Large nonlinear optical magnetoelectric response in a noncentrosymmetric magnetic Weyl semimetal

Weyl semimetals resulting from either inversion (P) or time-reversal (T) symmetry breaking have been revealed to show the record-breaking large optical response due to intense Berry curvature of Weyl-node pairs. Different classes of Weyl semimetals with both P and T symmetry breaking potentially exhibit optical magnetoelectric (ME) responses, which are essentially distinct from the previously observed optical responses in conventional Weyl semimetals, leading to the versatile functions such as directional dependence for light propagation and gyrotropic effects. However, such optical ME phenomena of (semi)metallic systems have remained elusive so far. Here, we show the large nonlinear optical ME response in noncentrosymmetric magnetic Weyl semimetal PrAlGe, in which the polar structural asymmetry and ferromagnetic ordering break P and T symmetry. We observe the giant second harmonic generation (SHG) arising from the P symmetry breaking in the paramagnetic phase, being comparable to the largest SHG response reported in Weyl semimetal TaAs. In the ferromagnetically ordered phase, it is found that interference between this nonmagnetic SHG and the magnetically induced SHG emerging due to both P and T symmetry breaking results in the magnetic field switching of SHG intensity. Furthermore, such an interference effect critically depends on the light-propagating direction. The corresponding magnetically induced nonlinear susceptibility is significantly larger than the prototypical ME material, manifesting the existence of the strong nonlinear dynamical ME coupling. The present findings establish the unique optical functionality of P- and T-symmetry broken ME topological semimetals.

Diamond lattice in single-component molecular crystals comprising tetrabenzoporphyrin neutral radicals

A single-component molecular radical crystal of CoIII(tbp˙-)(CN)2, where tbp = tetrabenzoporphyrinato ligand, exhibiting a diamond lattice was fabricated as a potential candidate for a three-dimensional Dirac electron system. Band structure calculations revealed that the Fermi energy level was located at the Dirac point. A small electrical resistivity of 160 Ω cm was observed at 2 K under the application of 2.4 GPa. Furthermore, substituting CoIII by FeIII or MnIII led to the introduction of local magnetic moments into the diamond-lattice system. MIII(tbp˙-)L2 crystals will open up uncharted fields in the study of the Dirac electron systems.

A large negative magnetoresistance effect in semiconducting crystals composed of an octahedrally ligated phthalocyanine complex with high-spin manganese(iii)

A design for an octahedrally ligated phthalocyanine complex with high-spin manganese(iii) (S = 2) and Mn^III(Pc)Cl₂ (Pc = phthalocyanine) is presented. The presence of high-spin state Mn^III in the fabricated Ph₄P[Mn^III(Pc)Cl₂]₂ (Ph₄P = tetraphenylphosphonium) semiconducting molecular crystal is indicated by the Mn–Cl distance, which suggests an electronic configuration of (d_yz, d_zx)²(d_xy)¹(d_z²)¹. This was confirmed by the Curie constant (C = 5.69 emu K mol⁻¹), which was found to be significantly larger than that of the isostructural Ph₄P[Mn^III(Pc)(CN)₂]₂, where Mn^III adopts a low-spin state (S = 1). The magnetoresistance (MR) effects of Ph₄P[Mn^III(Pc)Cl₂]₂ at 26.5 K under 9 T static magnetic fields perpendicular and parallel to the c-axis were determined to be −30% and −20%, respectively, which are significantly larger values than those of Ph₄P[Mn^III(Pc)(CN)₂]₂. Furthermore, the negative MR effect is comparable to that of Ph₄P[Fe^III(Pc)(CN)₂]₂ (S = 1/2), which exhibits the largest negative MR effect reported for [M^III(Mc)L₂]-based systems (Mc = macrocyclic ligand, L = axial ligand). This suggests that the spin state of the metal ion is the key to tuning the MR effect.

A Ph4P[Mn^III(Pc)Cl2]2 molecular crystal where Mn^III adopts a high-spin state (S = 2) was designed. The large magnetoresistance effect of fabricated Ph4P[Mn^III(Pc)Cl2]2 suggests that the spin state of the metal ion is the key to tuning the MR effect.

An electrically conducting molecular crystal composed of a magnetic iron(iii) complex (S = 1/2) with a large aromatic ligand, 1,2-naphthlalocyanine (C4h isomer): towards the development of molecular spintronics

The field of molecular spintronics has gained significant attention for the development of second-generation spintronic devices. Therefore, an electrically conducting molecular crystal, Ph₄P[Fe^III(1,2-Nc)(CN)₂]₂ (Ph₄P = tetraphenylphosphonium and 1,2-Nc = C_4h isomer of 1,2-naphthalocyanine), was fabricated as a new coordination compound with a strong π-d interaction. Furthermore, it is a mixed-valence compound with a local spin of S = 1/2 at the center of the conduction path. Crystal structure analysis revealed that Ph₄P[Fe^III(1,2-Nc)(CN)₂]₂ was isostructural to its non-magnetic analogue Ph₄P[Co^III(1,2-Nc)(CN)₂]₂ but possessed higher electrical resistivity, indicating that the strong intramolecular π-d interaction is present in the [Fe^III(1,2-Nc)(CN)₂] unit. Although the magnetic interaction between π-conduction electrons and Fe^III-d spins (π-d interaction) is crucial for the emergence of a negative magnetoresistance effect, the negative magnetoresistance effect of Ph₄P[Fe^III(1,2-Nc)(CN)₂]₂ was significantly smaller (-6% at 30 K under a static 9 T magnetic field) than those of Ph₄P[Fe^III(Pc)(CN)₂]₂ (-32%) and Ph₄P[Fe^III(tbp)(CN)₂]₂ (-13%) analogues (Pc = phthalocyanine and tbp = tetrabenzoporphyrin). This small negative magnetoresistance effect of Ph₄P[Fe^III(Pc)(CN)₂]₂ could be ascribed to the weak intermolecular antiferromagnetic interaction between its d spins. Hence, this study showed that constructing a molecular design for strengthening the intermolecular antiferromagnetic interaction is key to enhancing the negative magnetoresistance effect.

Intermolecular interactions of tetrabenzoporphyrin- and phthalocyanine-based charge-transfer complexes

The effect of molecular modification on the intermolecular interactions in tetrabenzoporphyrin-based charge transfer complexes is reported. TPP[Fe^III(tbp)Cl₂]₂, TPP[Co^III(tbp)Cl₂]₂ and TPP[Co^III(tbp)Br₂]₂ (TPP = tetraphenylphosphonium and tbp = tetrabenzoporphyrin) were synthesized and their crystal structures were compared to those of the reported TPP[M^III(tbp)(CN)₂]₂, TPP[Fe^III(tbp)Br₂]₂ and TPP[M^III(Pc)L₂]₂ complexes (Pc = phthalocyanine; and L = CN, Cl or Br). The prepared CT complexes were isostructural to reported systems. However, their intermolecular interactions were found to depend on the combination of the macrocyclic (Mc) and axial ligands (L). In Pc-based systems, the overlap integral between HOMOs of Pc decreased with the increase in the size of the axial ligand, which indicated that the intermolecular interactions in Pc-based systems were dominated by repulsive interactions. On the other hand, in tbp-based systems, attractive and repulsive interactions competed with each other. Furthermore, charge transport properties were found to depend on the central metal ion as well as the combination of Mc and L, which suggested that minor molecular modifications to porphyrin complexes will cause drastic changes in both inter- and intramolecular interactions.

Large Enhancement of Thermoelectric Efficiency Due to a Pressure-Induced Lifshitz Transition in SnSe

The Lifshitz transition, a change in Fermi surface topology, is likely to greatly influence exotic correlated phenomena in solids, such as high-temperature superconductivity and complex magnetism. However, since the observation of Fermi surfaces is generally difficult in the strongly correlated systems, a direct link between the Lifshitz transition and quantum phenomena has been elusive so far. Here, we report a marked impact of the pressure-induced Lifshitz transition on thermoelectric performance for SnSe, a promising thermoelectric material without a strong electron correlation. By applying pressure up to 1.6 GPa, we have observed a large enhancement of the thermoelectric power factor by more than 100% over a wide temperature range (10-300 K). Furthermore, the high carrier mobility enables the detection of quantum oscillations of resistivity, revealing the emergence of new Fermi pockets at ∼0.86  GPa. The observed thermoelectric properties linked to the multivalley band structure are quantitatively reproduced by first-principles calculations, providing novel insight into designing the SnSe-related materials for potential valleytronic as well as thermoelectric applications.

An electrically conducting crystal composed of an octahedrally ligated porphyrin complex with high-spin iron(iii)

A porphyrin-based octahedrally ligated complex with high-spin iron(iii) was designed, and the resulting electrically conducting crystal TPP[Fe^III(tbp)Br₂]₂ (TPP = tetraphenylphosphonium and tbp = tetrabenzoporphyrin) was synthesised. Although TPP[Fe(tbp)Br₂]₂ was isostructural to the reported TPP[Fe(Mc)L₂]₂ systems (Mc = macrocyclic ligands such as phthalocyanine (Pc) or tbp; and L = CN, Cl, or Br), the bond lengths between Fe and ligands in the [Fe(tbp)Br₂] unit were evidently longer than those in the other units, because of the different spin states of Fe: high-spin in TPP[Fe(tbp)Br₂]₂ and low-spin in others. The magnetic anisotropy observed in the low-spin state vanished when the Fe is in the high-spin state. Based on reports for Pc-based systems, the negative magnetoresistance (MR) effect for TPP[Fe(tbp)Br₂]₂ was expected to be smaller than that for TPP[Fe(tbp)(CN)₂]₂. However, the former showed a giant negative MR effect similar to or larger than the latter, suggesting that the nature of iron is a crucial factor for the electrical properties of porphyrin-based materials.

PVD thin film growth of M(Pc)(CN)2 axially substituted metal-phthalocyanines

We tested the feasibility of in-house synthesized M(Pc)(CN)[Formula: see text] type axially substituted metal-phthalocyanines (M [Formula: see text] metal ion Co or Fe; Pc [Formula: see text] phthalocyanine ligand) for thin film growth via physical vapor deposition. We performed optical and infrared spectroscopy on thin films deposited in high vacuum as well as thermal desorption experiments in ultrahigh vacuum using mass spectrometry. In contradiction to the expectation of a rather strongly bound CN ligand, the results indicate molecular dissociation under the loss of CN groups at a temperature that is below the evaporation temperature of the material needed to form a film on a substrate. Therefore it was concluded that PVD is not a suitable method to grow M(Pc)(CN)[Formula: see text] thin films. However, the obtained data yield valuable insight into the molecules’ stability.

A new strategy for inducing dipole moments in charge-transfer complexes: introduction of asymmetry into axially ligated iron phthalocyanines

Introduction of asymmetry into charge-transfer complexes composed of axially ligated iron phthalocyanines was achieved. In the obtained crystals of TPP[Fe^III(Pc)(CN)Cl]₂, TPP[Fe^III(Pc)(CN)Br]₂, and TPP[Fe^III(Pc)BrCl]₂ (TPP = tetraphenylphosphonium and Pc = phthalocyanine), the axial positions of the iron atoms were occupied by 50/50 ratios of the ligands CN/Cl, CN/Br, and Br/Cl, respectively. The crystal structures of the obtained CT complexes were isostructural to those composed of the symmetric analogues of the type [Fe^III(Pc)L₂] (L = CN, Cl or Br); the [Fe^III(Pc)LL'] units formed regular one-dimensional chains along the c-axis following the symmetry of the P4₂/n space group. Despite forming similar regular chains to the symmetric systems, the electrical resistivities and activation energies were enhanced in the obtained CT complexes compared to those in symmetric systems, indicating that the charge-ordered states were stabilised by the introduction of asymmetry. More specifically, the dielectric relaxation behaviour of the inhomogeneous disordered TPP[Fe^III(Pc)(CN)Cl]₂ probably suggests that a dipole moment was induced in this material.

A giant negative magnetoresistance effect in an iron tetrabenzoporphyrin complex

By measuring the electrical resistivity in TPP[Fe^III(tbp)(CN)₂]₂ (TPP = tetraphenylphosphonium and tbp = tetrabenzoporphyrin) under the application of a static magnetic field, a giant negative magnetoresistance (MR) effect with high anisotropy is observed. More specifically, the MR ratio at 13 K under a field of 9 T perpendicular to the c axis is -70%, whereas the MR ratio under a field parallel to the c axis is -40%. Furthermore, electron spin resonance (ESR) measurements indicate large anisotropy in the principal g-values of d spin (S = 1/2) in the [Fe^III(tbp)(CN)₂] unit; the g₁ value almost perpendicular to the tbp plane and the g₂ and g₃ values almost parallel to the tbp plane are 3.60, 1.24, and 0.39, respectively. It is revealed that the anisotropy in the MR effect arises from the anisotropy in the d spin, suggesting that the d spins in TPP[Fe^III(tbp)(CN)₂]₂ affect the π-conduction electron via the intramolecular π-d interaction. The anisotropy and magnitude in the giant negative MR effect for TPP[Fe^III(tbp)(CN)₂]₂ are smaller than the corresponding values for the isostructural phthalocyanine (Pc) analogue TPP[Fe^III(Pc)(CN)₂]₂. This is consistent with the fact that the intermolecular antiferromagnetic d-d interaction in TPP[Fe^III(tbp)(CN)₂]₂ (suggested by the Weiss temperature: Θ = -8.0 K) is weaker than that in TPP[Fe^III(Pc)(CN)₂]₂ (Θ = -12.3 K). This indicates that the minor modification in coordination complexes can significantly affect the MR effect via tuning the intermolecular d-d interaction as well as the intermolecular π-π overlap.

One-dimensional phthalocyanine-based conductor with S = 3/2 isotropic magnetic centers

The axially ligated phthalocyanine conductor of TPP [ Cr ( Pc )( CN )2]2 (TPP = tetraphenylphosphonium and Pc = phthalocyaninato) with d3 (S = 3/2) metal Cr III has been successfully prepared. The crystal is completely isomorphous with those of the Co and Fe analogs, and the degree of π–π interaction is almost the same in these three salts. The electrical conductivity at room temperature of the Cr system is almost the same with that of the Fe system in which S = 1/2 magnetic centers embedded in the conduction path, and the temperature dependence reveals the development of the charge disproportionation reflecting the existence of magnetic moments. Magnetic susceptibility of the Cr system shows Curie-like behavior with isotropic local moments of S = 3/2. In contrast with the Fe system, only weak antiferromagnetic interaction operates between the localized d-spins. The resistance decreases when magnetic fields are applied, but the magnetoresistance effect is not so large compared with the Fe system, indicating that the π– d interaction in the Cr system is somewhat different from that in the Fe system.

Emergence of a diffusive metal state with no magnetic order near the Mott transition in frustrated pyrochlore-type molybdates

The electron-correlation driven metal-insulator (Mott) transition in pyrocholore-type R2Mo2O7(R being rare-earth-metal ions) is accompanied by the change of the magnetic state from ferromagnetic to spin glass due to the competing double-exchange and superexchange interactions on the frustrated lattice. By application of high pressures on the compounds with Mott criticality, however, a new unique paramagnetic metal phase is observed to show up with nearly temperature-independent high resistivity close to the Ioffe-Regel limit. A possible non-Fermi-liquid character of this anomalously diffuse metallic state is argued in terms of the extended double-exchange model with the magnetically frustrated local S=1/2 spins.

Anomalous nernst effects in pyrochlore molybdates with spin chirality

The transverse thermoelectric (Nernst) effect on pyrochlore molybdates is investigated experimentally. In Nd(2)Mo(2)O(7) and Sm(2)Mo(2)O(7) with the spin chirality, the Nernst signal, which mostly arises from the transverse heat current (or equivalently the transverse Peltier coefficient alpha(xy)), shows a low-temperature (20-30 K) positive extremum, whereas it is absent in (Gd(0.95)Ca(0.05))(2)Mo(2)O(7) with no single-spin anisotropy of the rare-earth ion and hence with no spin chirality. The correlation between the Hall conductivity sigma(xy) and alpha(xy) in Nd(2)Mo(2)O(7) also indicates the spin chirality plays a significant role in the spontaneous (anomalous) Nernst effect.

Nature of the transition between a ferromagnetic metal and a spin-glass insulator in pyrochlore molybdates

The metal-insulator transition has been investigated for pyrochlore molybdates R(2)Mo(2)O(7) with nonmagnetic rare-earth ions R. The dynamical scaling analysis of ac susceptibility reveals that the geometrical frustration causes the atomic spin-glass state. The reentrant spin-glass phase exists below the ferromagnetic transition. The electronic specific heat is enhanced as compared to the band calculation result, perhaps due to the orbital fluctuation in the half-metallic ferromagnetic state. The large specific heat is rather reduced upon the transition, likely because the short-range antiferromagnetic fluctuation shrinks the Fermi surface.

Scaling of anomalous hall resistivity in Nd2(Mo(1-x)Nb(x))2O7 with spin chirality

We have investigated scaling of anomalous Hall resistivity with longitudinal resistivity (rho(xx)) in pyrochlore type Nd2(Mo(1-x)Nb(x))2O7 with spin chirality. Scattering rate of the conduction electron on the Mo sublattice can be varied with x from band transport to polaron hopping, while keeping the two-in-two-out structure of the Nd moments intact. The anomalous part of the Hall resistivity arising from the Mo spin chirality (rho(H)(chi)) shows a clear scaling behavior with rho(xx) (rho(H)(chi) proportional to rho(xx)0.39), in accord with a recent theoretical result based on the Berry phase mechanism in the hopping conduction regime.

Mott-Anderson transition controlled by a magnetic field in pyrochlore molybdate

The pyrochlore molybdate Gd2MO2O7 locates near the phase boundary between the ferromagnetic-metallic and the spin-glass insulating state. This metal-insulator transition is governed on a large energy scale by the electron-correlation effect, while the geometrical frustration causes the random potential. The magnetic field can tune the randomness of the potential and control, under a suitable pressure, the continuous Mott-Anderson transition precisely. The critical exponent (mu = 1.04 +/- 0.1) of the Mott-Anderson transition has been determined for this ferromagnetic orbital-degenerate electron system.

Charge dynamics near the electron-correlation induced metal-insulator transition in pyrochlore-type molybdates

The systematics of the bandwidth controlled metal-insulator transition (MIT) are investigated for R2Mo2O7 (R=Nd, Sm, Gd, Dy, and Ho) by measurements of dc and optical conductivity. The substantial role of electron correlation in driving the MIT is verified. With changing the R ionic radius (r) or equivalently the one-electron bandwidth, the T=0 K MIT occurs at rc approximately r(R=Gd). The T=0 K gap continuously vanishes as Delta proportional, variant(rc-r), while at the metallic side the decrease of Drude weight is followed towards rc. A high-temperature incoherent state is approached through crossover regions both from the metallic and the insulating state.

Relative potency in acute and chronic suppressive effects of prednisolone and betamethasone on the hypothalamic-pituitary-adrenal axis in man

The relative potency in the hypothalamic-pituitary-adrenal (HPA) suppression of both prednisolone and betamethasone was examined in an acute study with normal volunteers and in a chronic study with glucocorticoid-treated patients. Circadian rhythm of plasma cortisol was studied after a single dose administration of 5 to 30 mg prednisolone or 0.5 to 3.0 mg betamethasone at 8:00 hr. Morning-rise of plasma cortisol occurred on the morning after the administration of 30 mg or less prednisolone but no morning rise was noted after the administration of 1.0 mg or more betamethasone. Plasma ACTH was slightly elevated on the morning after 30 mg prednisolone administration but showed low levels throughout the night after 3.0 mg betamethasone administration. Plasma cortisol responsiveness to ACTH was examined in patients before and during therapy with either prednisolone or betamethasone. The basal cortisol level was not suppressed and the responsiveness to ACTH remained nearly normal during long-term 5 mg prednisolone therapy, but these were completely suppressed during long-term 5 mg betamethasone therapy. The responsiveness to ACTH was nearly normal in patients receiving alternate-day therapy with prednisolone in such large doses as 50 or 60 mg every other day, but was completely suppressed in patients receiving 1.0 mg betamethasone every other day. The relative potency of betamethasone in acute and chronic suppressive effects on the HPA system seems to be much stronger than that of prednisolone in equivalent doses with comparable anti-inflammatory effects. It is also suggested that the alternate-day therapy with such long-acting steroids as betamethasone are useless in preventing HPA suppression.

[Effect of biogenic amines on rat anterior pituitary cyclic AMP]

The effects of biogenic amines on the formation of adenosine-3',5' monophosphate (cAMP) in the anterior pituitary of rats were investigated. Pituitary halves were preincubated in a Krebs-Ringer bicarbonate buffer (pH 7.4) containing theophylline (10(-2)M) at 37 degrees C for 30 minutes and incubated in a fresh buffer for 15 minutes after the addition of test substances. Noradrenaline at 4 X 10(-4)M and serotonin at 3.3 X 10(-4)M were effective in elevating cAMP, but dopamine at 2 X 20(-4)M was ineffective. The beta adrenergic blocking agent, propranolol, effectively antagonized the cAMP response to noradrenaline, but the alpha adrenergic blocking agent, phentolamine, did not. The serotonin antagonist, methysergide, remarkably antagonized the cAMP response to serotonin, but the alpha and beta adrenergic agents did not.

The fate of ACTH released from rat anterior pituitary into the incubation medium in vitro: enzymatic degradation and acid activation

The bioactivity of ACTH released from isolated rat anterior pituitary glands into the incubation medium was determined. After the pituitaries were removed, ACTH activity in the medium decreased exponentially during further incubation at 37degreesC. The loss of ACTH activity was temperature- and pH-dependent and inhibited both by protease inhibitor (trasylol) and by preheating. Crude tissue extracts from median eminence, cerebral cortex and liver similarly inhibited the loss of ACTH activity. These results indicate that ACTH released into the medium may be destroyed by proteolytic enzyme(s) from the rat anterior pituitary. ACTH activity in the incubation medium was increased promptly by acidification of the medium to pH 1.5-2.5 with HC1, and reduced to the initial level by NaOH reneutralization of the medium (pH 6.8-7.8). These phenomena were not observed after the incubation medium had been heated at 100degreesC for 5 min.