Mixed Ground State Research Articles

Investigation of conventional exchange bias in (Nd1-xHox)2CoMnO6 (x = 0.0–0.4) double perovskite

In this article, we have investigated the characteristics related to structure, magnetism and exchange bias behavior in single phased polycrystalline samples of (Nd1-xHox)2CoMnO6 (x = 0.0–0.4) (NHCMO) series. The samples were prepared using the conventional solid state reaction technique and they crystalize in monoclinic structure with P21/n space group. The x = 0.0 sample shows the ferromagnetic transitions at TC1 = 139K and TC2 = 170K. These values decrease to 136K and 160K respectively for x = 0.1 sample. However, for other Ho substituted samples we observed only one transition. For x = 0.2, TC is 129K which decreases to 118K for x = 0.4 sample. The ferromagnetic super exchange interaction that arises within the network of Co2+ and Mn4+ ions bridged by O2− ions is the reason of occurring of these transitions. The presence of anti-site defects with antiferromagnetic interactions and subsequent formation of anti-phase boundaries within the system cannot be ignored. To gain a deeper insight into the underlying magnetic interactions, the M − T plots, under both the ZFC and FC conditions, were measured by applying different external magnetic fields. Saturation magnetization (MS) value of 3.68 μB/f.u. is observed in parent sample. The MS value increases to 7.22 μB/f.u. for x = 0.4 sample. Such mixed magnetic ground state of the samples motivates us to perform the exchange bias field measurement revealing an exchange bias field of 725Oe is found for x = 0.0 sample at 5K.

Two-dimensional miscible-immiscible supersolid and droplet crystal state in a homonuclear dipolar bosonic mixture

The recent realization of a binary dipolar Bose-Einstein condensate [A. Trautmann et al., Phys. Rev. Lett. 121, 213601 (2018)] presents new exciting aspects for studying quantum droplets and supersolids in a binary mixture. Motivated by this experiment, we study the ground-state phases and dynamics of a Dy-Dy mixture. The dipolar bosonic mixture exhibits qualitatively novel and rich physics. Relying on the three-dimensional numerical simulations in the extended Gross-Pitaevskii framework, we unravel the ground-state phase diagrams and characterize their different possible phases. The emergent phases include single-droplet (SD), multiple-droplets (MD), doubly supersolid (SS), and superfluid (SF) states in both miscible and immiscible phases. Intriguing mixed ground states are observed for an imbalanced binary mixture, including a combination of SS-SF, SS-MD, and SS-SS phases. We also explore the dynamics across the phase boundaries by linear quenches of interspecies and intraspecies scattering lengths. During these dynamical processes, we observe an abrupt change in phase which initially results in some instability in the system and forms some metastable states in the intermediate timescale. However, the state produced in long-time evolution is similar to our predicted ground state. Although we demonstrate the possible results for a Dy-Dy mixture and for a specific parameter range of intraspecies and interspecies scattering lengths, our results are in general valid for other dipolar mixtures and may become an important benchmark for future experimental scenarios.

Zero-mode entanglement across a conformal defect

We consider a free-fermion chain with a conformal defect that features an extended zero mode, and study the entanglement properties in its mixed ground state. The zero-mode induced degeneracy modifies the density of states in the single-particle entanglement spectrum, which can be calculated via the full counting statistics. For a homogeneous chain, the resulting change in the Rényi entropy is derived analytically for arbitrary subsystem ratios in the thermodynamic limit. For a conformal defect located in the center, analogous results can be obtained for the half-chain entanglement. In particular, we observe parity effects for half-chains with even/odd sites, which do not decay with size.

Crystal field effects in the zig-zag chain compound SrTm2O[formula omitted

The single ion properties of the zig-zag chain compound SrTm2O4 have been investigated using heat capacity, magnetic susceptibility, magnetization, inelastic neutron scattering, and polarized muon spectroscopy. Two crystal field models are employed to estimate the single ion properties; a Density Function Theory based model and an effective charge model based on the Hutchings point charge model. The latter describes our experimental results well. This model estimates an easy-axis anisotropy for one of the Tm3+ sites and an easy-plane anisotropy for the second site. It also predicts a mixed ground state with dominating J=0 characteristics for both sites. Additionally, muon spin rotation/relaxation (μ+SR) spectra reveal oscillations, typically a sign of long-range magnetic order. However, the temperature dependence of the precession frequency and the relaxation rates indicate that the system is in an extended critical regime and the observed relaxation is actually dynamic.

Mixed ground state in Fe-Ni Invar alloys

We investigate the ground state properties of Invar alloys via detailed study of the electronic structure of Fe1−xNix alloys (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) employing x-ray photoelectron spectroscopy (XPS). While all the alloys exhibit soft ferromagnetic behavior with Curie temperature much higher than the room temperature, the results for invar alloy, Fe0.6Ni0.4 exhibit anomalous behavior. Moreover, the magneto-resistance of the Invar alloy becomes highly negative while the end members possess positive magneto-resistance. The core level spectra of the Invar alloy exhibit emergence of a distinct new feature below 20 K while all other Fe-Ni alloys exhibit no temperature dependence down to 10 K. Interestingly, the shallow core level spectra (3s, 3p) of Fe and Ni of the Invar alloy reveal stronger deviation at low temperatures compared to the deep core levels (2s, 2p) indicating crystal field effect. It appears that there is a large precipitation of antiferromagnetic γ′ phase below 20 K possessing low magnetic moment (0.5μB) on Fe within the α phase. The discovery of negative magneto-resistance, anomalous magnetization at low temperature and the emergence of unusual new features in the core levels at low temperature provide an evidence of mixed phase in the ground state of Invar alloys.

Fe-Ni Invar alloys: A review

• Historical development of Invar alloys. • Explanation of Invar anomaly. • Review of magnetic properties of Invars. • Mixed ground state of Invars. Fe-Ni alloys with face centred cubic structure having Ni concentration around 36% exhibit extremely low or no coefficient of thermal expansion over a wide range of temperatures around room temperature which is called Invar behaviour. The Invar behaviour was first observed in Fe-Ni alloys by Charles Édouard Guillaume in the year 1896 and was awarded Nobel prize in physics in the year 1920. The observed Invar behaviour was puzzling and elusive for physicists. The origin of Invar phenomena has been the subject of serious concern for physicists since the observation of the phenomenon. The first model explained the Invar behaviour is called two gamma state model suggested by Weiss, considers two magnetic states of Fe in Invar alloys one with high spin high volume and another with low spin low volume states. When the temperature is increased low spin low volume states get populated at the cost of high spin high volume states and thus compensate the expected thermal expansion. However, this was shown to be incorrect and it seems that high moment to low moment transition is preceded by a frustrated ferromagnetic state. The moment volume instabilities in Invar alloys also lead to anomalous elastic properties. The Invar alloys find applications in the fabrication of watches, cryogenic storage dewars and aerospace engineering parts.

Interesting spin state properties of iron(II) polypyridine complex substituted by fluorine: A theoretical study

The special iron(II) polypyridyl complex substituted by fluorine ([Fe(dftpy)2]2+, dftpy = 6,6″-difluoro-2,2’; 6′2″-terpyridine) with uncommon mixed ground states has been detected recently, but the related explanation and characterization are not enough, especially for the influence of substituents. On the basis of previous studies, the key problem of mixed spin states is expected to be described very well by means of theoretical calculations. In this work, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been carried out to study the characteristics of the excited states in detail. Comparing with the parent complex [Fe(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) and corresponding bromine or chlorine substituted derivates, the ground states of [Fe(dftpy)2]2+ stay around the mixture of singlet state and quintet state, but also there is rare high spin excited state lifetime. Among the halogen substituted complexes, [Fe(dftpy)2]2+ has the shortest MLCT state lifetime of 14.0 ps which has been much longer than subpicosecond lifetime of [Fe(tpy)2]2+. The reason is explored by the combination of electronic structures, absorption properties, extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV) and potential energy curves (PECs). We can find that the bond lengths of Fe-Nt play a significant role on the change of metal centered (MC) ground states. With Fe-Nt extended by fluorine atoms, the quintet state becomes lower than the singlet state. Due to the deformation of structures, the interactions between metal and ligands diminish and give rise to weaker d orbital splitting than that of [Fe(tpy)2]2+, but slightly impact the pairwise orbital deformation density characteristics. And the PEC of 5MC intersects with 1,3MC which renders faster non-radiative deactivation through low-lying energy crossing points.

Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes.

Iron porphyrin carbenes (IPCs) are important reaction intermediates in engineered carbene transferase enzymes and homogeneous catalysis. However, discrepancies between theory and experiment complicate the understanding of IPC electronic structure. In the literature, this has been framed as whether the ground state is an open- vs closed-shell singlet (OSS vs CSS). Here we investigate the structurally dependent ground and excited spin-state energetics of a free carbene and its IPC analogs with variable trans axial ligands. In particular, for IPCs, multireference ab initio wave function methods are more consistent with experiment and predict a mixed singlet ground state that is dominated by the CSS (Fe(II) ← {:C(X)Y}0) configuration (i.e., electrophilic carbene) but that also has a small, non-negligible contribution from an Fe(III)-{C(X)Y}-• configuration (hole in d(xz), i.e., radical carbene). In the multireference approach, the "OSS-like" excited states are metal-to-ligand charge transfer (MLCT) in nature and are energetically well above the CSS-dominated ground state. The first, lowest energy of these "OSS-like" excited states is predicted to be heavily weighted toward the Fe(III)-{C(X)Y}-• (hole in d(yz)) configuration. As expected from exchange considerations, this state falls energetically above a triplet of the same configuration. Furthermore, potential energy surfaces (PESs) along the IPC Fe-C(carbene) bond elongation exhibit increasingly strong mixings between CSS/OSS characters, with the Fe(III)-{C(X)Y}-• configuration (hole in d(xz)) growing in weight in the ground state during bond elongation. The relative degree of electrophilic/radical carbene character along this structurally relevant PES can potentially play a role in reactivity and selectivity patterns in catalysis. Future studies on IPC reaction coordinates should evaluate contributions from ground and excited state multireference character.

Effect of short range ferromagnetic interaction on magnetocaloric properties of polycrystalline [formula omitted] compound

Magnetic and magnetocaloric properties of polycrystalline Eu0.55Sr0.45MnO3 compound have been investigated. In addition to the field induced meta-magnetic transition, magnetically a mixed ground state (ferromagnetic and antiferromagnetic) was observed especially at the low temperature (T < 30 K). Influence of the predominant short range ferromagnetic interaction reflects in its magnetocaloric effect, calculated from Maxwell’s thermodynamic relation. However, different nature of the magnetocaloric effect, calculated using Clausius-Clapeyron equation, discussed considering the phase diagram of this material. Our study suggest, in contrast to the Maxwell’s relation, magnetocaloric parameter (-ΔS) derived from Clausius-Clapeyron equation gives the reliable value, useful for magnetic refrigeration cycle.

Theoretical Study of Divalent Bis(Pentaisopropylcyclopentadienyl) Actinocenes.

The existence of divalent bis(pentaisopropylcyclopentadienyl) actinocene compounds, An(CpiPr5)2 (An = Th, U, Pu, Am, Bk, No, and Lr), is assessed by density functional theory (DFT) calculations with scalar-relativistic small-core pseudopotentials. The calculations predict ground states with significant 6d occupation for Th, U, and Lr, whereas Am, Bk, and No exhibit 5f ground states. A mixed ground state with predominant 5f character is found for Pu. The complexes exhibit a linear coordination geometry and high S10 symmetry except for Pu(CpiPr5)2 and Am(CpiPr5)2, which are found to be bent by 11 and 12°, respectively. Absorption spectra are simulated with time-dependent density functional theory (TD-DFT) and compared to experimental spectra of known tris(C4H4SiMe3) and tris(C5H3(SiMe3)2) compounds [ J. Am. Chem. Soc. 2015 , 137 , 369 - 382 . DOI: 10.1021/ja510831n ] as well as recently synthesized divalent lanthanide analogs Dy(CpiPr5)2 and Tb(CpiPr5)2 [ J. Am. Chem. Soc. , 2019 , 141 , 12967 - 12973 . DOI: 10.1021/jacs.9b05816 ]. Thermodynamic stability is assessed by calculation of adiabatic reduction potentials of the trivalent precursors [An(CpiPr5)2]+, and the feasibility of further reduction to obtain as yet unknown monovalent molecular actinide complexes is discussed.

K38 isomer production via fast fragmentation

In radioactive ion beam experiments, beams containing isomers can be of interest in probing nuclear structure and informing astrophysical reaction rates. While the production of mixed in-flight ground state and isomer beams using nucleon transfer can be generally understood through distorted wave Born approximation methodology, low-spin isomer production via fast fragmentation is relatively unstudied. To attain a practical understanding of low-spin isomer production using fast fragmentation beams, a test case of $^{38}\mathrm{K}/^{38m}\mathrm{K}$ was studied at the National Superconducting Cyclotron Laboratory's ReAccelerated Beam facility. Starting from lise++ predictions, the fragmentation momentum distribution was sampled to determine isomer production. In addition, the effects of the gas stopper gradient and charge breeding times were examined. In the case of $^{38}\mathrm{K}$, isomer production peaks at $\ensuremath{\sim}57%$. This maximum is observed just off the lise++ predicted optimum magnetic rigidity, with only small losses in beam intensity within a few percent of this optimum rigidity setting. Control of the isomer fraction was also achieved through the modification of charge breeding times. Fast fragmentation appears to be a feasible method for production of low-spin isomeric beams, but additional study is necessary to better describe the mechanism involved.

Time-of-flight expansion of binary Bose–Einstein condensates at finite temperature

Ultracold quantum gases provide a unique setting for studying and understanding the properties of interacting quantum systems. Here, we investigate a multi-component system of 87Rb–39K Bose–Einstein condensates (BECs) with tunable interactions both theoretically and experimentally. Such multi-component systems can be characterized by their miscibility, where miscible components lead to a mixed ground state and immiscible components form a phase-separated state. Here we perform the first full simulation of the dynamical expansion of this system including both BECs and thermal clouds, which allows for a detailed comparison with experimental results. In particular we show that striking features emerge in time-of-flight (TOF) for BECs with strong interspecies repulsion, even for systems which were separated in situ by a large gravitational sag. An analysis of the centre of mass positions of the BECs after expansion yields qualitative agreement with the homogeneous criterion for phase-separation, but reveals no clear transition point between the mixed and the separated phases. Instead one can identify a transition region, for which the presence of a gravitational sag is found to be advantageous. Moreover, we analyse the situation where only one component is condensed and show that the density distribution of the thermal component also shows some distinct features. Our work sheds new light on the analysis of multi-component systems after TOF and will guide future experiments on the detection of miscibility in these systems.

Electronic Structure and Magnetic Anisotropy in Lanthanoid Single-Ion Magnets with C3 Symmetry: The Ln(trenovan) Series.

We report the syntheses and the magnetic characterization of a new series of lanthanide complexes, in which the Ce, Nd, Gd, Dy, Er, and Yb derivatives show single-molecule magnet behavior. These complexes, named Ln(trenovan), where H3trenovan is tris(((3-methoxysalicylidene)amino)ethyl)amine, exhibit trigonal symmetry and the Ln(III) ion is heptacoordinated. Their molecular structure is then very similar to that of the previously reported Ln(trensal) series, where H3trensal is 2,2',2″-tris(salicylideneimino)triethylamine. This prompted us to use the spectroscopic and magnetic properties of the Ln(trensal) family (Ln = Nd, Tb, Dy, Ho, Er, and Tm) to obtain a set of crystal-field parameters to be used as starting point to determine the electronic structures and magnetic anisotropy of the analogous Ln(trenovan) complexes using the CONDON computational package. The obtained results were then used to discuss the electron paramagnetic resonance (EPR) and ac susceptibility results. As a whole, the obtained results indicate for this type of complexes single-molecule magnet behavior is not related to the presence of an anisotropy barrier, due to a charge distribution of the ligand around the lanthanoid, which results in highly mixed ground states in terms of MJ composition of the states. The crucial parameter in determining the slow relaxation of the magnetization is then rather the number of unpaired electrons (only Kramers ions showing in-field slow relaxation) than the shape of the charge distribution for different Ln(III).

Equilibration via Gaussification in Fermionic Lattice Systems.

In this Letter, we present a result on the nonequilibrium dynamics causing equilibration and Gaussification of quadratic noninteracting fermionic Hamiltonians. Specifically, based on two basic assumptions-clustering of correlations in the initial state and the Hamiltonian exhibiting delocalizing transport-we prove that non-Gaussian initial states become locally indistinguishable from fermionic Gaussian states after a short and well controlled time. This relaxation dynamics is governed by a power-law independent of the system size. Our argument is general enough to allow for pure and mixed initial states, including thermal and ground states of interacting Hamiltonians on large classes of lattices as well as certain spin systems. The argument gives rise to rigorously proven instances of a convergence to a generalized Gibbs ensemble. Our results allow us to develop an intuition of equilibration that is expected to be more generally valid and relates to current experiments of cold atoms in optical lattices.

Mn and Co Charge and Spin Evolutions in LaMn1–xCoxO3 Nanoparticles

The charge and spin-state evolution of manganese and cobalt in the LaMn1–xCoxO3 (x = 0.00, 0.25, 0.50, 0.75, and 1.00) perovskite nanoparticles have been studied with soft X-ray absorption spectroscopy. The results show a gradual increase in the average oxidation state of both Mn and Co ions with cobalt doping. The average valence of the LaMn1–xCoxO3 samples remains close to 3.0, with the Mn valence increasing from 3.1 to 4.0 and the Co valence increasing from 2.0 to 3.0. The symmetry of Mn and Co was determined using multiplet calculations. Calculating the intensity-area of the oxygen K pre-edge feature confirmed an increase in covalency with increasing Mn and Co oxidation state. The ground-state composition of Mn3+ in LaMnO3, and Co3+ in LaCoO3, was investigated, and it was found that Mn3+ (D4h) and Co3+ (Oh) are mainly in their low-spin state, with 10–20% admixture of high-spin state contributions into a mixed spin ground state.

Mixed configuration ground state in iron(II) phthalocyanine

We calculate the angular dependence of the x-ray linear and circular dichroism at the $L_{2,3}$ edges of $\alpha$-Fe(II) Phthalocyanine (FePc) thin films using a ligand field model with full configuration interaction. We find the best agreement with the experimental spectra for a mixed ground state of $^3E_{g}(a_{1g}^2e_g^3b_{2g}^1)$ and $^3B_{2g}(a_{1g}^1e_g^4b_{2g}^1)$ with the two configurations coupled by the spin-orbit interaction. The $^3E_{g}(b)$ and $^3B_{2g}$ states have an easy axis and plane anisotropies, respectively. Our model accounts for an easy-plane magnetic anisotropy and the measured magnitudes of the in-plane orbital and spin moments. The proximity in energy of the two configurations allows a switching of the magnetic anisotropy from easy plane to easy axis with a small change in the crystal field, as recently observed for FePc adsorbed on an oxidized Cu surface. We also discuss the possibility of a quintet ground state ($^5A_{1g}$ is 250~meV above the ground state) with planar anisotropy by manipulation of the Fe-C bond length by depositing the complex on a substrate that is subjected to a mechanical strain.

Control of Improper Ferroelectricity by Chemical Substitution and Magnetic Fields in Multiferroic Mn1-xCuxWO4

Multiferroic materials, like MnWO4, exhibit improper ferroelectricity driven by an inversion symmetry breaking magnetic order and strong spin lattice interactions. We show that the three magnetic phases in MnWO4, including one ferroelectric phase, can be controlled by substitution of copper for manganese and by application of external magnetic fields. Upon increasing Cu substitution, the paraelectric low-temperature phase is replaced by the ferroelectric phase which becomes the ground state at higher doping levels. We conclude that a mixed (para-/ferroelectric) ground state exists below 10% of Cu substitution.

Transient responses of transparency in a far-off resonant atomic system

Transient coherent oscillations in a closed Λ system under far-off resonant Raman fields were investigated theoretically. It has been found that the coherent superposition of the ground states can be formed due to the absorption even for initial maximal mixed ground states. The absorption oscillates with a period depending on the two-photon detuning when the system is initially in a transparent state and the two-photon Raman detuning is suddenly changed. The amplitude of the absorption decays with the decay rate of the ground states, which is different from the case when the lasers are applied resonantly. These transient coherent oscillations can be used to measure the relaxation rate of the ground states.

Quantum monte carlo methods for constrained systems

The torsional ground state for ethane, the torsional, rotational, and mixed torsional and rotational ground state of propane are computed with a version of diffusion Monte Carlo adapted to handle the geometric complexity of curved spaces such as the Ramachandra space. The quantum NVT ensemble average for the mixed torsional and rotational degrees of freedom of propane is computed, using a version of Monte Carlo path integral, also adapted to handle curved spaces. These three problems are selected to demonstrate the generality and the applicability of the approaches described. The spaces of coordinates can be best constructed from the parameters of continuous Lie groups, and alternative methods based on vector spaces, where extended Lagrangian terms would be too cumbersome to implement. We note that the geometric coupling between the torsions and the rotations of propane produces a substantial effect on the ground state energy of propane, and that the quantum effects on the energy of propane are quite large even well above room temperature. © 2014 Wiley Periodicals, Inc.

Frustration, entanglement, and correlations in quantum many-body systems

We derive an exact lower bound to a universal measure of frustration in degenerate ground states of quantum many-body systems. The bound results in the sum of two contributions: entanglement and classical correlations arising from local measurements. We show that average frustration properties are completely determined by the behavior of the maximally mixed ground state. We identify sufficient conditions for a quantum spin system to saturate the bound, and for models with twofold degeneracy we prove that average and local frustration coincide.