Decade In Temperature Research Articles

Real-space spectral simulation of quantum spin models: Application to generalized Kitaev models

The proliferation of quantum fluctuations and long-range entanglement presents an outstanding challenge for the numerical simulation of interacting spin systems with exotic ground states. Here, we present a toolset of Chebyshev polynomial-based iterative methods that provides a unified framework to study the thermodynamical properties, critical behavior and dynamics of frustrated quantum spin models with controlled accuracy. Similar to previous applications of the Chebyshev spectral methods to condensed matter systems, the algorithmic complexity scales linearly with the Hilbert space dimension and the Chebyshev truncation order. Using this approach, we study two paradigmatic quantum spin models on the honeycomb lattice: the Kitaev-Heisenberg (K-H) and the Kitaev-Ising (K-I) models. We start by applying the Chebyshev toolset to compute nearest-neighbor spin correlations, specific heat and entropy of the K-H model on a 24-spin cluster. Our results are benchmarked against exact diagonalization and a popular iterative method based on thermal pure quantum states. The transitions between a variety of magnetic phases, namely ferromagnetic, Néel, zigzag and stripy antiferromagnetic and quantum spin liquid phases are obtained accurately and efficiently. We also determine the temperature dependence of the spin correlations, over more than three decades in temperature, by means of a finite temperature Chebyshev polynomial method introduced here. Finally, we report novel dynamical signatures of the quantum phase transitions in the K-I model. Our findings suggest that the efficiency, versatility and low-temperature stability of the Chebyshev framework developed here could pave the way for previously unattainable studies of quantum spin models in two dimensions.

Two Decades of Temperature and Specific Humidity Variance Scaling With the Atmospheric Infrared Sounder

AbstractA two‐decade climatology of height‐resolved horizontal variance scaling exponents (𝛼) for temperature (T) and specific humidity (q) is described using Aqua Atmospheric Infrared Sounder (AIRS) sounding profiles. The AIRS Team Version 6 (V6), Version 7 (V7), and Community Long‐term Infrared Microwave Combined Atmospheric Product System (CLIMCAPS) retrieval algorithms are compared to European Centre for Medium‐Range Weather Forecasts Reanalysis v5 (ERA5) and Modern‐Era Retrospective analysis for Research and Applications, Version 2 (MERRA‐2) reanalyses. Large‐scale T exponents derived between 600 and 1,200 km (𝛼L) show close agreement between V6, V7, and CLIMCAPS algorithms. However, small‐scale q exponents derived between 150 and 400 km (𝛼S) are in poor agreement, including an unrealistically steep 𝛼S in the planetary boundary layer (PBL) in the V7 and CLIMCAPS algorithms that is caused by a combination of algorithm damping and overconstraint by the first guess fields. ERA5 and MERRA‐2 reanalyses have large values of 𝛼L and 𝛼S for both T and q that indicate reduced small‐scale variability in the reanalysis fields. Differences in 𝛼S between free‐running MERRA‐2 AMIP and MERRA‐2 are negligible, implying that suppressed small‐scale variability in reanalyses is imposed by the background model and not caused by the data assimilation process. AIRS has positively skewed T distributions in the tropical‐free troposphere that is consistent with positively buoyant air parcels in convection, and negative skewness in the PBL that is related to the existence of cold pools, behavior that is mostly absent in ERA5 and MERRA‐2. AIRS provides a global view of scale‐dependent variance and skewness that is useful for subgrid parameterization development and validation of weather and climate prediction models.

Increased dominance of heat-tolerant symbionts creates resilient coral reefs in near-term ocean warming

Climate change is radically altering coral reef ecosystems, mainly through increasingly frequent and severe bleaching events. Yet, some reefs have exhibited higher thermal tolerance after bleaching severely the first time. To understand changes in thermal tolerance in the eastern tropical Pacific (ETP), we compiled four decades of temperature, coral cover, coral bleaching, and mortality data, including three mass bleaching events during the 1982 to 1983, 1997 to 1998 and 2015 to 2016 El Niño heatwaves. Higher heat resistance in later bleaching events was detected in the dominant framework-building genus, Pocillopora, while other coral taxa exhibited similar susceptibility across events. Genetic analyses of Pocillopora spp. colonies and their algal symbionts (2014 to 2016) revealed that one of two Pocillopora lineages present in the region (Pocillopora "type 1") increased its association with thermotolerant algal symbionts (Durusdinium glynnii) during the 2015 to 2016 heat stress event. This lineage experienced lower bleaching and mortality compared with Pocillopora "type 3", which did not acquire D. glynnii. Under projected thermal stress, ETP reefs may be able to preserve high coral cover through the 2060s or later, mainly composed of Pocillopora colonies that associate with D. glynnii. However, although the low-diversity, high-cover reefs of the ETP could illustrate a potential functional state for some future reefs, this state may only be temporary unless global greenhouse gas emissions and resultant global warming are curtailed.

Comparing four hard-sphere approximations for the low-temperature WCA melting line.

By combining interface-pinning simulations with numerical integration of the Clausius-Clapeyron equation, we accurately determine the melting-line coexistence pressure and fluid/crystal densities of the Weeks-Chandler-Andersen system, covering four decades of temperature. The data are used for comparing the melting-line predictions of the Boltzmann, Andersen-Weeks-Chandler, Barker-Henderson, and Stillinger hard-sphere approximations. The Andersen-Weeks-Chandler and Barker-Henderson theories give the most accurate predictions, and they both work excellently in the zero-temperature limit for which analytical expressions are derived here.

Logarithmic criticality in transverse thermoelectric conductivity of the ferromagnetic topological semimetal CoMnSb

We report the results of our experimental studies on the magnetic, transport and thermoelectric properties of the ferromagnetic metal CoMnSb. Sizable anomalous Hall conductivity $\sigma_{yx}$ and transverse thermoelectric conductivity $\alpha_{yx}$ are found experimentally and comparable in size to the values estimated from density-functional theory. Our experiment further reveals that CoMnSb exhibits $-T\ln T$ critical behavior in $\alpha_{yx}(T)$, deviating from Fermi liquid behavior $\alpha_{yx}\sim T$ over a decade of temperature between 10 K to 400 K, similar to ferromagnetic Weyl and nodal-line semimetals. Our theoretical calculation for CoMnSb also predicts the $-T\ln T$ behavior when the Fermi energy locates near the Weyl nodes in momentum space.

Emergent moments in a Hund's impurity

Motivated by the relevance of Hund's coupling in the context of multiorbital superconductors, we revisit the problem of a multiorbital Kondo impurity with Hund's interaction. Using dynamical large-N techniques, we propose an efficient approach that retains the essential physics at play, while providing a pathway to scalable quantum impurity solvers. We are able to follow the ground state, dynamic, and thermodynamic properties of this system over many decades of temperature. Our approach captures the emergence of large moments, and follows the stretched evolution of the physics down to their exponentially suppressed Kondo temperature. We focus our analysis on the intermediate finite temperature phase which presents an alternate paramagnetic state due to the emergent moment, and discuss the relevance of this regime to Hund's metals.

Electron-phonon coupling of epigraphene at millikelvin temperatures measured by quantum transport thermometry

We investigate the basic charge and heat transport properties of charge neutral epigraphene at sub-kelvin temperatures, demonstrating a nearly logarithmic dependence of electrical conductivity over more than two decades in temperature. Using graphene's sheet conductance as an in situ thermometer, we present a measurement of electron-phonon heat transport at mK temperatures and show that it obeys the T4 dependence characteristic for a clean two-dimensional conductor. Based on our measurement, we predict the noise-equivalent power of ∼10−22 W/Hz of the epigraphene bolometer at the low end of achievable temperatures.

Caloric and isothermal equations of state of solids: empirical modeling with multiply broken power-law densities

Empirical equations of state (EoSs) are developed for solids, applicable over extended temperature and pressure ranges. The EoSs are modeled as multiply broken power laws, in closed form without the use of ascending series expansions; their general analytic structure is explained and specific examples are studied. The caloric EoS is put to test with two carbon allotropes, diamond and graphite, as well as vitreous silica. To this end, least-squares fits of broken power-law densities are performed to heat capacity data covering several logarithmic decades in temperature, the high- and low-temperature regimes and especially the intermediate temperature range where the Debye theory is of limited accuracy. The analytic fits of the heat capacities are then temperature integrated to obtain the entropy and caloric EoS, i.e. the internal energy. Multiply broken power laws are also employed to model the isothermal EoSs of metals (Al, Cu, Mo, Ta, Au, W, Pt) at ambient temperature, over a pressure range up to several hundred GPa. In the case of copper, the empirical pressure range is extended into the TPa interval with data points from DFT calculations. For each metal, the parameters defining the isothermal EoS (i.e. the density–pressure relation) are inferred by nonlinear regression. The analytic pressure dependence of the compression modulus of each metal is obtained as well, over the full data range.

Complex transport and magnetism in inhomogeneous mixed valence Ce3Ir4Ge13

We report the discovery of Ce$_3$Ir$_4$Ge$_{13}$, a new Remeika phase compound with a complex array of structural, electronic, and magnetic properties. Our single crystal x-ray diffraction measurements show that Ce$_3$Ir$_4$Ge$_{13}$ forms in the tetragonally distorted $I4_1/amd$ space group. The electrical resistivity is almost temperature independent over three decades in temperature, from 0.4 K to 400 K, while the Hall coefficient measurements are consistent with a low-carrier semimetal. Magnetic susceptibility measurements reveal an effective moment of $\mu^{\text{exp}}_{\text{eff}} = 1.87 \mu_B$/Ce, suggesting that this material has a mixture of magnetic Ce$^{3+}$ and non-magnetic Ce$^{4+}$. Upon cooling, Ce$_3$Ir$_4$Ge$_{13}$ first enters a short range magnetically ordered state below $T_{\text{SRO}}=10$ K, marked by a deviation from Curie-Weiss behavior in susceptibility and a broad field-independent heat capacity anomaly. At lower temperatures, we observe a second, sharper peak in the heat capacity at $T^* = 1.7$ K, concurrent with a splitting of the field-cooled and zero-field-cooled susceptibilities. A small resistivity drop at $T^*$ suggests a loss of spin disorder scattering consistent with a magnetic ordering or spin freezing transition. Ce$_3$Ir$_4$Ge$_{13}$ is therefore a rare example of an inhomogeneous mixed valence compound with a complex array of thermodynamic and transport properties.

Magnetoelastic coupling in URu2Si2 : Probing multipolar correlations in the hidden order state

Time-reversal symmetry and magnetoelastic correlations are probed by means of high-resolution volume dilatometry in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ at cryogenic temperatures, and magnetic fields sufficient to suppress the hidden order state at ${H}_{\mathrm{HO}}(T=0.66\phantom{\rule{0.28em}{0ex}}\mathrm{K})\ensuremath{\simeq}35$ T. We report a significant magnetoelastic volume expansion at and above ${H}_{\mathrm{HO}}(T)$, and even above ${T}_{\mathrm{HO}}$, possibly a consequence of field-induced $f$-electron localization. We investigate in detail the magnetostriction and magnetization as the temperature is reduced across two decades in temperature from 30 K where the system is paramagnetic, to 0.5 K in the realm of the hidden order state. We find a dominant quadratic-in-field dependence $\mathrm{\ensuremath{\Delta}}L/L\ensuremath{\propto}{H}^{2}$, a result consistent with a state that is symmetric under time reversal. The data shows, however, an incipient yet unmistakable asymptotic approach to linear ($\mathrm{\ensuremath{\Delta}}L/L\ensuremath{\propto}1\ensuremath{-}H/{H}_{0}$) for $15\phantom{\rule{4.pt}{0ex}}\text{T}<H<{H}_{\mathrm{HO}}(0.66\phantom{\rule{4.pt}{0ex}}\text{K})\ensuremath{\sim}40$ T at the lowest temperatures. We discuss these results in the framework of a Ginzburg-Landau formalism that proposes a complex order parameter for the HO phase to model the $(H,T,p)$ phase diagram.

Electrical resistivity across a nematic quantum critical point.

Correlated electron systems are highly susceptible to various forms of electronic order. By tuning the transition temperature towards absolute zero, striking deviations from conventional metallic (Fermi-liquid) behaviour can be realized. Evidence for electronic nematicity, a correlated electronic state with broken rotational symmetry, has been reported in a host of metallic systems1-5 that exhibit this so-called quantum critical behaviour. In all cases, however, the nematicity is found to be intertwined with other forms of order, such as antiferromagnetism5-7 or charge-density-wave order8, that might themselves be responsible for the observed behaviour. The iron chalcogenide FeSe1-xSx is unique in this respect because its nematic order appears to exist in isolation9-11, although until now, the impact of nematicity on the electronic ground state has been obscured by superconductivity. Here we use high magnetic fields to destroy the superconducting state in FeSe1-xSx and follow the evolution of the electrical resistivity across the nematic quantum critical point. Classic signatures of quantum criticality are revealed: an enhancement in the coefficient of the T2 resistivity (due to electron-electron scattering) on approaching the critical point and, at the critical point itself, a strictly T-linear resistivity that extends over a decade in temperature T. In addition to revealing the phenomenon of nematic quantum criticality, the observation of T-linear resistivity at a nematic critical point also raises the question of whether strong nematic fluctuations play a part in the transport properties of other 'strange metals', in which T-linear resistivity is observed over an extended regime in their respective phase diagrams.

Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal

In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE). Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co$_2$MnGa, reaching $S_{yx}\sim -6$ $\mu$V/K at room $T$, one order of magnitude larger than the maximum value reported for a magnetic conductor. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient $\alpha_{yx}$ shows a crossover between $T$-linear and $-T \log(T)$ behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions is responsible for the observed crossover properties and an enhanced $\alpha_{yx}$. The $T$ dependence of $\alpha_{yx}$ in experiments and numerical calculations can be understood in terms of a quantum critical scaling function predicted by the low energy effective theory over more than a decade of temperatures. Moreover, the observation of chiral anomaly or an unsaturated positive longitudinal magnetoconductance also provide evidence for the existence of Weyl fermions in Co$_2$MnGa.

Low temperature thermodynamic investigation of the phase diagram of Sr3Ru2O7

We studied the phase diagram of Sr$_3$Ru$_2$O$_7$ by means of heat capacity and magnetocaloric effect measurements at temperatures as low as 0.06 K and fields up to 12 T. We confirm the presence of a new quantum critical point at 7.5 T which is characterized by a strong non-Fermi-liquid behavior of the electronic specific heat coefficient $\Delta$C/T $\sim$ -logT over more than a decade in temperature,placing strong constraints on theories of its criticality. In particular logarithmic corrections are found when the dimension d is equal to the dynamic critical exponent z, in contrast to the conclusion proposed recently [Y. Tokiwa et al., Phys. Rev. Lett. 116, 226402 (2016)]. Moreover, we achieved a clear determination of the new second thermodynamic phase adjoining the first one at lower temperatures. Its thermodynamic features differ significantly from those of the dominant phase and characteristics expected of classical equilibrium phase transitions are not observed, indicating fundamental differences in the phase formation.

Persistent low-temperature spin dynamics in the mixed-valence iridate Ba3InIr2O9

Using thermodynamic measurements, neutron diffraction, nuclear magnetic resonance, and muon spin relaxation, we establish putative quantum spin-liquid behavior in Ba$_3$InIr$_2$O$_9$, where unpaired electrons are localized on mixed-valence Ir$_2$O$_9$ dimers with Ir$^{4.5+}$ ions. Despite the antiferromagnetic Curie-Weiss temperature on the order of 10 K, neither long-range magnetic order nor spin freezing are observed down to at least 20 mK, such that spins are short-range correlated and dynamic over nearly three decades in temperature. Quadratic power-law behavior of both the spin-lattice relaxation rate and specific heat indicates the gapless nature of the ground state. We envisage that this exotic behavior may be related to an unprecedented combination of the triangular and buckled honeycomb geometries of nearest-neighbor exchange couplings in the mixed-valence setting.

A Synoptic View of the Ventilation and Circulation of Antarctic Bottom Water from Chlorofluorocarbons and Natural Tracers.

Antarctic Bottom Water (AABW) is the coldest, densest, most prolific water mass in the global ocean. AABW forms at several distinct regions along the Antarctic coast and feeds into the bottom limb of the meridional overturning circulation, filling most of the global deep ocean. AABW has warmed, freshened, and declined in volume around the globe in recent decades, which has implications for the global heat and sea level rise budgets. Over the past three decades, the use of tracers, especially time-varying tracers such as chlorofluorocarbons, has been essential to our understanding of the formation, circulation, and variability of AABW. Here, we review three decades of temperature, salinity, and tracer data and analysis that have led to our current knowledge of AABW and how the southern component of deep-ocean ventilation is changing with time.

Heavy fermion behavior in the quasi-one-dimensional Kondo lattice CeCo2Ga8

Dimensionality plays an essential role in determining the anomalous non-Fermi liquid properties in heavy fermion systems. So far most heavy fermion compounds are quasi-two-dimensional or three-dimensional. Here we report the synthesis and systematic investigations of the single crystals of the quasi-one-dimensional Kondo lattice CeCo2Ga8. Resistivity measurements at ambient pressure reveal the onset of coherence at T * ≈ 20 K and non-Fermi liquid behavior with linear temperature dependence over a decade in temperature from 2 to 0.1 K. The specific heat increases logarithmically with lowering temperature between 10 and 2 K and reaches 800 mJ/mol K2 at 1 K, suggesting that CeCo2Ga8 is a heavy fermion compound in the close vicinity of a quantum critical point. Resistivity measurements under pressure further confirm the non-Fermi liquid behavior in a large temperature–pressure range. The magnetic susceptibility is found to follow the typical behavior for a one-dimensional spin chain from 300 K down to T *, and first-principles calculations predict flat Fermi surfaces for the itinerant f-electron bands. These suggest that CeCo2Ga8 is a rare example of the quasi-one-dimensional Kondo lattice, but its non-Fermi liquid behaviors resemble those of the quasi-two-dimensional YbRh2Si2 family. The study of the quasi-one-dimensional CeCo2Ga8 family may therefore help us to understand the role of dimensionality on heavy fermion physics and quantum criticality.

Two-Channel Kondo Physics due to As Vacancies in the Layered Compound ZrAs_{1.58}Se_{0.39}.

We address the origin of the magnetic-field-independent -|A|T^{1/2} term observed in the low-temperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ZrAs_{1.58}Se_{0.39}, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the nonmagnetic version of the two-channel Kondo effect, whose origin we ascribe to a dynamic Jahn-Teller effect operating at the vacancy-carrying As layer with a C_{4} symmetry. The pair-breaking nature of the dynamical defects in the square nets of As explains the low superconducting transition temperature T_{c}≈0.14 K of ZrAs_{1.58}Se_{0.39} compared to the free-of-vacancies homologue ZrP_{1.54}S_{0.46} (T_{c}≈3.7 K). Our findings should be relevant to a wide class of metals with disordered pnictogen layers.

Anomalous specific heat, magnetic, and electrical transport properties around magnetic instability in Ce3−xLaxCo4Sn13

Ce3Co4Sn13 exhibits interesting physical properties including a large increase in the magnetic specific heat, ΔC(T)/T, at low-temperatures with a maximum value of about 4JK−2molCe−1 and a cross-over from heavy fermion behavior with antiferromagnetic correlations at low magnetic field to single impurity Kondo behavior by applying the field larger than ∼3 T. In order to study the expected quantum criticality of Ce3Co4Sn13 we investigated electrical resistivity, magnetoresistivity, magnetic susceptibility and specific heat of Ce3−xLaxCo4Sn13 and showed that partial substitution of Ce by La does not lead to long range magnetic order, and the low-temperature properties are not typical of the quantum critical point (QCP). The magnetoresistivity of Ce3−xLaxCo4Sn13 has a conventional Kondo character. With increasing of the magnetic field resistivity follows power-law behavior over one decade in temperature for the samples with x ≠ 0 of the series, and exponent n in ρ ∼ Tn is strongly field dependent. The low-temperature specific heat of Ce3−xLaxCo4Sn13 exhibits evolution from a magnetically correlated heavy fermion state to a single impurity state at the critical concentration x = xC = 0.6, however, for xC the C(T)/T behavior is not typical for QCP. The low-temperature resistivity and magnetic susceptibility data give evidence for changeover from Kondo lattice to a single-ion Kondo behaviour at xC, when the La content in Ce3−xLaxCo4Sn13 increases.

Climate change and adaptation in the coastal areas of Europe's Northern Periphery Region

Adaptation to climate change in coastal regions of northern and western Norway, Scotland and Ireland and the coasts of Iceland must respond to trends and variability in a number of physical parameters that affect the regional life and economy. Historical trends and variability are apparent in historical data over several decades in temperature, precipitation and winds. Associated changes in waves, sea level and sea surges are also implied. Predictions of future climate change imply further trends in the same parameters. Those physical parameters act according to the local characteristics of various locations producing predictable impacts (both positive and negative) to which local communities will need to adapt.

Changes in ENACW observed in the Bay of Biscay over the period 1975–2010

Trends in Eastern North Atlantic Central Water (ENACW) were calculated in the Bay of Biscay over the period 1975–2010 using the Simple Ocean Data Assimilation (SODA) package. This approach, which reanalyzes data from different sources, allows obtaining information beneath the sea surface on a fine 0.5°×0.5° grid with 40 vertical layers, providing a complete view of the different hydrographic processes in the area. ENACW, which was associated to salinity and temperature values corresponding to the density interval 27.0–27.2kgm−3, was observed to warm and salinificate in most of the area at a maximum rate of 0.11°C per decade and 0.03psu per decade, respectively. Trends are more intense in the middle part of the Bay and near the northeastern boundary. The origin of the changes in ENACW observed inside the Bay of Biscay is not due to local effects. Actually, trends seem to be related to changes in the subpolar mode of ENACW (ENACWsp), affecting a wider North Atlantic area. The highest trends (0.3°C per decade in temperature and 0.05psu per decade in salinity) were observed north of the bay, especially at the shallow area that stretches from Brest to Ireland, which is characterized by deep winter mixing. The dependence of changes in ENACW on the two main modes of variability over the North Atlantic (NAO and EA) was also analyzed. Trends in salinity and temperature showed to be consistent with changes observed in EA. In addition, air temperature and precipitation minus evaporation (P−E) balance showed to contribute to warming and salinification of ENACW.