Nonmagnetic State Research Articles

Revisiting the link between magnetic properties and chemisorption at graphene nanoribbon zigzag edge.

Graphene has received tremendous interest in both chemical and physical fields. Among different edges of the graphene system, the zigzag edge terminated graphene nanoribbons (ZGNRs) show unique magnetic properties in the antiferromagnetic (AFM) state. However, to date, the understanding of ZGNR chemical properties is mainly based on the partial radical concept, and in previous studies, the energy differences between the ferromagnetic (FM) and AFM states are smaller than experimental evidence. Here, we report that the strongly constrained and appropriately normed functional gives a significantly larger energy difference, which matches the experimental observation. Furthermore, utilizing the energetics in the large difference case, we propose a conceptual supplement to the previous partial radical concept: the overall stabilization of the AFM state compared to the nonmagnetic (NM) state consists of two parts that affect the adsorption energy conversely. The NM-FM energy differences will strengthen the adsorption, being in line with the previous partial radical concept. The FM-AFM energy differences will instead weaken the adsorption. We perform calculations of H, OH, and LiS radical adsorption energies on ZGNRs to show that this weakening effect is numerically non-negligible: at least a ∼0.2eV difference in the adsorption energies is found. We expect that this refinement of the partial radical concept can provide a more comprehensive understanding of the chemical properties of ZGNRs. The differences in adsorption energies for the H, OH, and LiS radicals found here lead to significant changes in the predicted reactivity of the ZGNR models.

Structural and Magnetic Properties of Yb0.5Ce0.5Ni5

The rare-earth magnetism in the intermetallic compound Yb0.5Ce0.5Ni5 was studied using X-ray diffraction, magnetization, heat capacity, and electrical resistivity measurements. The effect of spin fluctuations (SF) was observed in M(T) at ~40 K. The measurement of thermal and transport properties supported the results obtained from magnetic measurements. Collected experimental data showed that Yb/Ce substitution shifts the maximum temperature for spin fluctuations to a lower temperature compared to that for pure CeNi5. Moreover, at low temperatures, an anomaly in the heat capacity of possible magnetic origin arising from Yb3+ was detected. Ce atoms seemed to remain in a non-magnetic valence state at almost 4+.

Electronic and magnetic properties of quasi-one-dimensional osmium halide OsCl4

Using ab initio density functional theory, we study the electronic and magnetic properties of the van der Waals chain material OsCl4. In the nonmagnetic state, a strongly anisotropic band structure was observed, in agreement with its anticipated one-dimensional crystal geometry. Based on Wannier functions, we found that the four electrons of the 5d Os atom form a low-spin S = 1 state, with a large crystal field between the dxz/yz and dxy orbitals, corresponding to a strong Jahn–Teller distortion (Q3< 0). As a consequence, the magnetic properties are mainly contributed by the dxz/yz states. Furthermore, when a Mott gap develops after the introduction of the Hubbard U and Hund coupling J, we found that the staggered spin order is the most likely magnetic state, namely, spins arranged as (↑-↓-↑-↓) with π wavevector along the chain. In addition, the energy differences between various spin states are small, suggesting a weak magnetic exchange coupling along the chain. Our results provide guidance to experimentalists and theorists working on quasi-one-dimensional osmium halides chain materials.

Structural, magnetic, electronic, thermoelectric, optical and elastic properties of Co2Mn1-xTixGe Heusler alloys

We have investigated the structural, electronic, magnetic, thermoelectric, optical, elastic and thermal properties of Co2Mn1-xTixGe using the first-principles full-potential linearized augmented plane wave method with the GGA and mBJ approximations. The ferromagnetic state was more stable than nonmagnetic state. These compounds have been found to be perfectly half-metallic ferromagnetic. The magnetic moments of all compounds are calculated. Co2Mn1-xTixGe with x = 0, 0.50 and 1 are mechanically anisotropic stable, while for x = 0.25 and 0.75 are not mechanically stable. These properties play an important role to understand the nature of material whether it can be used as optoelectronic devices.

Metamagnetic transition and a loss of magnetic hysteresis caused by electron trapping in monolayers of single-molecule magnet Tb2@C79N.

Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb2@C79N, showing a broad magnetic hysteresis in a bulk form. Surprisingly, monolayers of Tb2@C79N exhibited a completely different behavior, with the prevalence of a ground state with antiferromagnetic coupling at low magnetic field and a metamagnetic transition in the magnetic field of 2.5-4 T. Monolayers of Tb2@C79N were deposited onto Cu(111) and Au(111) by evaporation in ultra-high vacuum conditions, and their topography and electronic structure were characterized by scanning tunneling microscopy and spectroscopy (STM/STS). X-ray photoelectron spectroscopy (XPS), in combination with DFT studies, revealed that the nitrogen atom of the azafullerene cage tends to avoid metallic surfaces. Magnetic properties of the (sub)monolayers were then studied by X-ray magnetic circular dichroism (XMCD) at the Tb-M4,5 absorption edge. While in bulk powder samples Tb2@C79N behaves as a single-molecule magnet with ferromagnetically coupled magnetic moments and blocking of magnetization at 28 K, its monolayers exhibited a different ground state with antiferromagnetic coupling of Tb magnetic moments. To understand if this unexpected behavior is caused by a strong hybridization of fullerenes with metallic substrates, XMCD measurements were also performed for Tb2@C79N adsorbed on h-BN|Rh(111) and MgO|Ag(100). The co-existence of two forms of Tb2@C79N was found on these substrates as well, but magnetization curves showed narrow magnetic hysteresis detectable up to 25 K. The non-magnetic state of Tb2@C79N in monolayers is assigned to anionic Tb2@C79N- species with doubly-occupied Tb-Tb bonding orbital and antiferromagnetic coupling of the Tb moments. A charge transfer from the substrate or trapping of secondary electrons are discussed as a plausible origin of these species.

Computational study of phase stability, electronic, magnetic and thermoelectric properties of a new quaternary Heusler alloy CrCoIrGa

Exploring new materials with interesting physical properties plays a vital role in the development of modern technologies. Herein, using density functional theory based first-principles calculations, we present a study on the structural, electronic, magnetic and thermoelectric properties of a new quaternary Heusler alloy CrCoIrGa. The structure stability determined in terms of cohesive energy, formation energy and phonons dispersion curves confirms that alloy is stable and can be considered in experiments. The structural optimization in non-magnetic, ferromagnetic and antiferromagnetic state proves that alloy is ferromagnetic in the ground state with optimized lattice constant of 5.93 Å. The electronic band structure and density of states show that alloy is nearly half-metal (NHF) with 96% spin-polarization at the Fermi level. The effect of pressure on the electronic and magnetic properties investigated in the range 0–25 GPa, reveals that complete half-metallic character at the Fermi level can be obtained for pressure ≥10 GPa with a half-metallic gap of about 1 eV in the minority spin channel and a magnetic moment (Mt) of 3.0 μB/f.u., in agreement with Slater-Pauling rule (Mt = Zt-24). In addition, the electronic structures determined using DFT+U approach exhibit nearly half-metallic property such that amount of spin-polarization increases with increasing Hubbard parameter U. Finally, thermoelectric properties are examined by computing Seebeck coefficient (S), electrical conductivity (σ/τ), electronic thermal conductivity (κ) and power factor (PF) up to 800 K temperature. The positive value of S conveys that heat transport is carried by positive charge carriers. The room temperature values of S, σ/τ, κe/τ and PF are found to be about 5.9 μV/K, 3.06 × 1020 Ω−1m−1s−1, 2.27 × 1015 WK−1m−1s−1 and 1 WK−2m−1s−1, respectively. Collectively, these findings suggest that CrCoIrGa could play a useful role for the development of spintronics and energy production applications.

Frustration-induced Quantum Spin Liquid Behavior in the s = 1/2 Random-bond Heisenberg Antiferromagnet on the Zigzag Chain

Recent studies have revealed that the randomness-induced quantum spin liquid (QSL)-like state is stabilized in certain frustrated quantum magnets in two and three dimensions. In order to clarify the nature of this gapless QSL-like state, we investigate both zero- and finite-temperature properties of the random-bond one-dimensional (1D) $s=12$ Heisenberg model with the competing nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions, $J_1$ and $J_2$, by means of the exact diagonalization, density-matrix renormalization-group and Hams--de Raedt methods. We find that, on increasing the frustration $J_2$, the gapless nonmagnetic state stabilized in the unfrustrated model with $J_2=0$, the {\it unfrustrated\} random-singlet (RS) state, exhibits a phase transition into different gapless nonmagnetic state, the {\it frustrated\} RS state. This frustrated RS state in 1D has properties quite similar to the randomness-induced QSL-like state recently identified in 2D and 3D frustrated magnets exhibiting the $T$-linear low-temperature ($T$) specific heat, while the unfrustrated RS state is more or less specific to the unfrustrated 1D system exhibiting the $\sim 1(\log T)^3$ low-$T$ specific heat. Universal features and the robustness against perturbations of the frustrated RS state are emphasized.

Spin State Switching in Heptauthrene Nanostructure by Electric Field: Computational Study.

Recent experimental studies proved the presence of the triplet spin state in atomically precise heptauthrene nanostructure of nanographene type (composed of two interconnected triangles with zigzag edge). In the paper, we report the computational study predicting the possibility of controlling this spin state with an external in-plane electric field by causing the spin switching. We construct and discuss the ground state magnetic phase diagram involving (triplet) state, antiferromagnetic state and non-magnetic state and predict the switching possibility with the critical electric field of the order of 0.1 V/Å. We discuss the spin distribution across the nanostructure, finding its concentration along the longest zigzag edge. To model our system of interest, we use the mean-field Hubbard Hamiltonian, taking into account the in-plane external electric field as well as the in-plane magnetic field (in a form of the exchange field from the substrate). We also assess the effect of uniaxial strain on the magnetic phase diagram.

Competing magnetic states in transition metal dichalcogenide moiré materials

Small-twist-angle transition metal dichalcogenide (TMD) heterobilayers develop isolated flat moir\'e bands that are approximately described by triangular lattice generalized Hubbard models [PhysRevLett.121.026402]. In this article we explore the metallic and insulating states that appear under different control conditions at a density of one-electron per moir\'e period, and the transitions between them. By combining fully self-consistent Hartree-Fock theory calculations with strong-coupling expansions around the atomic limit, we identify four different magnetic states and one nonmagnetic state near the model phase diagram's metal-insulator phase-transition line. Ferromagnetic insulating states, stabilized by non-local direct exchange interactions, are surprisingly prominent.

DFT and Monte Carlo study of the structural, mechanical, electronic, magnetic and magnetocaloric properties of the Co2VGa Heusler alloy

We determined the structural, mechanical, electronic and magnetic properties of the Co2VGa by using full-potential linearized augmented plane-wave (FP-LAPW) method and Monte Carlo simulation. The structural stability studies were performed for nonmagnetic, ferromagnetic and antiferromagnetic states. The elastic parameters and their derivatives have been calculated. The investigated energy band structure shows that the Co2VGa is half metallic. The total magnetic moments, stiffness constant of spin wave and the Curie temperature are also calculated. The magnetocaloric effect of this material are studied using the results obtained by FP-LAPW. The transition temperature, magnetic entropy change and relative cooling power are calculated.

Nonmagnetic J=0 State and Spin-Orbit Excitations in K_{2}RuCl_{6}.

Spin-orbit Mott insulators composed of t_{2g}^{4} transition metal ions may host excitonic magnetism due to the condensation of spin-orbital J=1 triplons. Prior experiments suggest that the 4d antiferromagnet Ca_{2}RuO_{4} embodies this notion, but a J=0 nonmagnetic state as a basis of the excitonic picture remains to be confirmed. We use Ru L_{3}-edge resonant inelastic x-ray scattering to reveal archetypal J multiplets with a J=0 ground state in the cubic compound K_{2}RuCl_{6}, which are well described within the LS-coupling scheme. This result highlights the critical role of unquenched orbital moments in 4d-electron compounds and calls for investigations of quantum criticality and excitonic magnetism on various crystal lattices.

Modulation of electrical performance of zigzag edged tetra-penta-octagonal graphene nanoribbons based devices via boundary passivations

Recently, an sp2 hybridized planar two-dimensional graphene-based carbon allotrope composed of tetra-penta-octagonal rings has attracted considerable interest. The electronic structures and transport properties of zigzag-edged tetra-penta-octagonal-graphene nanoribbons (TPO-ZGNRs) modified via boundary passivation were investigated using density functional theory and the non-equilibrium Green’s function. TPO-ZGNRs were passivated with H, 2H, O, OH, and Cl on the edge to produce TPO-ZGNRs-X (X=H, 2H, O, OH, and Cl). TPO-ZGNRs-H exhibits metallic properties regardless of the nanoribbon width and exists in the non-magnetic state. Although TPO-ZGNRs-OH and TPO-ZGNRs-Cl remain metallic, TPO-ZGNRs-2H and TPO-ZGNRs-O are semiconductors. In addition, the current–voltage curves of three TPO-ZGNRs-X (X=H, OH, and Cl) homojunction device models contain a negative differential resistance region. Notably, the heterojunction device model in which TPO-ZGNRs are passivated with H and O atoms at different edge regions has negative differential resistance and rectification characteristics. This study is expected to facilitate the development of new carbon materials.

Binuclear s = 1/2 single molecular magnets with the symmetry restrictions

Exchange anisotropy effects in equal-spin s(1) = s(2) dimeric single-molecule magnets exhibiting C2 point- group symmetry have been studied. The Hamiltonian, which is written in two-spin 4 × 4 Pauli matrixes representation in the appropriate noncollinear local coordinates systems, has been transformed to common coordinates and used to derive energy levels and spin eigenstates. An anomalous variation of the magnetic system average spin depending on the misalignment of the local anisotropy axes has been found. Particularly, a fully nonmagnetic state of an s = 1/2 spin dimer and “silent” EPR mode has been predicted. The angular boundaries of the EPR existence have been determined for any possible mutual deflections of the local ion axes for each of the 6 possible transitions between the levels of the spin system, both in zero and in a nonzero magnetic field.

Field-induced anomalous magnetic state beyond the magnetically ordered state in the slightly distorted triangular S=12 rare-earth antiferromagnet CeZn3P3

Magnetic properties of ${\mathrm{CeZn}}_{3}{\mathrm{P}}_{3}$, which has been believed to have the same hexagonal ${\mathrm{ScAl}}_{3}{\mathrm{C}}_{3}$-type crystal structure as that of a spin dimer system of ${\mathrm{YbAl}}_{3}{\mathrm{C}}_{3}$ at room temperature, have been investigated by magnetization, magnetostriction, specific heat ($C$), and magnetocaloric effect measurements. In this research, we have found that ${\mathrm{CeZn}}_{3}{\mathrm{P}}_{3}$ certainly has a slightly deformed crystal structure from the hexagonal one even at room temperature, which is in contrast to the structural phase transition of ${\mathrm{YbAl}}_{3}{\mathrm{C}}_{3}$ occuring at around 80 K, although the very slight deformation along the $c$ plane, i.e., slightly deformed triangular lattice, and the formation of the multidomain structure are common to both compounds. ${\mathrm{CeZn}}_{3}{\mathrm{P}}_{3}$ is thought to maintain its deformed structure from a high-temperature region above room temperature to an extremely low-temperature region and shows a magnetic order below ${T}_{\mathrm{N}}=0.8$ K. The analysis of the temperature dependence of the magnetic susceptibility ($\ensuremath{\chi}$) and $C$ has revealed that a Kramers doublet ground state with an easy-plane type anisotropy on the $c$ plane is well isolated from the excited states by the crystal field splitting energy of more than 300 K. Above ${T}_{\mathrm{N}}, \ensuremath{\chi}$ makes a broad peak at around 2 K, which certainly originates from the dimer formation due to the slight deformation of the triangular lattice. On the other hand, below ${T}_{\mathrm{N}}$, a magnetic phase diagram reminiscent of a magnetic flower blooming on the $c$ plane was observed, which may have a close relation to a quantum effect of a quasi $S=\frac{1}{2}$ spin system and a contribution of the orbital component. With increasing magnetic field, we have found an anomalous magnetic state beyond the usual magnetically ordered state, where $C/T$ is anomalously enhanced. This anomalous magnetic state is similar to that observed in ${\mathrm{YbAl}}_{3}{\mathrm{C}}_{3}$ induced by the field, although the magnetic ground state in ${\mathrm{YbAl}}_{3}{\mathrm{C}}_{3}$ is a nonmagnetic dimer state different from the normal magnetically ordered state in ${\mathrm{CeZn}}_{3}{\mathrm{P}}_{3}$.

Topological Phase Transitions in Strongly Correlated Systems: Application to Co3Sn2S2

The topological transition in the strongly correlated half-metallic ferromagnetic compound Co$_3$Sn$_2$S$_2$ from Weyl semimetal (including chiral massless fermions) to a non-magnetic state is treated. This transition goes with a change in topological invariant, and is accompanied by a non-topological transition from saturated ferromagnetic to paramagnetic state, the minority Fermi surface being transformed from ghost (hidden) to real. A corresponding description is given in terms of slave fermion representation for the effective narrow-band Hubbard model. The system Co$_3$Sn$_2$S$_2$ provides a bright example of coexistence of non-trivial topology and strong low-dimensional ferromagnetism. A comparison is performed with other compounds where frustrations result in formation of a correlated paramagnetic state.

Multipolar nematic state of nonmagnetic FeSe based on DFT+U

Clarifying the origin of nematic state in FeSe is one of urgent problems in the field of iron-based superconductivity. Motivated by the discovery of a nematic solution in the density-functional theory implemented by on-site Coulomb interaction ($\mathrm{DFT}+U$) [npj Quantum Mater. 5, 50 (2020)], we reexamine the $U$ dependence of electronic states in the nonmagnetic normal state of FeSe and perform full multipolar analyses for the nematic state. We find that with increasing $U$ the normal state experiences a topological change in the Fermi surfaces before the emergence of a nematic ground state. The resulting nematic ground state is a multipolar state having both antiferrohexadecapoles in the $E$ representation and ferromultipoles in the ${B}_{2}$ representation on each Fe site. Cooperative coupling between the $E$ and the ${B}_{2}$ multipoles in the local coordinate with the ${D}_{2d}$ point group will play an important role in the formation of the ${d}_{xz},{d}_{yz}$ orbital-splitting nematic state not only in FeSe, but also in other iron pnictides.

Multiband [formula omitted] model and the electronic structure of quasi-two dimensional Nd[formula omitted]Sr[formula omitted]NiO[formula omitted

We study the tight-binding model of NiO2 planes in novel superconductors that takes into account anisotropic nickel–oxygen d−p hopping, oxygen–oxygen p−p hoppings, realistic crystal-field splittings, and the on-site Coulomb interactions {U,JH} both at nickel and oxygen ions. Electronic structure of the NiO2 layer is investigated for doping x=0 and x=0.25. For an ideal undoped system NdNiO2 (x=0) we get robust Mott insulator and no magnetism as various magnetic ground states (ferromagnetic, C-type and G-type antiferromagnetic) have all the same energy. For self-doping x=0.25 the system becomes conducting (zero HOMO–LUMO gap), with nonmagnetic ground state due to competing degenerate magnetic meta-stable states. These findings correlate well with experimental and theoretical data. However, it seems that the two-dimensional model with p and d orbitals only is insufficient for a realistic description of Nd1−xSrxNiO2.

Existence of spin-polarized Dirac cone in Sc[formula omitted]CrB[formula omitted]: A DFT study

First principles calculations on Sc2CrB6 crystal revealed the existence of Dirac Cone in its electronic structure for the first time. The compound has an interesting crystal structure with 2D boron layers with hexagonal, pentagonal, heptagonal boron rings sandwiched between the metal layers. Calculations show that the compound has a mixed covalent and metallic nature of the bonding. Also, ground state ferromagnetic behaviour is predicted for the metallic bonded Sc2CrB6. Here the magnetization arises mostly from the Cr atom with enhanced contribution from the remaining elements. Interestingly, band crossing is seen at the Fermi level with spin polarization indicating the existence of Dirac cone in Sc2CrB6. Apart from this, high-pressure calculations up to about 67 GPa reveal a pressure-induced transition from ferromagnetic state to non-magnetic state around 30.3 GPa.

Impurity-Induced Antiferromagnetism in S = 1/2 Alternating Chain System NaCu2VP2O10

Impurity-induced antiferromagnetism was observed in the S = 1/2 alternating chain compound NaCu2VP2O10 when a few nonmagnetic ions were substituted at the Cu sites. NaCu2VP2O10 showed a nonmagnetic spin-singlet ground state at low temperatures. The temperature dependence of the magnetic susceptibility and the specific heat of the Mg-doped compounds, Na(Cu1−xMgx)2VP2O10 \((x = 0.03,0.05)\), showed anomalies at ∼8 K, indicating antiferromagnetic ordering.

Tunable spin electronic and thermoelectric properties in twisted triangulene π -dimer junctions

In this work, we investigate the electronic properties and thermoelectric performance of triangulene π-dimer junctions with the twist angle from 0° to 60° by using first-principles calculations in combination with a non-equilibrium Green's function method. It is found that the triangulene π-dimer can be transformed between nonmagnetic state and antiferromagnetic or ferromagnetic state by varying the twist angle. The reason is that the relative rotation between the monomers weakens the overlap of two single occupied molecular orbital. More importantly, our theoretical analysis shows that the ferromagnetic states of the triangulene π-dimer junctions at the twist angle of 20°, 30°, and 60° have outstanding thermoelectric performance. The corresponding ZT value is as high as around 6, which is mainly contributed from the spin splitting nature. This work is instructive to improve the thermoelectric properties of π-stacking molecular junctions or organic polymers.