Even though abundant research has focused on metamagnetic critical end points in itinerant magnets, this critical phenomenon for magnetic insulators is still to be explored extensively. DyVO4, a magnetic insulator exhibits a field induced first order metamagnetic transition (MMT), which has previously been investigated via thermodynamic measurements. In this work, we extend our investigations down to mK temperatures and probe the metamagnetic tricritical point (TCP) directly via the magnetocaloric effect (MCE). Heat capacity reveals an inimitable phase diagram where the second order antiferromagnetic phase transition terminates at the TCP, beyond which it continues as a line of first order MMT as the temperature is lowered towards 0 K. The sample temperature and magnetic Grüneisen parameter (Γ𝑚) evaluated from MCE data show direct evidence of critical fluctuations and enhanced entropy in the vicinity of the TCP. These fluctuations are also supported by diverging susceptibility observed near the TCP. Critical analysis of magnetization suggests that the metamagnetic TCP in DyVO4 does not belong to any universal class. However, the extracted exponents agree with another insulating metamagnet, HoMnO3 [Phys. Rev. Lett. 110, 157202 (2013)], suggesting the presence of a unique universality class for metamagnetic critical systems. 

Dheeraj Ranaut et. al, Phys. Rev. B 111, 104409 (2025)

In this work, we describe the spinon spectrum obtained in deuterated singlecrystals of the spin 1/2 Heisenberg antiferromagnet [Cu(μ-C2O4)(4-aminopyridine)2(H2O)]nwhose ground state is a Tomonaga−Luttinger liquid (TLL). High-resolution mass spectroscopyindicated at least 99.9% deuteration achieved in [Cu(μ-C2O4)(4-aminopyridine)2(H2O)]n thatresulted in minimal background noise arising due to an incoherent scattering cross-section ofresidual H atoms in inelastic neutron-scattering measurements. The deuteration of startingreagents was performed by employing the method of H/D exchange using D2O as a solvent andPd/C as a catalyst. From single-crystal X-ray diffraction (SCXRD) measurements, the crystalstructure was found to be monoclinic with a C2 space group, unchanged from its undeuteratedversion. Calculated bond lengths and bond angles from density functional theory using dualbasis were found to match very well with those obtained from ScXRD data. The 3500 cm−1Infrared and Raman frequencies of the undeuterated crystals were found to be shifted in thelower range of 2000−3000 cm−1 in the deuterated crystals due to the C−H bonds of 4-aminopyridine. Magnetization measurements yielded the ground state of deuterated [Cu(μ-C2O4)(4-aminopyridine)2(H2O)]n to bethat of a spin 1/2 antiferromagnetic Heisenberg chain TLL, which was confirmed by the observation of a spinon spectrum that arecharacteristic excitations of the TLL state.

Athira Suresh et. al, ACS Omega https://pubs.acs.org/doi/full/10.1021/acsomega.4c08525) 

The ground state of a one-dimensional spin- 1 2 uniform antiferromagnetic Heisenberg chain (AfHc) is a Tomonaga-Luttinger liquid which is quantum-critical with respect to applied magnetic fields upto a saturation field µ0Hs beyond which it transforms to a fully polarised state. Wilson ratio has been predicted to be a good indicator for demarcating these phases [Phys. Rev. B 96, 220401 (2017)]. From detailed temperature and magnetic field dependent magnetisation, magnetic susceptibility and specific heat measurements in a metalorganic complex and comparisons with field theory and quantum transfer matrix method calculations, the complex was found to be a very good realisation of a spin- 1 2 AfHc. Wilson ratio obtained from experimentally obtained magnetic susceptibility and magnetic contribution of specific heat values was used to map the magnetic phase diagram of the uniform spin- 1 2 AfHc over large regions of phase space demarcating Tomonaga-Luttinger liquid, saturation field quantum critical, and fully polarised states. Luttinger parameter and spinon velocity were found to match very well with the values predicted from conformal field theory.

Sharath Kumar Channarayappa et. al, PNAS Nexus 3, 363 (2024) 

In this paper, we used a theoretical measure known as distance between the states, E(ρe), to determine the bipartite entanglement of a one dimensional magnetic dimer system. The calculation was compared with the well-known entanglement measure, concurrence, and found to be the same. E(ρe) was, then, expressed in terms of two thermodynamic quantities, namely, magnetic susceptibility and specific heat. Experimental verification of temperature variation of the bipartite entanglement measure in terms of magnetic susceptibility and specific heat was done on single crystals of copper acetate-an excellent one dimensional dimer system. The results showed the existence of bipartite entanglement till temperatures as high as room temperature! Large sized single crystals of copper acetate were grown by a new evaporation technique and characterised by TGA, IR and Raman spectroscopy measurements. Density functional theory calculations were done to calculate the delocalisation index which showed much lower values of δ(Cu, Cu) than other bonds, implying that the probability of direct Cu-Cu exchange in copper acetate is very small. Band structure calculations revealed the presence of flat bands at the Fermi level implying very weak intermolecular interactions in copper acetate. 

Athira S. et. al, New J. Physics 25, 103002 (2023).

This work reports the synthesis, structure and magnetic properties of a facile spin 1/2 one dimensional Heisenberg antiferromagnet bis(4-aminopyridinium) bis(oxalato)cuprate(II) dihydrate, (C5H7N2)2ijCuIJC2O4)2] ·2H2O. Single crystals of large sizes of the title compound were obtained using the technique liquid– liquid diffusion or layer diffusion with 100% yield. Single crystal X-ray diffraction measurements revealed the very good quality of the grown single crystals with a small value of goodness of fit R obtained (1.058). Powder X-ray diffraction showed the presence of peaks of the main phase with no impurity peaks, confirming the good quality of the crystals. The structure comprises corner sharing CuO6 octahedra resulting in Cu–Cu chains in the a-direction that are very well isolated in the b and c directions. Density functional theory (DFT) calculations with three different basis sets (B3LYP/6-311++G(d, p); B3LYP/LanL2DZ and B3LYP/6-311++G(d,p), B3LYP/LanL2DZ) generated the optimized geometry of a monomeric unit as well as its vibrational spectra. A vibrational frequency corresponding to the CuO6 octahedron was found in the experimentally obtained FTIR spectrum that matched very well with the theoretically obtained IR spectra incorporating the mixed basis. Temperature dependent dc magnetic susceptibility measurements revealed a low temperature peak, suggesting the presence of low dimensional magnetism in the system. Bonner–Fisher fit confirmed the one dimensional nature of the magnetic interaction with an exchange coupling constant of 1.23 K. Magnetisation measurements along with quantum Monte Carlo simulations confirm this metal–organic crystal to be a very good spin-1/2 Heisenberg antiferromagnet with a low saturation field Hs of 1.75 T. 

We present high resolution thermal expansion measurement data obtained with high relative sensitivity of ΔL/L = 10−9 and accuracy of ± 2 % using closed cycle refrigerators employing two different dilatometers. Experimental details of the set-up utilizing the multi-function probe integrated with the cold head of two kinds of closed cycle refrigerators, namely, pulse tube and GiffordMcMahon cryocoolers, has been described in detail. The design consists of decoupling the bottom sample puck and taking connections from the top of the multi-function probe to mitigate the vibrational noise arising from the cold heads, using which smooth and high quality thermal expansion data could be obtained. It was found that dilatometer#2 performs a better noise mitigation than dilatometer#1 due to the constrained movement of the spring in dilatometer#2. This was confirmed by finite element method simulations that were performed for understanding the spring movement in each dilatometer using which the effect of different forces/pressures and vibrations on the displacement of the spring was studied. Linear thermal expansion coefficient α obtained using both dilatometers was evaluated using derivative of a polynomial fit. The resultant α obtained using dilatometer#2 and either of the closed cycle cryostats on standard metals silver and aluminium showed excellent match with published values obtained using wet cryostats. Finally, thermal expansion measurements is reported on single crystals of two high temperature superconductors YBa2Cu3−xAlxO6+δ and Bi2Sr2CaCu2O8+x along the c-axis with very good match found with published data obtained earlier using wet liquid helium based cryostats.