions of its ground state during the past several decades. We calculate these physical properties within first-principle approaches based on the density functional theory and conclusively establish that all experimental data can be understood in terms of a rather unusual ground state of NiS that is best described as a self-doped, nearly compensated, antiferromagnetic metal, resolving the age-old controversy. We trace the origin of this novel ground state to the specific details of the crystal structure, band dispersions and a sizable Coulomb interaction strength that is still sub-critical to drive the system in to an insulating state. We also show how the specific antiferromagnetic structure is a consequence of the less-discussed 90 degree and less than 90 degree superexchange interactions built in to such crystal structures.