Weak Electron-phonon Interaction Research Articles

Possible Superconductivity for Layered Metal Boride Carbide Compounds MB2C2 (M = Alkali, Alkaline-Earth, or Rare-Earth Metals).

The possible emergence of superconductivity in layered metal boride carbide compounds MB2C2 (M = Sc, Y, Be, Ca) was investigated using density functional theory calculations upon the topology of a boron-carbon network and the nature of the metal. ScB2C2 and YB2C2 show metallic and superconductive properties with low critical temperatures (Tcs). The semiconducting BeB2C2 compound may show superconductivity upon carrier doping with a high Tc of 47.8 K by hole doping─comparable to the structurally related MgB2 superconductor─but with a low Tc by electron doping. In contrast, the semiconducting CaB2C2 compound is predicted to be a superconductor by hole and electron doping but with low Tcs. These differences arise from the spatial distribution of electrons at the Fermi level. For compounds with low Tcs, electrons at the Fermi level are localized primarily on B and C π states perpendicular to the BC layers, experiencing minimal influence from atomic oscillations and resulting in weak electron-phonon interactions. Conversely, for a high Tc, electrons are found in σ-bonding states, leading to strong electron-phonon interactions. Electrons at the Fermi level in boron-carbon σ-bonding states seem to be a prerequisite to expect high Tc superconductivity in this kind of compound.

Reprecipitation synthesis and spectroscopic characterization of Cs1−xFAxPbBr3 nanocrystals for backlight display devices

In this study, a new technique called the dual-solvent-assisted reprecipitation (DSAR) was developed for synthesizing high-quality Cs1−xFAxPbBr3 PQDs. This technique eliminated the need for high temperatures, an inert atmosphere, and the hazardous solvent (dimethylformamide). Moreover, XPS analysis of PQDs synthesized using LARP revealed the presence of residual DMF, leading to formation of defects in the PQDs. The Cs0.2FA0.8PbBr3 PQDs synthesized via the DSAR method exhibited a high PLQY of 99% and a bandwidth of 20 nm. The low-temperature photoluminescence results revealed that the PQDs prepared via the LARP method exhibited high nonradiative recombination, whereas PQDs prepared via the DSAR method exhibited weak electron–phonon interaction and high efficiency. The synthesized PQDs were encapsulated to fabricate PQD films. The obtained Cs0.2FA0.8PbBr3 PQD films exhibited high environmental stability, maintaining an initial intensity of 98% for 90 days under ambient conditions. In addition, the PQD films maintained an intensity of 75% for 300 h under conditions of 60 °C and 90% relative humidity. Moreover, the white-light-emitting device combining the synthesized PQD films with commercial K2SiF6:Mn4+ phosphors and blue LED chips achieved rec. 2020 coverage of 96.7%. Overall, the results indicate that the Cs0.2FA0.8PbBr3 PQDs exhibit excellent optical properties and environmental stability for display applications.

Growth, magnetic, transport and electronic properties of Co2TiSi Heusler alloy thin films

We have grown thin films of Co2TiSi (CTS) Heusler alloy on n-type Si (100) at different substrate temperatures by electron beam evaporator under a high vacuum. The structural, magnetic, and electric characterizations were carried out and found a strong correlation between the properties and the growth temperatures of the samples. The crystallinity and magnetic moment both were significantly improved with increasing growth temperatures from room temperature to 400 °C. The saturation magnetization of 1.85 µB at 10 K is close to the theoretical Slater–Pauling value. A large value of spin wave stiffness constant ( D = 7.2 meV- nm2) of CTS film manifests the robustness of magnetization. The temperature-dependent longitudinal resistivity demonstrated different scattering phenomenon like weak localization, electron-magnon, electron-election, and electron-phonon interactions. A decrease in residual resistivity (109–49 µΩ-cm) and an increase in residual resistivity ratio (1.2–1.39) indicate an enhancement of crystallinity and structural order of CTS films. By first principle study using DFT calculation we have analyzed the effect of atomic site disorder on electronic properties of both bulk and thin film (220) surface form of CTS Heusler alloy. It has been noticed that the ordered bulk structure shows perfect half-metallic nature but for thin film the spin polarization and moment both are deviated from identical value. The spin polarization and magnetic moment both are reduced a lot by introducing atomic site disorder in L21 ordered structure.

Cumulant expansion in the Holstein model: Spectral functions and mobility

We examine the range of validity of the second-order cumulant expansion (CE) for the calculation of spectral functions, quasiparticle properties, and mobility of the Holstein polaron. We devise an efficient numerical implementation that allows us to make comparisons in a broad interval of temperature, electron-phonon coupling, and phonon frequency. For a benchmark, we use the dynamical mean-field theory which gives, as we have recently shown, rather accurate spectral functions in the whole parameter space, even in low dimensions. We find that in one dimension, the CE resolves well both the quasiparticle and the first satellite peak in a regime of intermediate coupling. At high temperatures, the charge mobility assumes a power law $\ensuremath{\mu}\ensuremath{\propto}{T}^{\ensuremath{-}2}$ in the limit of weak coupling and $\ensuremath{\mu}\ensuremath{\propto}{T}^{\ensuremath{-}3/2}$ for stronger coupling. We find that, for stronger coupling, the CE gives slightly better results than the self-consistent Migdal approximation (SCMA), while the one-shot Migdal approximation is appropriate only for a very weak electron-phonon interaction. We also analyze the atomic limit and the spectral sum rules. We derive an analytical expression for the moments in CE and find that they are exact up to the fourth order, as opposed to the SCMA where they are exact to the third order. Finally, we analyze the results in higher dimensions.

Aromatic superconductors: Electron-phonon coupling and electronic correlations

<sec>Aromatic superconductors are a new type of high-temperature superconductor discovered in recent years. The superconducting transition temperature (<i>T</i><sub>c</sub>) increases with the size of aromatic molecule increasing, which has attracted widespread attention of experimental and theoretical researchers. The driving mechanism for such a superconductivity, whether it is dominated by electron-phonon coupling or electronic correlation effects, has aroused great interest of many research groups. This paper briefly introduces the rich superconducting phenomena of metal doped aromatic compounds. From the perspectives of electron-phonon coupling or electronic correlations, the superconductivity of aromatic compounds is discussed, which is helpful in exploring aromatic superconductors with higher <i>T</i><sub>c</sub>. The challenges currently faced in the field are also introduced.</sec><sec>The rest of this paper is organized as follows. We first introduce the existence of abundant superconducting phases in the experiment of metal doped aromatic compounds. Different doping concentrations of metal cause superconducting phases with different <i>T</i><sub>c</sub> values, especially the highest <i>T</i><sub>c</sub> value of the superconducting phase increases with the size of aromatic molecule increasing. Theoretical prediction shows that all aromatic hydrocarbon superconductors have a low-<i>T</i><sub>c</sub> superconducting phase in a range of 5–7 K, which is a common feature. For systems with few benzene rings (such as benzene, naphthalene, and phenanthrene crystals), only low-<i>T</i><sub>c</sub> phase of 5–7 K exists, while in systems with multiple benzene rings (such as picene, dibenzopentacene, and others with the number of benzene rings more than 5), there are multiple superconducting phases; the highest <i>T</i><sub>c</sub> in long-benzene-ring system depends not only on the number of benzene rings, but also on the chain size of organic molecule. Further research indicates that low-<i>T</i><sub>c</sub> phase is induced by doping about 2 electrons and has good stability, while high-<i>T</i><sub>c</sub> phase results from doping 3 electrons and has slightly poorer stability.</sec><sec>Then, the electron-phonon coupling characteristics and electron-electron exchange correlation effects in aromatic compound superconductors are discussed. For low-<i>T</i><sub>c</sub> phases, the values of electronic density of states at the Fermi level are comparable to each other and relatively low, resulting in weak electron-phonon interactions. However, the <i>T</i><sub>c</sub> value predicted by this electron-phonon coupling mechanism is in good agreement with experimental value, indicating that the electron-phonon coupling is sufficient to describe the superconductivity of low-<i>T</i><sub>c</sub> phases. For high-<i>T</i><sub>c</sub> phases, the big values of electron density of states at the Fermi level imply strong electron-phonon interactions, and this electron-phonon coupling increases with the size of organic molecule increasing. However, the <i>T</i><sub>c</sub> value predicted only by the electron-phonon mechanism is lower than the experimental value. The study of electron-electron exchange correlation effect of aromatic compounds shows that the electronic correlation effect increases with the size of aromatic molecule increasing, which is consistent with the increase of <i>T</i><sub>c</sub> maximum value with the size of aromatic molecule increasing in a long-benzene-ring system. This indicates that the superconductivity of high<i>-T</i><sub>c</sub> phase is driven by both the electron-phonon mechanism and the electronic correlation effect. This understanding of superconductivity is significant for exploring and discovering aromatic superconductors with higher transition temperatures.</sec><sec>Finally, comprehensive physical models and methods are required in this paper in order to gain a thorough understanding of the superconductivity of aromatic compound.</sec>

A terahertz detector based on superconductor-graphene-superconductor Josephson junction

Graphene is an attractive material for terahertz (THz) detectors due to its unique properties of extremely weak electron-phonon interaction and small electronic heat capacity. In this paper, we demonstrate a lens-antenna-coupled THz detector based on a diffusive superconductor-graphene-superconductor (SGS) junction, which has a sharp resistive transition at 1.4 K due to the proximity Josephson coupling. The SGS Josephson detector, measured with a dc superconducting quantum interference device (SQUID) readout, exhibits an electrical noise equivalent power (NEP) as low as 5 × 10−17 W/Hz0.5 at 0.1 K, which is still limited by the readout noise. Furthermore, the optical performance of the SGS Josephson detector is measured with a cryogenic blackbody radiation source, giving an optical NEP of 2.5–5 × 10−16 W/Hz0.5 at 1.4 THz in the temperature range from 0.1 K to 0.6 K.

Anomalously Isotropic Electron Transport and Weak Electron-Phonon Interactions in Hexagonal Noble Metals.

The electrical transport properties of typical hexagonal metals are anisotropic because of their anisotropic lattice structures. Unexpectedly, we show that the electron transport properties in hexagonal close-packed (hcp) noble metals are almost isotropic. Although the electron transport properties of an individual electronic band are highly anisotropic, the total contributions are almost equal in different crystalline orientations because of the complementary contributions of different bands. The electron transport is severely limited by phonons for metals with multisheet Fermi surfaces and optical phonon polarizations. However, it is found the electron-phonon interactions are weak in hcp noble metals, although their Fermi surfaces and phonon spectra are complicated. The electronic structure acts as a phonon filter, resulting in small electron-phonon scattering rates. The weak electron-phonon interactions are beneficial to electron and thermal transport, indicating hcp noble metals have great potential to be used in electronics and solar cells.

Intermediate polaronic charge transport in organic crystals from a many-body first-principles approach

Charge transport in organic molecular crystals (OMCs) is conventionally categorized into two limiting regimes − band transport, characterized by weak electron-phonon (e-ph) interactions, and charge hopping due to localized polarons formed by strong e-ph interactions. However, between these two limiting cases there is a less well understood intermediate regime where polarons are present but transport does not occur via hopping. Here we show a many-body first-principles approach that can accurately predict the carrier mobility in this intermediate regime and shed light on its microscopic origin. Our approach combines a finite-temperature cumulant method to describe strong e-ph interactions with Green-Kubo transport calculations. We apply this parameter-free framework to naphthalene crystal, demonstrating electron mobility predictions within a factor of 1.5−2 of experiment between 100 and 300 K. Our analysis reveals the formation of a broad polaron satellite peak in the electron spectral function and the failure of the Boltzmann equation in the intermediate regime.

Macroscopic assembled graphene nanofilms based room temperature ultrafast mid‐infrared photodetectors

AbstractGraphene with linear energy dispersion and weak electron–phonon interaction is highly anticipated to harvest hot electrons in a broad wavelength range. However, the limited absorption and serious backscattering of hot‐electrons result in inadequate quantum yields, especially in the mid‐infrared range. Here, we report a macroscopic assembled graphene (nMAG) nanofilm/silicon heterojunction for ultrafast mid‐infrared photodetection. The assembled Schottky diode works in 1.5–4.0 μm at room temperature with fast response (20–30 ns, rising time, 4 mm2 window) and high detectivity (1.6 × 1011 to 1.9 × 109 Jones from 1.5 to 4.0 μm) under the pulsed laser, outperforming single‐layer‐graphene/silicon photodetectors by 2–8 orders. These performances are attributed to the greatly enhanced photo‐thermionic effect of electrons in nMAG due to its high light absorption (~40%), long carrier relaxation time (~20 ps), low work function (4.52 eV), and suppressed carrier number fluctuation. The nMAG provides a long‐range platform to understand the hot‐carrier dynamics in bulk 2D materials, leading to broadband and ultrafast MIR active imaging devices at room temperature.image

Emission Spectroscopy Investigation of the Enhancement of Carrier Collection Efficiency in AgBiS2-Nanocrystal/ZnO-Nanowire Heterojunction Solar Cells.

Eco-friendly solar cells were fabricated using interdigitated layers comprising ZnO nanowires (NWs) and infrared absorbing AgBiS2 nanocrystals (ITO/ZnO NWs/AgBiS2/P3HT/Au). The quality of ZnO NWs was studied using photoluminescence and Raman spectroscopy to identify the defects in ZnO NWs influencing solar cell performance. Oxygen vacancies and Zn interstitial sites, among various recombination sites, were observed to be the main sites for carrier recombination, which hinders the carrier collection in the solar cells. Accordingly, the power conversion efficiency of AgBiS2 solar cells exhibited a good correlation with the number of oxygen vacancies. The structural order and electron-phonon interaction in ZnO NWs were also investigated via Raman scattering spectroscopy. A lower concentration of oxygen vacancies and zinc interstitials (Zni) resulted in a higher structural order as well as a weaker electron-phonon interaction in ZnO NWs. When ZnO NWs were treated at 500 °C in oxygen with the lowest oxygen vacancy concentration, the solar cells (500-O2 solar cell (SC)) demonstrated an external quantum efficiency of approximately 70% in the visible region and a corresponding internal quantum efficiency of more than 80%. The 500-O2 SC exhibited a power conversion efficiency of 5.41% (JSC = 22.21 mA/cm2, VOC = 0.41 V, and FF = 60%) under quasi one-sun illumination. New methods that can efficiently reduce oxygen vacancies and Zni without affecting the structural order of ZnO NWs would further enhance the carrier collection efficiency. Moreover, since ZnO is a key electron transport material for constructing not only colloidal quantum dot solar cells but also other emerging solar cells, such as organic thin-film solar cells, the present findings provide significant information for improving their performance.

Ultralong lifetime for fully photogenerated spin-polarized current in two-dimensional ferromagnetic/nonmagnetic semiconductor heterostructures

Photogenerated fully spin-polarized current has attracted considerable attention in spintronics owing to its high efficiency for information processing but low energy consumption. However, the short spin carrier lifetime of the primary materials significantly impedes the practical application of spin-polarized current and presents a major challenge. Here, on the basis of density functional theory and nonadiabatic molecular dynamics simulations, we design two prototypes of two-dimensional (2D) ferromagnetic/nonmagnetic semiconductor heterostructures ($\mathrm{Cr}{\mathrm{I}}_{3}/\mathrm{Mo}{\mathrm{Se}}_{2}$ and $\mathrm{Cr}{\mathrm{Br}}_{3}/\mathrm{Mo}{\mathrm{S}}_{2}$) with demonstrated ultralong photogenerated spin carrier lifetime. It is shown that the photogenerated spin holes can be effectively injected from the ferromagnetic layer to the nonmagnetic layer, and fully spin-polarized electrons will be confined in the ferromagnetic layer due to their type-II spin-polarized channel and large spin-flip gap. More surprisingly, the photogenerated spin carrier lifetime of these heterostructures is up to the time scale of nanoseconds. Such an ultralong spin carrier lifetime is mainly originated from small nonadiabatic coupling induced by weak spin electron-phonon interaction and slow nuclear velocity. Our results provide insights into the design of 2D semiconductor spintronic devices with an ultralong fully spin-polarized current lifetime.

Weak localization and electron-phonon interaction in layered Zintl phase SrIn2P2 single crystal

Recently, the Zintl phase SrIn2P2 single crystal was proposed to be a topological insulator candidate under lattice strain. Here, we report systematic electrical transport studies on the unstrained layered SrIn2P2 single crystals. The resistance presents a minimum value around T c = 136 K and then increases remarkably at low temperature. Distinct negative magnetoresistance below T c, combined with the anomalous resistance, implies the carriers are weak localized at low temperature due to strong quantum coherence. Further analysis based on three-dimensional weak localization (WL) model suggests that the electron-phonon interaction dominates the phase decoherence process. Moreover, Hall measurements indicate that the transport properties are mainly dominated by hole-type carriers, and the WL effect is obviously affected by the carrier transport. These findings not only provide us a promising platform for the fundamental physical research but also open up a new route for exploring the potential electronic applications.

Demonstration of a high-sensitivity and wide-dynamic-range terahertz graphene hot-electron bolometer with Johnson noise thermometry

Graphene hot-electron bolometer (HEB) detectors, with weak electron–phonon interaction and a wide working temperature range, are of particular interest for terahertz (THz) applications including astronomy. In this paper, we report on the performance of a zero-biased THz HEB detector using Johnson noise thermometry based on bilayer graphene (BLG) of high charge carrier density. Two BLG HEB devices with respective normal-metal and superconducting electrodes are compared particularly for their thermal conductance and detection sensitivity (noise equivalent power, NEP) in a low-temperature regime (0.3–10 K). With electron out-diffusion largely suppressed by Andreev reflection, the device with superconducting electrodes outperforms the device with normal-metal electrodes, giving an electrical NEP of 15 fW/Hz0.5 and a dynamic range of 47 dB at 0.3 K. Moreover, its optical coupling efficiency is found to be 58% and can reach as high as 92% with the optical losses of the measurement system calibrated out. Graphene detectors of this kind can achieve better detection performance given lower readout noise and decreased electron–phonon thermal conductance.

A novel Mn4+-activated layered oxide-fluoride perovskite-type KNaMoO2F4 red phosphor for wide gamut warm white light-emitting diode backlights.

Mn4+-activated oxide-fluoride phosphors are attractive for application in a wide range of solid-state lighting devices because of their distinct red emission at about 630 nm and the abundant storage of Mn ions. However, the zero-phonon line (ZPL) of Mn4+ ions is too weak to be detected in most host materials due to the magnetic dipole nature. In this article, we introduce a co-precipitation method for synthesizing a Mn4+-doped oxyfluoride perovskite KNaMoO2F4 phosphor containing [MoO2F4]2- building units. The electron paramagnetic resonance (EPR) spectra are consistent with the presence of a MnF62- species at g = 1.991. The KNaMoO2F4:0.01Mn4+ phosphor exhibits strong absorption under blue light and an internal quantum yield (IQE) of 65.8%. Attributed to the distorted octahedral environment of the Mn4+ ions, visible ZPL emission was detected at 625 nm. Based on theoretical calculations, the Mn4+ ions in the KNaMoO2F4 host exist in a strong crystal field with a high Dq/B value of ∼3.86. A series of photoluminescence-dependent low-temperature spectra indicates that the Mn4+ emissive state experiences weak electron-phonon interactions upon calculating the Huang-Rhys factor. Benefiting from the ZPL, a warm white-light-emitting diode is achieved using YAG:Ce3+ and KNaMoO2F4:Mn4+ as color converters, in which the color rendering index was Ra = 83.5 and CCT = 4490 K.

Profiling novel high-conductivity 2D semiconductors

When complex mechanisms are involved, pinpointing high-performance materials within large databases is a major challenge in materials discovery. We focus here on phonon-limited conductivities, and study 2D semiconductors doped by field effects. Using state-of-the-art density-functional perturbation theory and Boltzmann transport equation, we discuss 11 monolayers with outstanding transport properties. These materials are selected from a computational database of exfoliable materials providing monolayers that are dynamically stable and that do not have more than six atoms per unit cell. We first analyze electron-phonon scattering in two well-known systems: electron-doped InSe and hole-doped phosphorene. Both are single-valley systems with weak electron-phonon interactions, but they represent two distinct pathways to fast transport: a steep and deep isotropic valley for the former and strongly anisotropic electron-phonon physics for the latter. We identify similar features in the database and compute the conductivities of the relevant monolayers. This process yields several high-conductivity materials, some of them only very recently emerging in the literature (GaSe, Bi2SeTe2, Bi2Se3, Sb2SeTe2), others never discussed in this context (AlLiTe2, BiClTe, ClGaTe, AuI). Comparing these 11 monolayers in detail, we discuss how the strength and angular dependency of the electron-phonon scattering drives key differences in the transport performance of materials despite similar valley structure. We also discuss the high conductivity of hole-doped WSe2, and how this case study shows the limitations of a selection process that would be based on band properties alone.

First-principles calculations of physical properties and superconductivity of orthorhombic ScRuSi and ZrRhSi

We have performed ab initio pseudopotential calculations of the structural, electronic, elastic, mechanical, vibrational, electron-phonon interaction, and superconducting properties of ScRuSi and ZrRhSi. An assessment of their elastic and mechanical properties reveals that ScRuSi is a ductile compound, while ZrRhSi is a brittle compound. For both compounds electronic energy bands in the vicinity of the Fermi level originate from the $d$ states of transition-metal atoms. The Eliashberg spectral function reveals that low-frequency phonon modes due to coupled motions of transition-metal atoms are more involved in the process of scattering of electrons than the high-frequency modes due to the vibrations of Si atoms. The average electron-phonon coupling parameter is calculated to be 0.54 for ScRuSi and 0.84 for ZrRhSi from the integration of the Eliashberg spectral function. It is found that the relatively weaker electron-phonon interaction in ScRuSi makes its superconducting transition temperature value $({T}_{c}=3.23 \mathrm{K})$ considerably lower than that of ZrRhSi $({T}_{c}=10.45 \mathrm{K})$.

High figure-of-merit in the heavy-fermion UN2 system for radioisotope thermoelectric applications

The design of uranium-based thermoelectric (TE) materials presents a novel and intriguing strategy for directly converting nuclear heat into electrical power. Using high-level first-principles approach combined with accurate solution of Boltzmann transport equation, we demonstrate that a giant n-type power factor of 13.8 mW m−1 K−2 and a peak ZT value of 2.2 can be realized in the heavy-fermion UN2 compound at 700 K. Such promising TE performance arises from the large degeneracy (N v = 14) of heavy conduction band coupled with weak electron–phonon interactions, which is in principle governed by the strong Coulomb correlation among the partially filled U-5f electrons in the face-centered cubic structure. Collectively, our theoretical work suggests that the energetic UN2 could serve as both excellent heat source and efficient power convertor, which also uncovers an underexplored area for TE research.

Giant Enhancement of Fluorescence Emission by Fluorination of Porous Graphene with High Defect Density and Subsequent Application as Fe3+ Ion Sensors.

Defect-mediated nonradiative recombination in traditional semiconductors, such as porous graphene, tremendously lowers the fluorescence emission, thus greatly restricting their applications in more extensive fields. Here, we report that the fluorescence emission of porous graphene with a high defect density has a giant enhancement (about two orders of magnitude) by a direct and simple fluorination strategy, showing a fine defect-tolerance characteristic. Meanwhile, the corresponding fluorocarbon bonds with excellent thermostability (over 500 °C in N2 even air) also bring about good stability. The photophysical origins during the whole photoluminescence evolution are further investigated. In the excitation process, the coexistence of fluorine and aromatic regions in fluorinated porous graphene (FPG) contributes to producing a new electronic band gap structure to match the maximum excitation wavelength, then numerous excitons generate, which is a precondition for strong fluorescence emission. In the emission process, weak electron-phonon interactions, large rigidity, and constrained electron at the defects in FPG greatly reduce nonradiative recombination loss. Moreover, fluorine at the defects also reduces interlayer interactions among FPG nanosheets and resists the influence of absorbed impurities, thereby further restricting nonradiative recombination pathway. Highly fluorescent FPG has been utilized as a fascinating tool to achieve sensitive and naked-eye detection of Fe3+ ions with a high selectivity. The fluorescence quenching efficiency reaches 24% even with an ultralow concentration of Fe3+ (0.06 μM), and that increases to 84% when the concentration of Fe3+ is 396 μM.

Spectroscopic signatures of electron-phonon coupling in silicon-vacancy centers in diamond

Vacancy centers in diamond have proven to be a viable solid-state platform for quantum coherent opto-electronic applications. Among the variety of vacancy centers, silicon-vacancy (SiV) centers have recently attracted much attention as an inversion-symmetric system that is less susceptible to electron-phonon interactions. Nevertheless, phonon-mediated processes still degrade the coherent properties of SiV centers, however characterizing their electron-phonon coupling is extremely challenging due to their weak spectroscopic signatures and remains an open experimental problem. In this paper we theoretically investigate signatures of electron-phonon coupling in simulated linear and nonlinear spectra of SiV centers. We demonstrate how even extremely weak electron-phonon interactions, such as in SiV centers, may be completely characterized via nonlinear spectroscopic techniques and even resolved between different fine-structure transitions.

Low-temperature properties of pyrochlore Lu2Ir2O7

Low-temperature properties of Lu2Ir2O7 were studied by means of magnetization, ac-susceptibility and specific heat measurements. Our low-temperature measurements reveal a bifurcation of ZFC and FC magnetization data below 140 K, which is further pronounced as a broad anomaly in the temperature development of specific heat and ac-susceptibility. The anomaly can be attributed to the same physical mechanism as discussed for other A2Ir2O7 with A = magnetic rare-earth element, i.e. to short-range correlations among Ir4+ magnetic moments, rather than to a long-range order of Ir sublattice reported earlier. The ac-susceptibility data revealed a strong frequency dependence of the signal, a paradigmatic sign of a spin-glass transition. The analysis of low-temperature specific heat data brought important information on physical properties of Lu2Ir2O7, in striking agreement with our density functional theoretical (DFT) calculations, concluding relatively weak electron-phonon interaction and semimetallic ground state in Lu2Ir2O7.