Spin Seebeck Effect near the Antiferromagnetic Spin-Flop Transition

We develop a low-temperature, long-wavelength theory for the interfacial spin Seebeck effect (SSE) in easy-axis antiferromagnets. The field-induced spin-flop (SF) transition of N\'eel order is associated with a qualitative change in SSE behavior: Below SF, there are two spin carriers with opposite magnetic moments, with the carriers polarized along the field forming a majority magnon band. Above SF, the low-energy, ferromagnetic-like mode has magnetic moment opposite the field. This results in a sign change of the SSE across SF, which agrees with recent measurements on Cr$_2$O$_3$/Pt and Cr$_2$O$_3$/Ta devices [Li $\textit{et al.,}$ $\textit{Nature}$ $\textbf{578,}$ 70 (2020)]. In our theory, SSE is due to a N\'eel spin current below SF and a magnetic spin current above SF. Using the ratio of the associated N\'eel to magnetic spin-mixing conductances as a single constant fitting parameter, we reproduce the field dependence of the experimental data and partially the temperature dependence of the relative SSE jump across SF.Comment: 5 pages, 3 figure

Observation of nuclear-spin Seebeck effect

Thermoelectric effects have been applied to power generators and temperature sensors that convert waste heat into electricity. The effects, however, have been limited to electrons to occur, and inevitably disappear at low temperatures due to electronic entropy quenching. Here, we report thermoelectric generation caused by nuclear spins in a solid: nuclear-spin Seebeck effect. The sample is a magnetically ordered material MnCO3 having a large nuclear spin (I = 5/2) of 55Mn nuclei and strong hyperfine coupling, with a Pt contact. In the system, we observe low-temperature thermoelectric signals down to 100 mK due to nuclear-spin excitation. Our theoretical calculation in which interfacial Korringa process is taken into consideration quantitatively reproduces the results. The nuclear thermoelectric effect demonstrated here offers a way for exploring thermoelectric science and technologies at ultralow temperaturesThis work was supported by JST ERATO “Spin Quantum Rectification Project” (JPMJER1402), JST CREST (JPMJCR20C1 and JPMJCR20T2), JSPS KAKENHI (JP19H05600, JP19K21031, JP20H02599, JP20K22476, and JP20K15160), MEXT [Innovative Area “Nano Spin Conversion Science” (JP26103005)], and Daikin Industries, Ltd. The work at UCLA was supported by the US Department of Energy, Office of Basic Energy Sciences under Award number DE-SC0012190. K.O. acknowledges support from GP-Spin at Tohoku University. R.R. acknowledges support from the European Commission through the project 734187-SPICOLOST (H2020-MSCA-RISE-2016), the European Union’s Horizon 2020 research and innovation program through the Marie Sklodowska-Curie Actions grant agreement SPEC number 894006 and the Spanish Ministry of Science (RYC 2019-026915-I)S

Analysis of shared heritability in common disorders of the brain

ience, this issue p. eaap8757 Structured Abstract INTRODUCTION Brain disorders may exhibit shared symptoms and substantial epidemiological comorbidity, inciting debate about their etiologic overlap. However, detailed study of phenotypes with different ages of onset, severity, and presentation poses a considerable challenge. Recently developed heritability methods allow us to accurately measure correlation of genome-wide common variant risk between two phenotypes from pools of different individuals and assess how connected they, or at least their genetic risks, are on the genomic level. We used genome-wide association data for 265,218 patients and 784,643 control participants, as well as 17 phenotypes from a total of 1,191,588 individuals, to quantify the degree of overlap for genetic risk factors of 25 common brain disorders. RATIONALE Over the past century, the classification of brain disorders has evolved to reflect the medical and scientific communities' assessments of the presumed root causes of clinical phenomena such as behavioral change, loss of motor function, or alterations of consciousness. Directly observable phenomena (such as the presence of emboli, protein tangles, or unusual electrical activity patterns) generally define and separate neurological disorders from psychiatric disorders. Understanding the genetic underpinnings and categorical distinctions for brain disorders and related phenotypes may inform the search for their biological mechanisms. RESULTS Common variant risk for psychiatric disorders was shown to correlate significantly, especially among attention deficit hyperactivity disorder (ADHD), bipolar disorder, major depressive disorder (MDD), and schizophrenia. By contrast, neurological disorders appear more distinct from one another and from the psychiatric disorders, except for migraine, which was significantly correlated to ADHD, MDD, and Tourette syndrome. We demonstrate that, in the general population, the personality trait neuroticism is significantly correlated with almost every psychiatric disorder and migraine. We also identify significant genetic sharing between disorders and early life cognitive measures (e.g., years of education and college attainment) in the general population, demonstrating positive correlation with several psychiatric disorders (e.g., anorexia nervosa and bipolar disorder) and negative correlation with several neurological phenotypes (e.g., Alzheimer's disease and ischemic stroke), even though the latter are considered to result from specific processes that occur later in life. Extensive simulations were also performed to inform how statistical power, diagnostic misclassification, and phenotypic heterogeneity influence genetic correlations. CONCLUSION The high degree of genetic correlation among many of the psychiatric disorders adds further evidence that their current clinical boundaries do not reflect distinct underlying pathogenic processes, at least on the genetic level. This suggests a deeply interconnected nature for psychiatric disorders, in contrast to neurological disorders, and underscores the need to refine psychiatric diagnostics. Genetically informed analyses may provide important "scaffolding" to support such restructuring of psychiatric nosology, which likely requires incorporating many levels of information. By contrast, we find limited evidence for widespread common genetic risk sharing among neurological disorders or across neurological and psychiatric disorders. We show that both psychiatric and neurological disorders have robust correlations with cognitive and personality measures. Further study is needed to evaluate whether overlapping genetic contributions to psychiatric pathology may influence treatment choices. Ultimately, such developments may pave the way toward reduced heterogeneity and improved diagnosis and treatment of psychiatric disorders

Anoxic development of sapropel S1 in the Nile Fan inferred from redox sensitive proxies, Fe speciation, Fe and Mo isotopes

Redox conditions and the mechanisms of redox development are a critical aspect of Eastern Mediterranean sapropels, whose formation in oxygen-depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima. Sapropels in the Nile Fan formed at relatively shallow water depths under the influence of the monsoon-driven freshwater output from the River Nile. This work evaluates the redox evolution of Holocene sapropel S1 in VALPAMED cruise core MD9509, recovered at 880 mbsl in the NE Nile Fan, using a combination of geochemical element proxies, Fe speciation, Fe and Mo isotopes studies. The productivity and redox proxies (Ba/Al, Mo/Al, U/Al, V/Al, Sb/Al) show well-defined enrichments in the sapropel, but with a marked minimum at ca 8.2 ka indicative of reventilation corresponding to a well known global cooling event. Peak productivity and reducing signals occur close to the initiation of sapropel formation. The proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel S1, recovered at the same depth, but 380 km to the north (MD9501), supporting the notion (suggested in previous studies) of more reduced conditions in the Nile Fan. The MoEF vs. UEF enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non-sapropel sediments to anoxic non-sulphidic water column conditions in the sapropel. Correspondingly, the highly reactive Fe pool (FeHR) measured in Fe speciation studies is dominated by Fe(oxyhydr) oxide minerals in the background sediments, whereas pyrite (Fepy) becomes the dominant component of the FeHR pool in the sapropel. Maximum Fepy values in the sapropel coincide with peak productivity and reducing conditions, implying a clear link between trace element uptake, diagenetic bacterial sulphate reduction in anoxic porewater and Fe mobilization in the sapropel. Iron isotope compositions (δ56Fe) in the sapropel do not show any departure from primary (marine and detrital) source sediment values, and the absence of an Fe/Al vs. δ56Fe trend strongly argues against an Fe shuttle. Molybdenum isotopes, however, show marked non-conservative fractionation patterns. Background sediment δ98/95Mo values (0.2 to 0.7‰) are compatible with fractionation upon absorptive uptake by Fe (oxyhydr)oxides and pyrite. In contrast, minimum δ98/95Mo values exhibited at peak sapropel (reducing and pyrite producing) conditions are most closely modeled by Mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species. The conservative Fe isotope behavior/Mo isotope fractionation minima in the sapropel may be a characteristic of organic-rich sediment diagenesis below an anoxic non-sulphidic water body, without the operation of a benthic Fe shuttle

Spin transport in ferromagnets, antiferromagnets, paramagnets, and nuclear spins

The central scientific objective in this thesis is to develop experimentally-testable theoriesfor the dynamical behavior of magnetic systems. Our second objective is to use our understanding of these systems to investigate their potential for generating thermally-induced spin currents, ultimately having thermoelectric applications. This phenomena is known as the spin Seebeck effect. In Chapter 1, we derive the low-energy, long-wavelength spectra of Heisenberg ferromagnets (FMs) and antiferromagnets (AFs), in their strongly-ordered regimes, from an intuitive starting point in a classical theory. Once quantized, these spin excitations become magnons. The strongly ordered phases are understood as a dilute magnon gas, but interactions become increasingly complex as we approach the magnetic transition temperature in 3D. In order to pursue a theory of magnetism which treats all phases–both ordered and disordered–on equal footing, we turn to Schwinger boson mean field theory (SBMFT). The Schwinger boson transformation fractionalizes the spin operators into two bosonic field operators, and the language initially appears to be less intuitive. Nonetheless, we will show how two-spinon excitations reproduce magnons, and then press on to regimes where both magnon-like and paramagnetic-like excitations proliferate in thermal equilibrium. In Chapter 2, we investigate the dynamical linear response of insulating magnetic systems in equilibrium. In the ordered phases, where time-reversal symmetry is broken, we find that spin currents are present in equilibrium. The thermally-averaged spin-spin correlators, which make up spin currents, may be evaluated from the dissipative part of the dynamic susceptibility tensor using the semiclassical fluctuation-dissipation theorem (FDT). Since spin currents are not directly measurable, we interface the insulating magnet with a metal. The interface acts as weak link, and when the two are out-of-equilibrium with each other, a spin current flows across the interface. By the inverse spin Hall effect, this is picked up as a voltage drop across the metal. In the FDT method, we compute the interfacial spin current up to an overall phenomenological parameter called the (dissipative part of the) spin-mixing conductance. An alternative method, used for Schwinger bosons, is to compute the interfacial spin current directly using Fermi’s golden rule. In Chapter 3, we summarize and analyze our final results for the spin Seebeck coefficients in FMs, AFs, PMs, and discuss a novel SSE which is relevant at temperatures below where electronic spin dynamics freeze out – the nuclear SSE. In Chapter 4, we compare our results to experiments on the AF SSE in chromium oxide (Cr2O3), the paramagnetic SSE in gadolinium gallium garnet (GGG), and the nuclear SSE in manganese carbonate (MnCO3). We discovered remarkable quantitative agreement between our theory and the Cr2O3 data after comparing the SSE across a metamagnetic phase transition. Here, the overall thermal and electronic transport properties such as thermal conductivities and metallic resistivities which affect the measured SSE were eliminated in this comparison, because they are unaffected by the magnetic configuration of the AF. We then applied this technique to the paramagnetic and nuclear SSE analysis to extract additional, specific information from the overall magnetic field profile of the SSE. Finally, we conclude with predictions from SBMFT for future experimental proposals. These investigate the strength of paramagnetic fluctuations in ordered magnetic phases and signatures of magnetic correlations in disordered phases

Spin Seebeck effect near the antiferromagnetic spin-flop transition

We develop a low-temperature, long-wavelength theory for the interfacial spin Seebeck effect (SSE) in easy-axis antiferromagnets. The field-induced spin-flop (SF) transition of N\'eel order is associated with a qualitative change in SSE behavior: Below SF, there are two spin carriers with opposite magnetic moments, with the carriers polarized along the field forming a majority magnon band. Above SF, the low-energy, ferromagnetic-like mode has magnetic moment opposite the field. This results in a sign change of the SSE across SF, which agrees with recent measurements on Cr$_2$O$_3$/Pt and Cr$_2$O$_3$/Ta devices [Li $\textit{et al.,}$ $\textit{Nature}$ $\textbf{578,}$ 70 (2020)]. In our theory, SSE is due to a N\'eel spin current below SF and a magnetic spin current above SF. Using the ratio of the associated N\'eel to magnetic spin-mixing conductances as a single constant fitting parameter, we reproduce the field dependence of the experimental data and partially the temperature dependence of the relative SSE jump across SF

Modeling Charge Preparation And Combustion In Diesel Fuel, Ethanol, And Dual-Fuel PCCI Engines

In this work, multi-dimensional computational fluid dynamics modeling predictions are compared for three different methods of achieving high-efficiency, low NOT, and soot premixed charge compression ignition (PCCI) combustion. The first method is early injection, highly dilute (i.e., low oxygen concentration), diesel fuel PCCI operation. In this method, the oxygen concentration is reduced to extend the ignition delay to allow adequate time for mixing prior to autoignition. The second method is early injection PCCI operation using neat ethanol. In this method, the fuel reactivity is sufficiently low such that PCCI combustion can be achieved without using external dilution. The final method, dual-fuel reactivity controlled compression ignition (RCCI) combustion, blends fuels with different ignition qualities in the combustion chamber to tailor the auto-ignition properties of the mixture for the specific operating condition. In this study, RCCI operation was investigated using in-cylinder fuel blending of diesel fuel and gasoline as well as diesel fuel and an E85 blend (i.e., 85% ethanol and 15% gasoline). It was found that the modeling approach used in this work is capable of capturing the bulk combustion characteristics (e.g., cylinder pressure) as well as the details of the injection event (e.g., liquid penetration) and ignition processes. The simulations were shown to provide accurate predictions of the differences in combustion characteristics of diesel fuel, ethanol, and fuel blends (i.e., gasoline + diesel fuel and E85 + diesel fuel). It was found that the ethanol PCCI and dual-fuel (gasoline + diesel fuel and E85 + diesel fuel) RCCI cases have significantly reduced rates of energy release compared to neat diesel fuel PCCI operation. The reduced energy release rates of the ethanol PCCI and dual-fuel RCCI cases may allow these modes of PCCI combustion to achieve higher engine loads than that of neat diesel PCCI