Ground State Changes Research Articles

Structure, magnetism and magnetocaloric properties in performance GdClWO4 compound

Rare earth elements possessing a diverse range of optical, electrical, and magnetic properties has attracted more attention. In the present work, GdClWO4 compound with monoclinic structure was prepared, the structure, magnetism and magnetocaloric effect (MCE) were investigated. The magnetic phase transition temperature (TN), magnetic ground state and maximum magnetic entropy change (−ΔSMmax) were derived from magnetization measurements. Both first principles calculation and experimental results suggest that the compound displays antiferromagnetic (AFM) interactions, with a magnetic transition occurring at 0.6 K, shifting from a paramagnetic (PM) state to an AFM state. The −ΔSMmax is up to 35.2 J kg−1 K−1 (205.9 mJ cm−3 K−1) for GdClWO4 at T = 1.3 K under a magnetic field change of 0–50 kOe. In particular, the volumetric magnetic entropy change still remains 83.1 mJ cm−3 K−1 at 0.5 K and μ0ΔH = 20 kOe, which is almost twice as much as that of commercial refrigerant Gd3Ga5O12 (GGG) under μ0ΔH = 20 kOe, establishing GdClWO4 compound as a potential candidate for cryogenic refrigeration applications.

Topological magnon–polaron transport in a bilayer van der Waals magnet

The stacking of intrinsically magnetic van der Waals materials provides a fertile platform to explore tunable transport effects of magnons, presenting significant prospects for spintronic applications. The possibility of having topologically nontrivial magnons in these systems can further expand the scope of exploration. In this work, we consider a bilayer system with intralayer ferromagnetic exchange and a weak interlayer antiferromagnetic exchange and study the topological magnon-polaron excitations induced by magnetoelastic couplings. Under an applied magnetic field, the system features a metamagnetic transition, where the magnetic ground state changes from antiparallel layers to parallel. We show that the metamagnetic transition is accompanied by a transition of the topological structure of the magnon polarons, which results in discernible changes in the topology induced transport effects. The magnetic-field dependence of the thermal Hall conductivity and spin Nernst coefficient is analyzed with linear response theories.

Tracking a spin-polarized superconducting bound state across a quantum phase transition

The magnetic exchange coupling between magnetic impurities and a superconductor induce so-called Yu-Shiba-Rusinov (YSR) states which undergo a quantum phase transition (QPT) upon increasing the exchange interaction beyond a critical value. While the evolution through the QPT is readily observable, in particular if the YSR state features an electron-hole asymmetry, the concomitant change in the ground state is more difficult to identify. We use ultralow temperature scanning tunneling microscopy to demonstrate how the change in the YSR ground state across the QPT can be directly observed for a spin-1/2 impurity in a magnetic field. The excitation spectrum changes from featuring two peaks in the doublet (free spin) state to four peaks in the singlet (screened spin) ground state. We also identify a transition regime, where the YSR excitation energy is smaller than the Zeeman energy. We thus demonstrate a straightforward way for unambiguously identifying the ground state of a spin-1/2 YSR state.

Fe-Fe bonding in the rhombic Fe4 cores of the Zintl clusters [Fe4E18]4- (E = Sn and Pb).

We report here the synthesis and characterization of two endohedral Zintl-ion clusters, [Fe4Sn18]4- and [Fe4Pb18]4-, which contain rhombic Fe4 cores. The Fe-Fe bond lengths are all below 2.5 Å, distinctly shorter than in the corresponding Cu clusters, indicating the presence of Fe-Fe bonding. Subtle differences in the structure of the Fe4 core between the two clusters suggest that the change in tetrel element causes a change in electronic ground state, with a very short Fe-Fe bond length of 2.328 Å present across the diagonal of the rhombus in the lead case.

Bulk Behaviour of Ground States for Relativistic Schrödinger Operators with Compactly Supported Potentials

We propose a probabilistic representation of the ground states of massive and massless Schrödinger operators with a potential well in which the behaviour inside the well is described in terms of the moment-generating function of the first exit time from the well and the outside behaviour in terms of the Laplace transform of the first entrance time into the well. This allows an analysis of their behaviour at short to mid-range from the origin. In a first part, we derive precise estimates on these two functionals for stable and relativistic stable processes. Next, by combining scaling properties and heat kernel estimates, we derive explicit local rates of the ground states of the given family of non-local Schrödinger operators both inside and outside the well. We also show how this approach extends to fully supported decaying potentials. By an analysis close-by to the edge of the potential well, we furthermore show that the ground state changes regularity, which depends qualitatively on the fractional power of the non-local operator.

Low-temperature giant coercivity in Co6.2Ga3.8−x Ge x (x = 2.4 to 3.2)

The observation of giant coercivity exceeding 20 kOe at low temperatures in several transition-metal-based compounds has attracted significant attention from a fundamental perspective. This research is also relevant to developing rare-earth-free permanent magnets, wherein cobalt is one of the primary elements used. To facilitate easy fabrication, rare-earth-free and Co-based inorganic bulk magnets that exhibit giant coercivity are highly demanded but rarely reported. Herein, we report the observation of low-temperature giant coercivity in polycrystalline metallic Co6.2Ga3.8−x Ge x (x = 2.4 to 3.2) with the hexagonal Fe13Ge8-type structure composed of Kagome and triangular lattices. As the Ge content x decreases from 3.2, the magnetic ground state changes from ferrimagnetism to ferromagnetism at x = 2.6. In the ferrimagnetic state, we observed a signature of spin frustration arising from the Kagome and/or triangular lattices of Co atoms. The ferromagnetic ordering temperatures for the x = 2.6 and 2.4 samples are 46 K and 60 K, respectively. The coercive fields rapidly increase upon cooling and reach values of 26 kOe and 44 kOe in the x = 2.6 and 2.4 samples, respectively, at 2 K.

Magnetic relaxations in La0.80Ag0.15MnO[formula omitted] nanoparticles

We have performed the study of spin dynamic on two samples of La0.80Ag0.15MnO3+δ nanoparticles with different ratio of double exchange and superexchange interaction due to heat treatment in Ar or in O2. Our study reports that the change of ground state by annealing affects relaxation processes only slightly. Cole–Cole analysis of the AC susceptibility data revealed the simultaneous existence of two separated relaxation processes with two different relaxation times τ1 of 10−4 s and τ2 of 10−1 s, which can be assigned to the relaxation of different magnetic entities. Faster relaxation processes connected with τ1 are associated with smaller value of Cole–Cole parameter α1 and slower relaxation are characterized by τ2 connected with higher value of Cole–Cole parameter α2. The temperature dependence of τ1 (T) differs from τ2 (T). The aging effect in magnetic relaxation we demonstrate on sample prepared in Ar atmosphere using measurements of the magnetization as a function of observation time which were analysed by stretched exponential expression at low and high temperatures. Our results indicate two different relaxation processes at low and high temperatures, the system of nanoparticles exhibits slow magnetic relaxation, i.e., spin glassy behaviour.

Evolution of the magnetic structure in overdoped antiferromagnetic La1−xCaxMnO3(0.51≤x≤0.69) manganites: A neutron diffraction study

The ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ series of compounds with antiferromagnetic ground states ($x\ensuremath{\ge}1/2$) have been extensively studied due to the novel spin, orbital, and charge-ordering states observed when the calcium concentration is a simple fraction ($x=1/2,\phantom{\rule{0.28em}{0ex}}2/3$, and 3/4). The ground states of these compositions have been explained by the Goodenough charge, orbital, and spin ordering model. An important issue remaining is the elucidation of how the ground state changes when $x$ is not a simple number. Here we study the magnetic structure of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ for $0.51\ensuremath{\le}x\ensuremath{\le}0.69$ using powder neutron diffraction measurements supported by magnetization data. For compositions with $0.51\ensuremath{\le}x\ensuremath{\le}0.56$, the magnetic structure, which we term as an incommensurate charge exchange (CE) structure can be described by two propagation vectors ${\mathbf{k}}_{\mathrm{C}}=[1/2,0,1/2]$ and ${\mathbf{k}}_{\mathrm{E}}=[{\ensuremath{\varepsilon}}_{\mathrm{E}},0,1/2]$. In the second one, the component parallel to the ${\mathbf{a}}^{*}$ axis of the reciprocal lattice changes with the ${\mathrm{Mn}}^{4+}$ concentration $x$ as ${\ensuremath{\varepsilon}}_{\mathrm{E}}\ensuremath{\approx}x\ensuremath{-}1/2$ providing, thus, an unambiguous signature of the adoption of an incommensurate magnetic structure. As $x$ gradually increases, the diffraction data reveal that two magnetic phases---one adopting the incommensurate CE, and one adopting the commensurate ``2/3'' magnetic structure--co-exist in the concentration regime of $0.57\ensuremath{\le}x\ensuremath{\le}0.61$. Around the simple fraction $x=2/3$, the magnetic structure can be also described by three propagation vectors, the commensurate ${\mathbf{k}}_{\mathrm{E}}=[0,0,1/2]$, ${\mathbf{k}}_{\mathrm{C}}=[1/2,0,1/2]$, and an incommensurate ${\mathbf{k}}_{2/3}=[1/3+{\ensuremath{\varepsilon}}_{2/3},0,1/2]$ propagation vector with ${\ensuremath{\varepsilon}}_{2/3}$ taking negative/zero/positive values for $x$ smaller than/equal to/larger than 2/3, respectively. Our experimental results for $0.51\ensuremath{\le}x\ensuremath{\le}0.56$ are neither in favor of a stripe structure consisting of a fine mixture of $x=1/2$ and $x=2/3$ phases (phase separation) nor of a defect structure in which an appropriate amount of ${\mathrm{Mn}}^{3+}$-O sheets have been replaced by ${\mathrm{Mn}}^{4+}$-O sheets (defect structure). A sinusoidal modulated structure has been used as a possible candidate in explaining the experimental neutron diffraction magnetic Bragg peaks. This result may be linked to the presence of a mixed orbital state of the manganese ions.

Josephson current via spin and orbital states of a tunable double quantum dot

Supercurrent transport is experimentally studied in a Josephson junction hosting a double quantum dot (DQD) with tunable symmetries. The QDs are parallel-coupled to two superconducting contacts and can be tuned between strong inter-dot hybridization and a ring geometry where hybridization is suppressed. In both cases, we observe supercurrents when the two interacting orbitals are either empty or filled with spins, or a combination. However, when each QD hosts an unpaired spin, the supercurrent depends on the spin ground state. It is strongly suppressed for the ring geometry with a spin-triplet ground state at zero external magnetic field. By increasing the QD hybridization, we find that a supercurrent appears when the ground state changes to spin-singlet. In general, supercurrents are suppressed in cases of spin doublet ground state, but an exception occurs at orbital degeneracy when the system hosts one additional spin, as opposed to three, pointing to a broken particle-hole symmetry.

Mechanical control of physical properties in the van der Waals ferromagnet Cr2Ge2Te6 via application of electric current

Cr2Ge2Te6 is a van der Waals ferromagnet with a Curie temperature at 66 K. Here we report a swift change in the magnetic ground state upon application of small DC electric current, a giant yet anisotropic magnetoelectric effect, and a sharp, lattice-driven quantum switching manifested in the I-V characteristic of the bulk single-crystal Cr2Ge2Te6. At the heart of these observed phenomena is a newly uncovered, strongly anisotropic magnetoelastic coupling that enables strongly anisotropic responses of the lattice to application of electric current and/or magnetic field, thus the exotic phenomena in Cr2Ge2Te6. Such a rare mechanical tunability in the magnetic semiconductors promises tantalizing prospects for unique functional materials and devices.

Consecutive topological phase transitions and colossal magnetoresistance in a magnetic topological semimetal

The combination of magnetic symmetries and electronic band topology provides a promising route for realizing topologically nontrivial quasiparticles, and the manipulation of magnetic structures may enable the switching between topological phases, with the potential for achieving functional physical properties. Here, we report measurements of the electrical resistivity of EuCd2As2 under pressure, which show an intriguing insulating dome at pressures between pc1 ~ 1.0 GPa and pc2 ~ 2.0 GPa, situated between two regimes with metallic transport. The insulating state can be fully suppressed by a small magnetic field, leading to a colossal negative magnetoresistance on the order of 105%, accessible via a modest field of ~ 0.2 T. First-principles calculations reveal that the dramatic evolution of the resistivity under pressure can be attributed to consecutive transitions of EuCd2As2 from a magnetic topological insulator to a trivial insulator, and then to a Weyl semimetal, with the latter resulting from a pressure-induced change in the magnetic ground state. Similarly, the colossal magnetoresistance results from a field-induced polarization of the magnetic moments, transforming EuCd2As2 from a trivial insulator to a Weyl semimetal. These findings underscore weak exchange couplings and weak magnetic anisotropy as ingredients for discovering tunable magnetic topological materials with desirable functionalities.

Density Functional Theory Study of Oxygen Deficiencies in SrCoO25

We report the effects of oxygen deficiency to the electronic and magnetic properties of SrCoO3 calculated using spin-polarized Density Functional Theory. The correlation effects of the Co 3d orbitals are considered explicitly using the effective Hubbard U eff parameter. The U eff consists of the subtraction between the U value of 3 eV to denote the Coulomb interaction and the Hund’s coupling parameter J of 0.95 eV. Our results show that the localized character of the Co 3d orbitals strongly depends on the oxygen content. The magnetic exchange interaction between two neighboring Co ions is therefore altered. As a consequence, a change of magnetic ground state using the same U eff value from ferromagnetic ordering in full oxygen content to antiferromagnetic ordering in SrCoO2.5 is obtained.

Quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry: Magnetism, correlations, and phase transitions

We describe here a quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry. The simulator consists of a structured lateral gate confining two dimensional electrons in a quantum well into artificial minima arranged in a hexagonal lattice. The sublattice symmetry breaking is generated by forming an artificial triangular graphene quantum dot (ATGQD) with zigzag edges. The resulting extended Hubbard model generates tunable ratio of tunneling strength to electron-electron interactions and of sublattice symmetry with control over shape. The validity of the simulator is confirmed for small systems using mean-field and exact diagonalization many-body approaches which show that the ground state changes from a metallic to an antiferromagnetic (AF) phase by varying the distance between sites or depth of the confining potential. The one-electron spectrum of these triangular dots contains a macroscopically degenerate shell at the Fermi level. The shell persists at the mean-field level for weak interactions (metallic phase) but disappears for strong interactions, in the AF phase. We determine the effects of electron-electron interactions on the ground state, the total spin, and the excitation spectrum as a function of filling of the ATGQD. We find that the half-filled charge neutral shell leads to a partially spin polarized state in both metallic and AF regimes in accordance with Liebs theorem. In both regimes a relatively large gap separates the spin polarized ground state to the first excited many-body state at half filling of the degenerate shell. By adding or removing an electron, this gap drops dramatically, and alternate total spin states emerge with energies nearly degenerate to a spin polarized ground state.

First demonstration of tuning between the Kitaev and Ising limits in a honeycomb lattice.

Recent observations of novel spin-orbit coupled states have generated interest in 4d/5d transition metal systems. A prime example is the state in iridate materials and α-RuCl3 that drives Kitaev interactions. Here, by tuning the competition between spin-orbit interaction (λSOC) and trigonal crystal field (ΔT), we restructure the spin-orbital wave functions into a previously unobserved state that drives Ising interactions. This is done via a topochemical reaction that converts Li2RhO3 to Ag3LiRh2O6. Using perturbation theory, we present an explicit expression for the state in the limit ΔT ≫ λSOC realized in Ag3LiRh2O6, different from the conventional state in the limit λSOC ≫ ΔT realized in Li2RhO3. The change of ground state is followed by a marked change of magnetism from a 6 K spin-glass in Li2RhO3 to a 94 K antiferromagnet in Ag3LiRh2O6.

Structure and magnetic studies of geometrically frustrated disordered pyrochlores A2Zr2O7: (A [formula omitted] Eu, Gd, Er)

Dy2Ti2O7 and Ho2Ti2O7 are the well-known spin ice systems where the replacement of Ti with Zr destabilizes the pyrochlore lattice and leads to the dynamic ground state. We have studied a series of disordered pyrochlore oxides A2Zr2O7 (A = Eu, Gd, Er) and compared their crystal structure, magnetic, and heat capacity behavior with that of Dy2Zr2O7 and Ho2Zr2O7 systems. The analysis of the dc susceptibility curves suggests these systems as a frustrated antiferromagnet with negative Curie–Weiss temperature. Eu2Zr2O7 exhibited a temperature-independent dc susceptibility in the range T = 20–90 K, which persist even at high fields due to the Van Vleck contribution. Gd2Zr2O7 and Er2Zr2O7 shows an unusual field-induced spin freezing anomaly near T≈ 10 K which is similar to (Dy/Ho)2Zr2O7 systems. These studies show that depending on the disorder parameter, the spin-freezing behavior can be induced by slowing down the spin dynamic with a suitable choice of the magnetic field. We observed that, unlike titanates, modification at the rare-earth site does not make a considerable change in the magnetic ground state of these Dy, Ho, Er, Gd based zirconates compounds.

Stacking order and Coulomb correlation effect in the layered charge density wave phase of 1T−NbS2

Based on first-principles calculations, we explored the interplay between stacking effect and electron-electron correlation in the layered vdW material of bulk 1T-NbS2 with a 2D charge density wave (CDW) order. Without considering the Coulomb correlation, two energetically favorable out-of-plane stacking configurations are identified: one is a metallic phase with a single-layer stacking pattern, another is a band insulator with a paired-bilayer stacking configuration. Even though the Coulomb correlation is taken into account, the two energetic favorable stacking orders are still far more stable than other stacking orders. Furthermore, increasing the Coulomb interaction, the paired-bilayer stacking configuration transforms from nonmagnetic band insulator to antiferromagnetic insulator, while the single-layer stacking undergoes a Slater-Mott metal-insulator transition, which indicates the non-negligible role of electron-electron correlation interactions. In addition, the electronic structure and magnetic ground state change drastically among different stacking configurations, providing a platform to tune the electronic structures and interlayer magnetic interactions by altering the stacking order. In contrast to the widely accepted scenario of Mott localization as the driving force behind the gap formation in the CDW phase of layered transition metal dichalcogenides, our results not only highlight the crucial role of stacking order in the electronic structures of 1T-NbS2, but also shed fresh light on the distinct effects of Coulomb interaction in different stacking arrangements.

Magnetic order transition in monolayerMoS2induced by strong intervalley correlation

In this work, we study a model for monolayer molybdenum disulfide with including the intravalley and intervalley electron-electron interaction. We solve the model at a self-consistent mean-field level and get three solutions $L_{0}$, $L_{+}$ and $L_{-}$. As for $L_{0}$, the spin polarizations are opposite at $\textbf{K}$ and $\textbf{K}^{\prime}$ valley and the total magnetization is zero. $L_{\pm}$ describe two degenerate spin-polarized states, and the directions of polarization are opposite for the states of $L_{+}$ and $L_{-}$. Based on these results, the ground state can be deduced to be spin polarized in domains in which their particular states can be randomly described by $L_{+}$ or $L_{-}$. Therefore, a zero net magnetization is induced for zero external magnetic field $\mathbf{B}$, but a global ferromagnetic ground state for a nonzero $\mathbf{B}$. We estimate the size of domains as several nanometers. As the increase of the chemical potential, the ground state changes between $L_{0}$ and $L_{\pm}$, indicating first order phase transitions at the borders, which is coincident with the observation of photoluminescence experiments in the absence of the external magnetic field [J. G. Roch $\it{et}$ $\it{al.}$, Phys. Rev. Lett. ${\bf 124}$, 187602 (2020)].

Trends of higher-order exchange interactions in transition metal trilayers

We present a systematic study of higher-order exchange interactions beyond the pair-wise Heisenberg exchange in transition-metal trilayers based on density functional theory calculations. We show that these terms can play an important role in magnetic trilayers composed of a single hexagonal Fe or Co atomic layer sandwiched between $4d$ and $5d$ transition-metal layers. We study the dependence of the biquadratic and the three-site and four-site four spin interaction on the band filling of the $4d$ and $5d$ layers as well as the stacking sequence, i.e. fcc vs. hcp stacking. Our calculations reveal relatively small higher-order interactions for Co based trilayers. For Fe based trilayers with a Rh or Ir layer the higher-order terms can be on the same order of magnitude as pair-wise Heisenberg exchange. The trends obtained for freestanding trilayers are used to understand the higher-order interactions in ultrathin film systems on surfaces that are experimentally accessible. It is shown that hcp Rh/Fe/Ir(111) and hcp-Rh/Fe/Rh(111) exhibit the largest values for the biquadratic and the three-site four spin interaction of all systems under study. We further demonstrate that the three-site four spin interaction is responsible for the experimentally observed change of the magnetic ground state of Rh/Fe/Ir(111) from a spin spiral (single-Q) for fcc-Rh to a 2Q state for hcp-Rh. We find similar trends for Rh/Fe/Rh(111), i.e. replacing the Ir surface by the isoelectronic Rh surface. For Rh/Co/Ir(111), we obtain a negative value for the four-site four spin interaction which will lead to a reduced stability of magnetic skyrmions which are metastable in this film at zero magnetic field. In contrast, for Pd/Fe/Ir(111), the four-site four spin interaction is positive which leads to an enhanced stability of skyrmions.

Study of the Structure, Magnetic, Thermal and Electrical Characterisation of ZnCr2Se4: Ta Single Crystals Obtained by Chemical Vapour Transport.

The new series of single-crystalline chromium selenides, Ta-doped ZnCr2Se4, was synthesised by a chemical vapour transport method to determine the impact of a dopant on the structural and thermodynamic properties of the parent compound. We present comprehensive investigations of structural, electrical transport, magnetic, and specific heat properties. It was expected that a partial replacement of Cr ions by a more significant Ta one would lead to a change in direct magnetic interactions between Cr magnetic moments and result in a change in the magnetic ground state and electric transport properties of the ZnCr2−xTaxSe4 (x = 0.05, 0.06, 0.07, 0.08, 0.1, 0.12) system. We found that all the elements of the cubic system had a cubic spinel structure; however, the doping gain linearly increased the ZnCr2−xTaxSe4 unit cell volume. Doping with tantalum did not significantly change the semiconductor and magnetic properties of ZnCr2Se4. For all studied samples (0 ≤ x ≤ 0.12), an antiferromagnetic order (AFM) below TN~22 K was observed. However, a small amount of Ta significantly reduced the second critical field (Hc2) from 65 kOe for x = 0.0 (ZnCr2Se4 matrix) up to 42.2 kOe for x = 0.12, above which the spin helical system changed to ferromagnetic (FM). The Hc2 reduction can lead to strong competition among AFM and FM interactions and spin frustration, as the specific heat under magnetic fields H < Hc2 shows a strong field decrease in TN.

Low-temperature breakdown of many-body perturbation theory for thermodynamics

It is shown analytically and numerically that the finite-temperature many-body perturbation theory in the grand canonical ensemble has zero radius of convergence at zero temperature when the energy ordering or degree of degeneracy for the ground state changes with the perturbation strength. When the degeneracy of the reference state is partially or fully lifted at the first-order Hirschfelder-Certain degenerate perturbation theory, the grand potential and internal energy diverge as $T \to 0$. Contrary to earlier suggestions of renormalizability by the chemical potential $\mu$, this nonconvergence, first suspected by W. Kohn and J. M. Luttinger, is caused by the nonanalytic nature of the Boltzmann factor $e^{-E/k_\text{B}T}$ at $T=0$, also plaguing the canonical ensemble, which does not involve $\mu$. The finding reveals a fundamental flaw in perturbation theory, which is deeply rooted in the mathematical limitation of power-series expansions and is unlikely to be removed within its framework.