arXiv subjects

A. M. dos Santos

Publications and source records attributed to A. M. dos Santos.

14 recordsLinked to original sources

arXiv2025

Atomistic Description of Spin Crossover Under Pressure and its Giant Barocaloric Effect

The pressure-dependent evolution of the spin crossover (SCO) transition has garnered significant interest due to its connection to the giant barocaloric effect (BCE) near room temperature. Pressure alters both the molecular and solid-state structures of SCO materials, affecting the relative stability of low- and high-spin states and, consequently, the transition temperature ($T_{1/2}$). Crucially, the shape of the $T_{1/2}$ vs. pressure curve dictates the magnitude of the BCE, making its accurate characterization essential for identifying high-performance materials. In this work, we investigate the nonlinear $T_{1/2}$ vs. pressure behavior of the prototypical SCO complex [FeL$_2$][BF$_4$]$_2$ [L = 2,6-di(pyrazol-1-yl)pyridine] using solid-state PBE+U computations. Our results unveil the mechanisms by which pressure influences its SCO transition, including the onset of a phase transition, as well as the key role of low-frequency phonons in the BCE. Furthermore, we establish a computational protocol for accurately modeling the BCE in SCO crystals, providing a powerful tool for the rapid and efficient discovery of new materials with enhanced barocaloric performance.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional S-orbital state of Tb and cooperative Ru(4d)-Tb(4f) spin-ordering in strongly correlated 4d-4f system, Ba3TbRu2O9

The 6H-perovskite Ba3RRu2O9 (R = rare-earth), composed of Ru2O9 dimers connected through RO6 octahedra, exhibits an intriguing variety of magnetic ground states, ranging from non-magnetic to ferromagnetic and antiferromagnetic, depending on the specific R ion. In this study, we investigate the compound Ba3TbRu2O9 using magnetic susceptibility measurements and time-of-flight neutron diffraction experiments. Our combined bulk and microscopic analyses reveal that the Tb4+ (4f7) electronic configuration results in an s-like state with an orbital moment L=0 and spin-only value of S=7/2, and Ru4+ exhibits a spin-only value of S=1 despite the presence of strong spin-lattice coupling in this compound, representing a sharp contrast to other reported members of this family. A cooperative 4d-4f spin ordering is observed below the Neel temperature (around 9.5 K), indicating strong Ru(4d)=Tb(4f) correlations in the system. The Tb-moments order antiferromagnetically in the bc-plane, whereas the Ru-moments are aligned antiferromagnetically along the b-axis. Furthermore, a collinear antiferromagnetic arrangement of spins is observed within the Ru2O9 dimers throughout the structure, unlike other reported members of this series (e.g., Ho and Nd).

cond-mat.mtrl-sci↗

arXiv2024

Helical Spin Dynamics in Commensurate Magnets: a Study on Brochantite, Cu$_4$SO$_4$(OH)$_6$

We report the direct observation of a commensurate-ordered antiferromagnetic (AFM) state but incommensurate helical spin dynamics in the natural mineral brochantite Cu$_4$SO$_4$(OH)$_6$ through neutron diffraction and neutron spectroscopy measurements. Inelastic neutron scattering measurements reveal magnon-like excitations with considerable dispersion along the c-axis and almost flat branches in other principal directions, indicating the strong one-dimensional character of the magnetic correlations. We experimentally observe the effect of the uniform Dzyaloshinskii-Moriya (DM) interaction, which elevates the degeneracy of the spin-wave modes shifting them in opposite directions in reciprocal space. The system has a commensurate AFM ground state, stabilized by the anisotropic symmetric Heisenberg exchange interactions, and quasi-one-dimensional chiral spin dynamics due to the antisymmetric DM interaction. Employing linear spin-wave theory, we were able to construct an effective Heisenberg Hamiltonian. We quantify both the symmetric exchange parameters and the DM vector components in Cu$_4$SO$_4$(OH)$_6$ and determine the mechanism of the magnetic frustration. Our work provides detailed insights into the complex dynamics of the spin chain in the presence of uniform DM interaction.

cond-mat.str-el↗

arXiv2024

Origin of spin-driven ferroelectricity and effect of external pressure on the complex magnetism of 6H-perovskite Ba3HoRu2O9

The compound Ba3HoRu2O9 magnetically orders at 50 K (TN1) followed by another complex magnetic ordering at 10.2 K (TN2). The 2nd magnetic phase transition was characterized by the co-existence of two competing magnetic ground states associated with two different magnetic wave vectors (K1=1/2 0 0 and K2=1/4 1/4 0). Here, we have discussed the origin of spin-driven ferroelectricity, which is not known yet. We demonstrate through time-of-flight Neutron diffraction and theoretical calculation that the non-collinear structure involving two different magnetic ions, Ru(4d) and Ho(4f), break the spatial inversion symmetry via inverse Dzyaloshinskii-Moriya (D-M) interaction through strong 4d-4f magnetic correlation, which shifts the oxygen atoms and results in non-zero polarization. Such an observation of inverse D-M interaction from two different magnetic ions which caused ferroelectricity is rarely observed. We have systematically studied the spin and dipolar dynamics, which exhibit intriguing behavior with shorter coherence lengths of 2nd magnetic phase associated with the k2-wave vector. The results manifest the development of finite-size magnetoelectric domains instead of true long-range ordering which justifies the experimentally obtained low value of ferroelectric polarization. The synchrotron XRD analysis predicts a non-centrosymmetric space group P-62c. Furthermore, we have investigated the effect of external pressure on this complex magnetism. The result reveals an enhancement of ordering temperature by the application of external pressure (1.6 K/GPa). The external pressure might favor stabilizing the magnetic ground state associated with 2nd magnetic phase. Our study shows an unconventional mechanism of spin-driven ferroelectricity.

cond-mat.str-el↗

arXiv2022

Stronger quantum fluctuation with larger spins: Emergent magnetism in the pressurized high-temperature superconductor FeSe

A counter-intuitive enhancement of quantum fluctuation with larger spins, together with a few novel physical phenomena, is discovered in studying the recently observed emergent magnetism in high-temperature superconductor FeSe under pressure. Starting with experimental crystalline structure from our high-pressure X-ray refinement, we analyze theoretically the stability of the magnetically ordered state with a realistic spin-fermion model. We find surprisingly that in comparison with the magnetically ordered Fe-pnictides, the larger spins in FeSe suffer even stronger long-range quantum fluctuation that diminishes their ordering at ambient pressure. This "fail-to-order" quantum spin liquid state then develops into an ordered state above 1GPa due to weakened fluctuation accompanying the reduction of anion height and carrier density. The ordering further benefits from the ferro-orbital order and shows the observed enhancement around 1GPa. We further clarify the controversial nature of magnetism and its interplay with nematicity in FeSe in the same unified picture for all Fe-based superconductors. In addition, the versatile itinerant carriers produce interesting correlated metal behavior in a large region of phase space. Our study establishes a generic exceptional paradigm of stronger quantum fluctuation with larger spins that complements the standard knowledge of insulating magnetism.

cond-mat.str-el↗

arXiv2020

Pressure induced topological quantum phase transition in Weyl semimetal T_d-MoTe_2

We report the pressure (p_max = 1.5 GPa) evolution of the crystal structure of the Weyl semimetal T_d-MoTe_2 by means of neutron diffraction experiments. We find that the fundamental non-centrosymmetric structure T_d is fully suppressed and transforms into a centrosymmertic 1T' structure at a critical pressure of p_cr = 1.2 GPa. This is strong evidence for a pressure induced quantum phase transition (QPT) between topological to a trivial electronic state. Although the topological QPT has strong effect on magnetoresistance, it is interesting that the superconducting critical temperature T_c, the superfluid density, and the SC gap all change smoothly and continuously across p_cr and no sudden effects are seen concomitantly with the suppression of the T_d structure. This implies that the T_c, and thus the SC pairing strength, is unaffected by the topological QPT. However, the QPT requires the change in the SC gap symmetry from non-trivial s+- to a trivial s++ state, which we discuss in this work. Our systematic characterizations of the structure and superconducting properties associated with the topological QPT provide deep insight into the pressure induced phase diagram in this topological quantum material.

cond-mat.mtrl-sci↗

arXiv2018

Soft antiphase tilt of oxygen octahedra in the hybrid improper multiferroic Ca3Mn1.9Ti0.1O7

We report a single crystal neutron and x-ray diffraction study of the hybrid improper multiferroic Ca3Mn1.9Ti0.1O7 (CMTO), a prototypical system where the electric polarization arises from the condensation of two lattice distortion modes.With increasing temperature (T ), the out-of-plane, antiphase tilt of MnO6 decreases in amplitude while the in-plane, in-phase rotation remains robust and experiences abrupt changes across the first-order structural transition. Application of hydrostatic pressure (P) to CMTO at room temperature shows a similar effect. The consistent behavior under both T and P reveals the softness of antiphase tilt and highlights the role of the partially occupied d orbital of the transition-metal ions in determining the stability of the octahedral distortion. Polarized neutron analysis indicates the symmetry-allowed canted ferromagnetic moment is less than the 0.04 μB/Mn site, despite a substantial out-of-plane tilt of the MnO6 octahedra.

cond-mat.str-el↗

arXiv2017

Pressure tuning of structure, superconductivity and novel magnetic order in the Ce-underdoped electron-doped cuprate T'-Pr_1.3-xLa_0.7Ce_xCuO_4 (x = 0.1)

High-pressure neutron powder diffraction, muon-spin rotation and magnetization studies of the structural, magnetic and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system T'-Pr_1.3-xLa_0.7Ce_xCuO_4 with x = 0.1 are reported. A strong reduction of the lattice constants a and c is observed under pressure. However, no indication of any pressure induced phase transition from T' to T structure is observed up to the maximum applied pressure of p = 11 GPa. Large and non-linear increase of the short-range magnetic order temperature T_so in T'-Pr_1.3-xLa_0.7Ce_xCuO_4 (x = 0.1) was observed under pressure. Simultaneously pressure causes a non-linear decrease of the SC transition temperature T_c. All these experiments establish the short-range magnetic order as an intrinsic and a new competing phase in SC T'-Pr_1.2La_0.7Ce_0.1CuO_4. The observed pressure effects may be interpreted in terms of the improved nesting conditions through the reduction of the in-plane and out-of-plane lattice constants upon hydrostatic pressure.

cond-mat.supr-con↗

arXiv2016

Carboxylate based molecular magnet: one path toward achieving stable quantum correlations at room temperature

The control of quantum correlations in solid state systems by means of material engineering is a broad avenue to be explored, since it makes possible steps toward the limits of quantum mechanics and the design of novel materials with applications on emerging quantum technologies. In this context, this Letter explores the potential of molecular magnets to be prototypes of materials for quantum information technology. More precisely, we engineered a material and from its geometric quantum discord we found significant quantum correlations up to 9540 K (even without entanglement); and, in addition, a pure singlet state occupied up to around 80 K (above liquid nitrogen temperature). These results could only be achieved due to the carboxylate group promoting a metal-to-metal huge magnetic interaction.

quant-ph↗

arXiv2015

Pressure-induced collapsed-tetragonal phase in SrCo2As2

We present high-energy x-ray diffraction data under applied pressures up to p = 29 GPa, neutron diffraction measurements up to p = 1.1 GPa, and electrical resistance measurements up to p = 5.9 GPa, on SrCo2As2. Our x-ray diffraction data demonstrate that there is a first-order transition between the tetragonal (T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6 GPa at T = 7 K. The pressure for the onset of the cT phase and the range of coexistence between the T and cT phases appears to be nearly temperature independent. The compressibility along the a-axis is the same for the T and cT phases whereas, along the c-axis, the cT phase is significantly stiffer, which may be due to the formation of an As-As bond in the cT phase. Our resistivity measurements found no evidence of superconductivity in SrCo2As2 for p <= 5.9 GPa and T >= 1.8 K. The resistivity data also show signatures consistent with a pressure-induced phase transition for p >= 5.5 GPa. Single-crystal neutron diffraction measurements performed up to 1.1 GPa in the T phase found no evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K. Spin-polarized total-energy calculations demonstrate that the cT phase is the stable phase at high pressure with a c/a ratio of 2.54. Furthermore, these calculations indicate that the cT phase of SrCo2As2 should manifest either A-type antiferromagnetic or ferromagnetic order.

cond-mat.supr-con↗

arXiv2012

Evidence for entanglement at high temperatures in an engineered molecular magnet

The molecular compound [Fe$_{2}$($μ_{2}$-oxo)(C$_{3}$H$_{4}$N$_{2}$)$_{6}$(C$_{2}$O$_{4}$)$_{2}$] was designed and synthesized for the first time and its structure was determined using single-crystal X-ray diffraction. The magnetic susceptibility of this compound was measured from 2 to 300 K. The analysis of the susceptibility data using protocols developed for other spin singlet ground-state systems indicates that the quantum entanglement would remain at temperatures up to 732 K, significantly above the highest entanglement temperature reported to date. The large gap between the ground state and the first-excited state (282 K) suggests that the spin system may be somewhat immune to decohering mechanisms. Our measurements strongly suggest that molecular magnets are promising candidate platforms for quantum information processing.

quant-ph↗

arXiv2009

Temperature and Pressure Dependence of the Fe-specific Phonon Density of States in Ba(Fe(1-x)Co(x))2As2

The {57}Fe-specific phonon density of states of Ba(Fe(1-x)Co(x))2As2 single crystals (x=0.0, 0.08) was measured at cryogenic temperatures and at high pressures with nuclear-resonant inelastic x-ray scattering. Measurements were conducted for two different orientations of the single crystals, yielding the orientation-projected {57}Fe-phonon density of states (DOS) for phonon polarizations in-plane and out-of-plane with respect to the basal plane of the crystal structure. In the tetragonal phase at 300 K, a clear stiffening was observed upon doping with Co. Increasing pressure to 4 GPa caused a marked increase of phonon frequencies, with the doped material still stiffer than the parent compound. Upon cooling, both the doped and undoped samples showed a stiffening, and the parent compound exhibited a discontinuity across the magnetic and structural phase transition. These findings are generally compatible with the changes in volume of the system upon doping, increasing pressure, or increasing temperature, but an extra softening of high-energy modes occurs with increasing temperature. First-principles computations of the phonon DOS were performed and showed an overall agreement with the experimental results, but underestimate the Grueneisen parameter. This discrepancy is explained in terms of a magnetic Grueneisen parameter, causing an extra phonon stiffening as magnetism is suppressed under pressure.

cond-mat.supr-con↗

arXiv2009

Entanglement temperature in molecular magnets composed of S-spin dimers

In the present work, we investigate the quantum thermal entanglement in molecular magnets composed of dimers of spin $S$, using an Entanglement Witness built from measurements of magnetic susceptibility. An entanglement temperature, $T_{e}$, is then obtained for some values of spin $S$. From this, it is shown that $T_{e}$ is proportional to the intradimer exchange interaction $J$ and that entanglement appears only for antiferromagnetic coupling. The results are compared to experiments carried on three isostructural materials: KNaMSi$_{4}$O$_{10}$ (M$=$Mn, Fe or Cu).

quant-ph↗

arXiv2009

Entanglement and Bell's inequality violation above room temperature in metal carboxylates

In the present work we show that a special family of materials, the metal carboxylates, may have entangled states up to very high temperatures. From magnetic susceptibility measurements, we have estimated the critical temperature below which entanglement exists in the cooper carboxylate \{Cu$_2$(O$_2$CH)$_4$\}\{Cu(O$_2$CH)$_2$(2-methylpyridine)$_2$\}, and we have found this to be above room temperature ($T_e \sim 630$ K). Furthermore, the results show that the system remains maximally entangled until close to $\sim 100$ K and the Bell's inequality is violated up to nearly room temperature ($\sim 290$ K).

quant-ph↗