Spin-singlet Formation Research Articles

Magnetoresistance Recovery in the Amorphous Dielectric Material SiCOH

The use of a magnetoresistance in the characterization of transport properties in the amorphous low-k dielectric material SiCOH is demonstrated. The double occupancy of charge carriers in trap states within the dielectric material can only exist in spin singlet formation due to Pauli Exclusion. The trap-assisted negative magnetoresistance (MR) in amorphous SiCOH, driven by an applied electric field that results in an observed increase in magnitude of the current in the conduction band is due to singly occupied trap spin-mixing suppression of carriers with the application of an external magnetic field. The material MR decays with time under electrical bias and temperature stress as traps are filled by charge carriers and from space charge accumulation. The MR can be reinstated by the ionization of these traps via the conduction mechanisms of nonthermally activated tunneling and thermal ionization with the assistance of an applied coulombic potential barrier lowering electric field. In this work a direct correlation is shown between a material MR and the trapping, de-trapping, and trap avoidance of singly occupied traps in the transport of charge carriers in the amorphous low-k dielectric material SiCOH (a-SiCOH).

Robust atomic orbital in the cluster magnet LiMoO2

In this study, we present a rutile-related material, LiMoO2, that becomes a cluster magnet and exhibits a spin singlet formation on a preformed molybdenum dimer upon cooling. Unlike ordinary cluster magnets, the atomic dyz orbital robustly survives despite the formation of molecular orbitals, thereby affecting the magnetic properties of the selected material. Such hybrid cluster magnets with the characters of molecular and atomic orbitals realize multiple independent spins on an isolated cluster, leading to an ideal platform to study the isolated spin dimers physics.

Occupation switching of d orbitals in vanadium dioxide probed via hyperfine interactions

Metal-insulator transition was microscopically investigated by orbital-resolved nuclear magnetic resonance (OR-NMR) spectroscopy in a single crystal of vanadium dioxide VO$_2$. Observations of the anisotropic $^{51}$V Knight shift and the nuclear quadrupole frequency allow us to evaluate orbital-dependent spin susceptibility and $d$ orbital occupations. The result is consistent with the degenerated $t_{2g}$ orbitals in a correlated metallic phase and the $d$ orbital ordering in a nonmagnetic insulating phase. The predominant orbital pointing along the chain facilitates a spin-singlet formation triggering metal-insulator transition. The asymmetry of magnetic and electric hyperfine tensors suggests the $d$ orbital reformation favored by a low-symmetry crystal field, forming a localized molecular orbital. The result highlights the cooperative electron correlation and electron-phonon coupling in Mott transition with orbital degrees of freedom.

Spin-dipole mode in a trapped Fermi gas near unitarity

We theoretically investigate the spin-dipole oscillation of a strongly interacting Fermi gas in a harmonic trap. By using a combined diagrammatic strong-coupling theory with a local density approximation and a sum rule approach, we clarify the temperature dependence of the spin-dipole frequency near the unitarity, which is deeply related to the spin susceptibility, as well as pairing correlations. While the spin-dipole frequency exactly coincides with the trap frequency in a non-interacting Fermi gas, it is shown to remarkably be enhanced in the superfluid state, because of the suppression of the spin degree of freedom due to the spin-singlet Cooper-pair formation. In strongly interacting Fermi gases, this enhancement occurs even above the superfluid phase transition temperature, due to the strong pairing correlations.

Spin-singlet formation in the spin-tetramer layered organic-inorganic hybridCH3NH3Cu2Cl5

We report spin-singlet formation in an organic-inorganic hybrid identified as ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{Cu}}_{2}{\mathrm{Cl}}_{5}$. The hybrid adopts a layered structure in which ${{\mathrm{Cu}}_{2}{\mathrm{Cl}}_{5}}^{\ensuremath{-}}$ layers are separated by ${{\mathrm{CH}}_{3}{\mathrm{NH}}_{3}}^{+}$ layers. The inorganic layers consist of corner- and edge-sharing ${\mathrm{CuCl}}_{6}$ octahedra, forming edge-sharing tetramers. Magnetic susceptibility measurements indicate strong antiferromagnetic interactions within the tetramers, whereas the coupling between the tetramers is weak. Low-temperature magnetic susceptibility data as well as electron spin resonance measurements suggest a nonmagnetic ground state with a large spin gap of $\ensuremath{\sim}130\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, in apparent contradiction with ferromagnetic interactions between nearest-neighbor spins. We discuss a spin-tetramer model in which antiferromagnetic next-nearest-neighbor interactions lead to a spin-singlet state.

Cuprate phase diagram and the influence of nanoscale inhomogeneities

The phase diagram associated with the high Tc superconductors is complicated by an array of different ground states. The parent material represents an antiferromagnetic insulator but with doping superconductivity becomes possible with transition temperatures previously thought unattainable. The underdoped region of the phase diagram is dominated by the so-called pseudogap phenomena whereby in the normal state the system mimics superconductivity in its spectra response but does not show the complete loss of resistivity associated with the superconducting state. An understanding of this regime presents one of the great challenges for the field. In the present study we revisit the structure of the phase diagram as determined in photoemission studies. By careful analysis of the role of nanoscale inhomogeneities in the overdoped region we are able to more carefully separate out the gaps due to the pseudogap phenomena from the gaps due to the superconducting transition. Within a mean field description we are thus able to link the magnitude of the gap directly to the Heisenberg exchange interaction term, $J\sum{s_i \cdot s_j}$, contained in the $t-J$ model. This approach provides a clear indication that the pseudogap is that associated with spin singlet formation.

Spin-Singlet Transition in the Magnetic Hybrid Compound from a Spin-Crossover Fe(III) Cation and π-Radical Anion

To develop a new spin-crossover functional material, a magnetic hybrid compound [Fe(qsal)2][Ni(mnt)2] was designed and synthesized (Hqsal = N-(8-quinolyl)salicylaldimine, mnt = maleonitriledithiolate). The temperature dependence of magnetic susceptibility suggested the coexistence of the high-spin (HS) Fe(III) cation and π-radical anion at room temperature and a magnetic transition below 100 K. The thermal variation of crystal structures revealed that strong π-stacking interaction between the π-ligand in the [Fe(qsal)2] cation and [Ni(mnt)2] anion induced the distortion of an Fe(III) coordination structure and the suppression of a dimerization of the [Ni(mnt)2] anion. Transfer integral calculations indicated that the magnetic transition below 100 K originated from a spin-singlet formation transformation in the [Ni(mnt)2] dimer. The magnetic relaxation of Mössbauer spectra and large thermal variation of a g-value in electron paramagnetic resonance spectra below the magnetic transition temperature implied the existence of a magnetic correlation between d-spin and π-spin.

Correlation Effects on Charge Order and Zero-Gap State in the Organic Conductor α-(BEDT-TTF)2I3

The effects of electron correlation in the quasi-two-dimensional organic conductor \alpha-(BEDT-TTF)2I3 are investigated theoretically by using an extended Hubbard model with on-site and nearest-neighbor Coulomb interactions. A variational Monte Carlo method is applied to study its ground-state properties. We show that there appears a nonmagnetic horizontal-stripe charge order in which nearest-neighbor correlation functions indicate a tendency toward a spin-singlet formation on the bonds with large transfer integrals along the charge-rich stripe. Under uniaxial pressure, a first-order transition from the nonmagnetic charge order to a zero-gap state occurs. Our results on a spin correlation length in the charge-ordered state suggest that a spin gap is almost unaffected by the uniaxial pressure in spite of the suppression of the charge disproportionation. The relevance of these contrasting behaviors in spin and charge degrees of freedom to recent experimental observations is discussed.

Tetramer spin singlet instability in the fluorine-substituted pyrochlore superconducting system Cd2Re2O7−xFx

We synthesized polycrystalline samples of the fluorine-substituted pyrochlore rhenates Cd2Re2O7−xFx, and investigated their magnetic, transport and structural properties. The transition temperature Ts1, where each Re4 tetrahedron in the Re pyrochlore network alternately expands and contracts, decreases with increasing x from 200 K at x = 0 to 100 K at x = 0.5. The strong x dependence of the magnetic and transport properties at the low-temperature phase indicates that the driving force of structural phase transition is fluctuations of the tetramer spin singlet formation in order to release the spin frustration in the pyrochlore lattice. Furthermore, we found unconventional superconducting properties in Cd2Re2O7−xFx. It was found that the superconducting phase transition temperature Tc markedly decreases with increasing x, suggesting that the addition of imperfection suppresses a condensation of Cooper-pair. In addition, the estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit and increases with increasing x in spite of the reduction of Tc. Hence, Cd2Re2O7−xFx is suggested to be an exotic superconductor realized in the itinerant electron systems on a spin frustrated lattice.

Magnetic and thermodynamic properties of the spin-dimer systemLa2RuO5induced by Mn substitution

Magnetic properties as well as the specific heat of the spin-dimer system ${\mathrm{La}}_{2}{\mathrm{Ru}}_{1\ensuremath{-}y}{\mathrm{Mn}}_{y}{\mathrm{O}}_{5}$ were investigated for manganese concentrations $0\ensuremath{\le}y\ensuremath{\le}0.25$. The magnetic (dc) susceptibility of the unsubstituted ($y=0$) system shows a steplike decrease close to 160 K reflecting a magnetostructural transition into a dimerized ground state. With increasing manganese concentration this behavior (typical for singlet formation) becomes continuously suppressed and the susceptibility bears the signatures of the emergence of new magnetic ground states. The high-temperature Curie-Weiss susceptibility can be described by ${\mathrm{Ru}}^{4+}$ ($S=1$) and ${\mathrm{Mn}}^{4+}$ ($S=\frac{3}{2}$) spin moments, with a dramatic decrease of the Curie-Weiss temperatures by almost 30% close to $y=0.1$, indicating significant changes in the average mean magnetic exchange. Field-cooled and zero-field-cooled experiments as well as ac-susceptibility measurements provide clear evidence for the formation of a spin-glass state, well below the characteristic dimerization temperature. The relaxation dynamics can be described by a Vogel-Fulcher-Tammann behavior and indicates high fragility when characterized in terms of glassy dynamics of canonical supercooled liquids. Additional electron-spin resonance experiments indicate different spin-glass regimes and a rather dynamic nature of the dimerized phase. In the Mn-substituted compounds, a linear contribution to the heat capacity at low temperatures can be ascribed to the spin-glass formation. With increasing manganese concentration, the anomaly in the specific heat caused by the spin-singlet formation is shifted to lower temperatures and becomes continuously suppressed and smeared out. On the basis of these results, we propose a ($y,T$)-phase diagram indicating the competition of the spin-glass and the dimerized states. We stress the similarities with doped $\mathrm{Cu}\mathrm{Ge}{\mathrm{O}}_{3}$, the canonical inorganic spin-Peierls system.

Nonmagnetic spin-singlet dimer formation and coupling to the lattice in the 6H perovskite Ba3CaRu2O9

Non-conservation of magnetic neutron scattering from the six-layer hexagonal perovskite Ba3CaRu2O9 on cooling reveals that (Ru5+)2 dimers form nonmagnetic spin-singlet ground states. This is in contrast to Ba3BRu2O9 (B=Co or Ni) analogues which display long range spin ordered ground states containing antiferromagnetic (Ru5+)2 dimers (Lightfoot and Battle 1990 J. Solid State Chem. 89 174). Dimer formation is weakly coupled to the lattice, resulting in an excess [101] microstrain broadening of diffraction peaks at low temperatures. Non-conservation of magnetic neutron scattering is also observed around the charge ordering transition in Ba3NaRu2O9 showing that the ground state structure is coupled to spin-singlet formation in charge ordered Ru2O9 dimers.

Magnetic transition in dimerized radical cation salt of (BPDT-TTF)2ICl2 studied by heat capacity measurements

Thermodynamic investigation using the relaxation calorimetry technique and the microchip calorimetry technique is performed to clarify low-temperature behaviors of a radical cation salt consisting of a donor molecule of bispropylenedithiotetrathiafulvalene (BPDT-TTF) and a linear anion of \({{\text{ICl}}_{2}^{-}}\). This compound has a layered structure similar to numerous BEDT-TTF compounds. The donor molecules form a dimerized arrangement in the layer. Temperature dependence of heat capacity obtained by the relaxation technique shows a broad hump structure around 20–25 K corresponding to the temperature where the magnetic susceptibility shows a drastic decrease due to the formation of the singlet spin state. The microchip calorimetry technique detected a step-like anomaly around 23 K in the temperature dependence of CpT−1 of which entropy is evaluated as only few % of Rln2 corresponding to the full entropy of localized π-electrons located on each dimer unit. The negligibly small T-linear term in the low-temperature heat capacity and absence of magnetic fields dependence below 3.2 K predict opening of rigid gap structure in the spin excitations, which is consistent with a spin-singlet formation due to the formation of spin-Peierls type ordering or charge ordering state.

3d Electron Quadrupole Moments in Vanadium Oxides

We report the local spin susceptibility and the 3d quadrupole moment in V5O9. The metal-insulator transition accompanies with the spin singlet formation without an apparent dimerization in the V lattice. Based on the angle dependence of the 51V quadrupole frequency, we suggest that the orbital ordering assists the spin singlet formation in the quasi-one-dimensional chain in V5O9. At high temperatures, the paramagnetic metallic state has nonuniform spin susceptibility depending on V sites, which indicates that the magnetically active and inactive V sites are already separated in the metallic state due to the strong electron correlation.

Orbital ordering and spin-singlet clusters in triangular-based vanadates

We report 51V NMR spectra in the triangular-lattice vanadates, AV10O15, (3d2.2), AV13O18 (3d2.38), and A2V13O22 (3d1.92) (A = Ba, Sr). These compounds have orbital degrees of freedom and itinerant electrons arising from the partially occupied t2g bands. We show microscopic observations of spin-singlet trimers and antiferromagnetic tertramers with orbital orders in BaV10O15, but no indication of the orbital order and the spin singlet in SrV10O15. The paramagnetic and nonmagnetic spins coexist in BaV13O22 but only the nonmagnetic spins are present in AV13O18 (A = Ba, Sr). The spin singlet formation in the incommensurate band filling is understood as the incommensurate charge-density wave induced by the 3d orbital order.

Spin-dimerization in rare-earth substituted La2RuO5

The Ru-Ru spin-singlet formation in La2 − x L n x RuO5 (Ln = Pr, Nd, Sm, Gd, Dy) was investigated by measurements of the specific heat and magnetic susceptibility. After subtraction of the lattice contribution from the specific heat (C p ), similar excess entropy values were obtained for all compounds. These entropies can be explained by the formation of antiferromagnetic Ru-spin dimers at low temperatures and provide a lower estimate for the intradimer exchange strength. Pronounced changes in the transition temperatures and a broadening of the corresponding peak in C p were observed. These changes depend on the rare-earth element and are due to local structural changes and heterogeneities caused by the substitution. The magnetic susceptibilities can be described by the sum of a rare-earth paramagnetic moment and the susceptibility of the unsubstituted La2RuO5. Density functional theory (DFT) calculations were performed for various compounds to investigate the origin of the magnetic transition and the relationship between structural changes and the spin-dimerization temperature. The combination of the present results with previous structural investigations supports the model of a spin-pairing of the Ru moments which occurs as a reason of the structural phase transition in La2 − x L n x RuO5.

13C-NMR studies of the paramagnetic and charge-ordered states of the organic superconductorβ′′-(BEDT−TTF)3Cl2·2H2O under pressure

${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-(BEDT$\ensuremath{-}$TTF)${}_{3}$Cl${}_{2}$$\ifmmode\cdot\else\textperiodcentered\fi{}$2H${}_{2}$O [BEDT$\ensuremath{-}$TTF: $\underline{\mathrm{b}}$is-($\underline{\mathrm{e}}$thylene$\underline{\mathrm{d}}$i$\underline{\mathrm{t}}$hio)$\underline{\mathrm{t}}$etra$\underline{\mathrm{t}}$hia$\underline{\mathrm{f}}$ulvalene] is superconductive under pressures, whereas the salt exhibits metal-insulator (MI) transition under ambient pressure. The insulator phase in the salt was examined using the charge density wave (CDW) phase that was obtained from band calculation. The charge-ordered (CO) state was recently proposed as the insulator phase of the salt, and the mechanism of superconductivity intermediated by charge fluctuation was suggested. We accessed ${}^{13}$C-NMR on ${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-(BEDT$\ensuremath{-}$TTF)${}_{3}$Cl${}_{2}$$\ifmmode\cdot\else\textperiodcentered\fi{}$2H${}_{2}$O at ambient pressure and under pressure up to 1.6 GPa. At ambient pressure, the NMR spectrum changed at approximately 100 K. Three isolated peaks appeared at low temperatures, suggesting that the CO state exists below 100 K, and spin-gap behavior was observed. By analyzing the chemical shift, the charges on the three sites were estimated as $\ensuremath{\sim}$+0.4$e$, $\ensuremath{\sim}$+0.6$e$, and $\ensuremath{\sim}$+1.0$e$. The ratio of peak intensity and unsymmetrical peak position suggest the CO state with some symmetry breaking. When pressure is applied, the splitting of the NMR peaks in the CO state is reduced. The salt finally exhibits superconductivity at 1.6 GPa, spin-gap behavior observed at ${({T}_{1}T)}^{\ensuremath{-}1}$ below 1.3 GPa suddenly disappears, whereas the NMR spectrum predicts that charge disproportionation coexists with superconductivity. The suppression of the spin-singlet formation observed in ${({T}_{1}T)}^{\ensuremath{-}1}$ at 1.6 GPa suggests the metallic state with the charge disproportionation and the CO instability with some symmetry breaking.

Metal-insulator transition in the hollanditeK2V8O16with a frustrated zigzag ladder probed byV51NMR

We report the experimental results of $^{51}\mathrm{V}$ NMR measurements on the hollandite ${\mathrm{K}}_{2}$V${}_{8}$O${}_{16}$ consisting of a frustrated zigzag ladder with the orbital degrees of freedom. The metal-insulator transition is found to involve the spin-singlet formation by the $^{51}\mathrm{V}$ Knight shift $K$, the nuclear spin-lattice relaxation rate $1/{T}_{1}$, and the spin-echo decay rate $1/{T}_{2}$ measurements. In the insulating state, the anisotropic electric-field gradient supports the ${d}_{\mathit{xy}}$ orbital order with the spin singlet along the chain. The ${d}_{\mathit{xy}}$ orbital is magnetically most active in the metallic state, as observed by the anisotropic Knight shift, which suggests the strong electron correlation in the ${d}_{\mathit{xy}}$ band. Despite the large enhancement of the spin susceptibility, no apparent spin correlation is developed in the frustrated metallic state. Pressure suppresses the electron correlation continuously, as highlighted in the decrease of the metal-insulator transition and the spin susceptibility keeping the largest ${d}_{\mathit{xy}}$ contribution in the metallic state. A robust spin-singlet insulating phase with the large spin gap and paramagnetic spins appears above 1 GPa, which suggests a competition of the charge-orbital ordering pattern.

Variational Monte Carlo Study of the Kondo Necklace Model with Geometrical Frustration

We investigate the ground state of the Kondo necklace model on geometrically-frustrated lattices by the variational Monte Carlo simulation. To explore the possibility of a partially-ordered phase, we employ an extension of the Yosida-type wave function as a variational state, which can describe a coexistence of spin-singlet formation due to the Kondo coupling and magnetic ordering by the Ruderman-Kittel-Kasuya-Yosida interaction. We show the benchmark of the numerical simulation to demonstrate the high precision brought by the optimization of a large number of variational parameters. We discuss the ground-state phase diagram for the model on the kagome lattice in comparison with that for the triangular-lattice case.

Metal-insulator transitions inTiO2/VO2superlattices

We have fabricated a series of superlattices composed of ${\text{V}}_{1\ensuremath{-}x}{\text{W}}_{x}{\text{O}}_{2}$ ($x=0$ or 0.08 with $1+2x$ $d$ electron per V atom) and ${\text{TiO}}_{2}$ (no $d$ electron) to investigate the interface and carrier-confinement effects of the metal-insulator phenomena of ${\text{VO}}_{2}$. This study was also motivated by the prediction of a half-metallic state with a semi-Dirac point at the ${\text{TiO}}_{2}/{\text{VO}}_{2}$ interface [V. Pardo and W. E. Pickett, Phys. Rev. Lett. 102, 166803 (2009)]. The growth conditions of the superlattices were optimized so that we could reproduce the known electronic states for the constituent compounds in case of the single-layer films, namely, ${\text{VO}}_{2}$ exhibiting a paramagnetic metal to spin-singlet insulator transition at around room temperature, ${\text{V}}_{0.92}{\text{W}}_{0.08}{\text{O}}_{2}$ $(\text{W}:{\text{VO}}_{2})$ being metallic down to the lowest temperature, and ${\text{TiO}}_{2}$ being a wide-gap band insulator. We found no metallic ground state in these superlattices in contradiction with the theoretical prediction. The ${\text{TiO}}_{2}/{\text{VO}}_{2}$ superlattices always show a resistive transition, corresponding to the metal-insulator transition in the ${\text{VO}}_{2}$ single layer, at around 300 K for the ${\text{VO}}_{2}$-layer thickness varying from 15 monolayers (ML) down to 3 ML. The resistive transition is accompanied with the structural change, consistent with the V-V dimerization in ${\text{VO}}_{2}$ along $c$ axis, suggesting the robust spin-singlet bond formation as well as the dimeric lattice distortion persistent even at the ${\text{TiO}}_{2}/{\text{VO}}_{2}$ interface. In the case of ${\text{TiO}}_{2}/\text{W}:{\text{VO}}_{2}$ superlattices, the insulating ground state revives while the metal-insulator transition temperature decreases from 230 to 95 K as the $\text{W}:{\text{VO}}_{2}$ thickness is increased from 10 to 40 ML. These results indicate the persistent competition between the spin-singlet bond formation and the kinetic energy of correlated electrons regulated by the $\text{W}:{\text{VO}}_{2}$ thickness.

Non-BCS superconductivity for underdoped cuprates by spin-vortex attraction

Within a gauge approach to the t–J model, we propose a new, non-BCS mechanism of superconductivity for underdoped cuprates. The gluing force of the superconducting mechanism is an attraction between spin vortices on two different Néel sublattices, centered around the empty sites described in terms of fermionic holons. The spin fluctuations are described by bosonic spinons with a gap generated by the spin vortices. Due to the no-double occupation constraint, there is a gauge attraction between holon and spinon binding them into a physical hole. Through gauge interaction the spin vortex attraction induces the formation of spin-singlet (RVB) spinon pairs with a lowering of the spinon gap. Lowering the temperature, the approach exhibits two crossover temperatures: at the higher crossover a finite density of incoherent holon pairs are formed leading to a reduction of the hole spectral weight, while at the lower crossover a finite density of incoherent spinon RVB pairs are also formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic vortices appear in the plasma phase. Finally, at a even lower temperature the hole pairs become coherent, the magnetic vortices becoming dilute and superconductivity appears. The superconducting mechanism is not of BCS-type since it involves a gain in kinetic energy (for spinons) coming from the spin interactions.