Magnetic field dependence of the neutron spin resonance in CeB6

In zero magnetic field, the famous neutron spin resonance in the f-electron superconductor CeCoIn5 is similar to the recently discovered exciton peak in the non-superconducting CeB6. Magnetic field splits the resonance in CeCoIn5 into two components, indicating that it is a doublet. Here we employ inelastic neutron scattering (INS) to scrutinize the field dependence of spin fluctuations in CeB6. The exciton shows a markedly different behavior without any field splitting. Instead, we observe a second field-induced magnon whose energy increases with field. At the ferromagnetic zone center, however, we find only a single mode with a non-monotonic field dependence. At low fields, it is initially suppressed to zero together with the antiferromagnetic order parameter, but then reappears at higher fields inside the hidden-order phase, following the energy of an electron spin resonance (ESR). This is a unique example of a ferromagnetic resonance in a heavy-fermion metal seen by both ESR and INS consistently over a broad range of magnetic fields.Comment: 7 pages, 6 figures including one animation, accepted to Phys. Rev.

ЭНДОГЕННАЯ ОПИОИДНАЯ СИСТЕМА КАК ЗВЕНО СРОЧНОЙ И ДОЛГОВРЕМЕННОЙ АДАПТАЦИИ ОРГАНИЗМА К ЭКСТРЕМАЛЬНЫМ ВОЗДЕЙСТВИЯМ. ПЕРСПЕКТИВЫ КЛИНИЧЕСКОГО ПРИМЕНЕНИЯ ОПИОИДНЫХ ПЕПТИДОВ

It has been well established that opioid peptides (OPs) affect various hormonal systems. Opioids exhibit stress-limiting and gastro-protective effects in stressed animals, acting via μ- and δ-opioid receptors (OR). Peripheral μ-OR stimulation by endogenous and exogenous opioids increases cardiac tolerance to pathological consequences of stress. Enhancement of prostacyclin synthesis, decrease of thromboxane production as well as suppression of lipid peroxidation can be directly responsible for cardioprotective effects of OPs in stressed animals. Adaptive responses are accompanied by increased OP levels in blood and tissues. Reduction of ventricular arrhythmias induced by repeated short-term immobilization stress is mediated via μ-OR stimulation by endogenous opioids, while δ-OR account for an antiarrhythmic effect of adaptation to chronic intermittent hypobaric hypoxia. The mechanism of infarct size-limiting effect of continuous normobaric hypoxia involves both μ- and δ-OR stimulation. Peptide OR agonists can be considered in future clinical practice for treatment of withdrawal syndrome, stress-related cardiac disease or myocardial injury caused by ischemia-reperfusion insult. Установлено, что опиоидные пептиды (ОП) влияют на различные гормональные системы. Опиоиды оказывают стресс-лимитирующий и гастропротекторный эффект на стрессированных особей, действуя на μ- и δ-опиоидные рецепторы (ОР). Стимуляция периферических μ-ОР эндо- и экзогенными опиоидами способствует повышению толерантности сердца к патогенному действию стресса. Усиление синтеза простациклина, снижение продукции тромбоксана, а также подавление процессов пероксидного окисления липидов (ПОЛ) могут иметь прямое отношение к кардиопротекторному эффекту ОП у стрессированных животных. Адаптация сопровождается увеличением содержания ОП в крови и тканях экспериментальных животных. Снижение частоты возникновения желудочковых аритмий, индуцированное повторными иммобилизациями, опосредуется через стимуляцию μ-ОР эндогенными опиоидами, в то время как δ-ОР ответственны за антиаритмический эффект адаптации к периодической гипобарической гипоксии. Инфаркт-лимитирующий эффект адаптации к непрерывной нормобарической гипоксии зависит от активации μ- и δ-ОР. Пептидные агонисты ОР в будущем могут найти применение в клинической практике в лечении абстинентного синдрома, стресс-индуцированных заболеваний сердца или повреждений миокарда, вызванных ишемией и реперфузией.

Superconducting properties and pseudogap from preformed Cooper pairs in the triclinic (CaFe1−x_{1-x}Ptx_xAs)10_{10}Pt3_3As8_8

Using a combination of muon-spin relaxation ($\mu$SR), inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR), we investigated the novel iron-based superconductor with a triclinic crystal structure (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$ (T$_{\rm c}$ = 13 K), containing platinum-arsenide intermediary layers. The temperature dependence of the superfluid density obtained from the $\mu$SR relaxation-rate measurements indicates the presence of two superconducting gaps, $\Delta_\text{1}\gg\Delta_\text{2}$. According to our INS measurements, commensurate spin fluctuations are centered at the ($\pi$, 0) wave vector, like in most other iron arsenides. Their intensity remains unchanged across T$_\text{c}$, indicating the absence of a spin resonance typical for many Fe-based superconductors. Instead, we observed a peak in the spin-excitation spectrum around $\hslash\omega_0=\,$7 meV at the same wave vector, which persists above T$_{\rm c}$ and is characterized by the ratio $\hslash\omega_0/k_\text{B}T_\text{c}\approx\,$6.2, which is significantly higher than typical values for the magnetic resonant modes in iron pnictides (~4.3). The temperature dependence of magnetic intensity at 7 meV revealed an anomaly around T* = 45 K related to the disappearance of this new mode. A suppression of the spin-lattice relaxation rate, $1/T_1T$, observed by NMR immediately below T* without any notable subsequent anomaly at T$_{\rm c}$, indicates that T* could mark the onset of a pseudogap in (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$, which is likely associated with the emergence of preformed Cooper pairs

Magnon spectrum of the helimagnetic insulator Cu2OSeO3

Complex low-temperature-ordered states in chiral magnets are typically governed by a competition between multiple magnetic interactions. The chiral-lattice multiferroic Cu2OSeO3 became the first insulating helimagnetic material in which a long-range order of topologically stable spin vortices known as skyrmions was established. Here we employ state-of-the-art inelastic neutron scattering to comprehend the full three-dimensional spin-excitation spectrum of Cu2OSeO3 over a broad range of energies. Distinct types of high- and low-energy dispersive magnon modes separated by an extensive energy gap are observed in excellent agreement with the previously suggested microscopic theory based on a model of entangled Cu4 tetrahedra. The comparison of our neutron spectroscopy data with model spin-dynamical calculations based on these theoretical proposals enables an accurate quantitative verification of the fundamental magnetic interactions in Cu2OSeO3 that are essential for understanding its abundant low-temperature magnetically ordered phases

Destruction of long range magnetic order in an external magnetic field and the associated spin dynamics in Cu2GaBO5 and Cu2AlBO5 ludwigites

The quantum spin systems Cu2M BO5 M Al,Ga with the ludwigite crystal structure consist of a structurally ordered Cu2 sublattice in the form of three leg ladders, interpenetrated by a structurally disordered sublattice with a statistically random site occupation by magnetic Cu2 and nonmagnetic Ga3 or Al3 ions. A microscopic analysis based on density functional theory calculations for Cu2GaBO5 reveals a frustrated quasi two dimensional spin model featuring five inequivalent antiferromagnetic exchanges. A broad low temperature 11B nuclear magnetic resonance points to a considerable spin disorder in the system. In zero magnetic field, antiferromagnetic order sets in below TN approximation 4.1 K and 2.4 K for the Ga and Al compounds, respectively. From neutron diffraction, we find that the magnetic propagation vector in Cu2GaBO5 is commensurate and lies on the Brillouin zone boundary in the H0L plane, qm 0.45, 0, 0.7 , corresponding to a complex noncollinear long range ordered structure with a large magnetic unit cell. Muon spin relaxation is monotonic, consisting of a fast static component typical for complex noncollinear spin systems and a slow dynamic component originating from the relaxation on low energy spin fluctuations. Gapless spin dynamics in the form of a diffuse quasielastic peak is also evidenced by inelastic neutron scattering. Most remarkably, application of a magnetic field above 1 T destroys the static long range order, which is manifested in the gradual broadening of the magnetic Bragg peaks. We argue that such a crossover from a magnetically long range ordered state to a spin glass regime may result from orphan spins on the structurally disordered magnetic sublattice, which are polarized in magnetic field and thus act as a tuning knob for field controlled magnetic disorde

Magnon spectrum of the helimagnetic insulator Cu₂OSeO₃

Complex low-temperature-ordered states in chiral magnets are typically governed by a competition between multiple magnetic interactions. The chiral-lattice multiferroic Cu2OSeO3 became the first insulating helimagnetic material in which a long-range order of topologically stable spin vortices known as skyrmions was established. Here we employ state-of-the-art inelastic neutron scattering to comprehend the full three-dimensional spin-excitation spectrum of Cu2OSeO3 over a broad range of energies. Distinct types of high-and low-energy dispersive magnon modes separated by an extensive energy gap are observed in excellent agreement with the previously suggested microscopic theory based on a model of entangled Cu-4 tetrahedra. The comparison of our neutron spectroscopy data with model spin-dynamical calculations based on these theoretical proposals enables an accurate quantitative verification of the fundamental magnetic interactions in Cu2OSeO3 that are essential for understanding its abundant low-temperature magnetically ordered phases