We investigate the interplay between the lattice, charge, and spin dynamics in charge-ordered high $T_c$ cuprate ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4\left(T_c=4\mathrm{K}\right)$ based on the inverse Laplace transform (ILT) analysis of the ${}^{139}\mathrm{La}$ nuclear spin-lattice relaxation rate $1/T_1$ (dubbed ${\mathrm{ILTT}}_1$ analysis hereafter). A major thrust of the ${\mathrm{ILTT}}_1$ analysis is that one can deduce the probability density function $P(1/T_1)$ of distributed $1/T_1$. We demonstrate that $1/T_1^{\text{lm}}$, defined as the log mean (i.e., the center of gravity on a logarithmic scale) of $P(1/T_1)$, can be well approximated by $1/T_1^{\text{str}}$ deduced from the phenomenological stretched fit; however, $P(1/T_1)$ can provide much richer insight into how the lattice, charge, and spin fluctuations and their distribution develop near and below the long-range charge order at $T_{\text{charge}}\sim 54\mathrm{K}$. Upon entering the charge-ordered state, a divergent increase of $1/T_1^{\text{lm}}$ toward the spin ordering at $T_{\text{spin}}^{\mu \text{SR}}\simeq 35\mathrm{K}$ is accompanied by an asymmetric broadening of $P(1/T_1)$. Even deep inside the charge-ordered state, $1/T_1$ at a gradually diminishing fraction of ${}^{139}\mathrm{La}$ sites continues to slow down as temperature is lowered, as expected for canonical superconducting ${\mathrm{CuO}}_2$ planes without enhanced spin fluctuations. The fraction of such canonical ${}^{139}\mathrm{La}$ sites almost disappears by $\simeq 40\mathrm{K}$. In contrast, nearly half of the ${}^{139}\mathrm{La}$ sites in ${\mathrm{La}}_{1.885}{\mathrm{Sr}}_{0.115}{\mathrm{CuO}}_4\left(T_{\text{charge}}\simeq 80\mathrm{K}\right)$ still exhibit the canonical behavior without enhanced spin fluctuations even near its $T_c=31\mathrm{K}$. These contrasting behaviors explain why superconductivity in ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$ is more strongly suppressed than in ${\mathrm{La}}_{1.885}{\mathrm{Sr}}_{0.115}{\mathrm{CuO}}_4$ despite the lower onset temperature of the charge order.

We discuss the influence of momentum-dependent correlations on the superconducting gap structure in iron-based superconductors. Within the weak coupling approach including self-energy effects at the one-loop spin-fluctuation level, we construct a dimensionless pairing strength functional which includes the effects of quasiparticle renormalization. The stationary solution of this equation determines the gap function at $T_c$. The resulting equations represent the simplest generalization of spin fluctuation pairing theory to include the effects of an anisotropic quasiparticle weight. We obtain good agreement with experimentally observed anisotropic gap structures in LiFeAs, indicating that the inclusion of quasiparticle renormalization effects in the existing weak-coupling theories can account for the observed anomalies in the gap structure of Fe-based superconductors.

The canonical spin glass (SG) is an enigmatic system in condensed matter physics. In spite of the intense activities of the last five decades, several questions regarding the nature of the SG phase transition and the SG ground state are yet to be resolved completely. In this backdrop we have revisited the field-cooled state of the canonical spin glass. We have experimentally studied the magnetic response in two canonical spin-glass systems AuMn(1.8%) and AgMn(1.1%), both in the field-cooled (FC) as well as zero-field-cooled (ZFC) state. We show that the well-known magnetic memory effect, which clearly established earlier the metastable nature of the ZFC state in SG, is also present in the FC state. The results of our experimental study indicate that the FC state also is a nonequilibrium state, contrary to common understanding, and hence the energy landscape involved is a nontrivial one. This in turn seriously questions the picture of the spin-glass transformation as a simple second-order thermodynamic phase transition.

Recent theoretical studies predict the suppressed ferroelectric instability in orthorhombic $Pnma$ perovskites and experimental evidence is due. We observed significant softening at cooling of the lowest-frequency polar phonon at the Brillouin zone center in the $Pnma$ antiferromagnetic fluoroperovskite ${\mathrm{NaMnF}}_3$ that is the direct proof of the theoretically predicted ferroelectric instability. In contrast to oxides where the hybridization plays the dominant role, the effective ionic charges in fluoroperovskites are close to their nominal valencies that confirms the geometric origin of the observed incipient ferroelectricity. Furthermore, below the Néel temperature, the softening phonon clearly shows a strong coupling with the magnetic subsystem as a result of dynamical modulation of the superexchange interaction. Our findings clarify microscopic mechanisms of the incipient multiferroicity in the $Pnma$ fluoroperovskites and reveal still unexplored opportunities of this class of materials for further research and potential applications.

We derive an effective tight-binding model that captures, in real space and with only two parameters, the dominant Coulomb interactions and superconducting pairing near half-filling of magic-angle twisted bilayer graphene. We show that, in an antiferromagnetic Mott insulating ground state with intervalley coherence, magnetic fluctuations and doping mediate superconducting pairing. We find the pairing wave function to have chiral $d$-wave symmetry and obtain a self-consistent mean-field phase diagram in line with experiments on the doping-induced insulator-to-superconductor transition. We further reveal the existence of chiral Majorana edge modes implied by the nontrivial pairing symmetry, which establishes twisted bilayer graphene as a potential platform for topological superconductivity. This effective model opens the door to systematic scrutiny of the competition between correlated states in this system.

A series of shock wave experiments were conducted to measure the optical properties of single-crystal diamond $\langle 100\rangle$ in the pressure regime between 60 and 550 GPa. The results show that the transparency limit of diamond at 532 nm is $\sim 170\mathrm{GPa}$. When the applied pressure in diamond is lower than its Hugoniot elastic limit (HEL), diamond remains transparent during both compression and release processes. At the pressures between the HEL and the limit of its transparency, however, diamond is found to be transparent only while the compression is maintained and gradually loses its transparency during the subsequent release process. We also found that the refractive index of single-crystal diamond 〈100〉 monotonically increases as density increases to the limit of its transparency, in contrast to the previous static reports on continuous decrease of refractive index with increasing pressure up to 40 GPa.

The understanding of the symmetry breakings within crystals that are aperiodic by construction is actually very limited. Quasicrystals and incommensurate composite crystals may potentially allow such studies. We focus on the phase transitions of the aperiodic $n$-nonadecane/urea which recovers a translational symmetry within a four-dimensional space at room temperature. High-resolution neutron and synchrotron studies are reported as a function of the temperature on this organic crystal which presents an exceptional mosaicity. They reveal the richness of such approach, showing the appearance of very long wavelength supplementary intermodulations. This work generalizes the Landau theory to incommensurate composite crystals.

Combining electron paramagnetic resonance (EPR) spectroscopy and first-principles density functional theory calculations we have identified the carbon monovacancy center and a second carbon vacancy-related defect, the carbon vacancy–carbon antisite defect in $3C$-SiC. In close analogy to the vacancy in silicon, the carbon vacancy in $3C$-SiC with its four potentially equivalent silicon dangling bonds shows a strong Jahn-Teller effect, confirming previously predicted negative-$U$ properties. High-temperature annealing (above 700 °C) of electron or proton irradiated samples anneals out the primary silicon monovacancies and generates a new defect, which we assign to the carbon vacancy–carbon antisite complex. As predicted by theory, this defect is the result of a structural instability of the silicon vacancy in the positive charge state, which transforms at high temperatures according to $V_{\mathrm{Si}}{\mathrm{C}}_4\to V_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}}{\mathrm{C}}_3$. Both defects are deep donors and thus not suitable for achieving semi-insulating properties.

Recent experiments demonstrate that antiferromagnets exhibit the spin Hall effect. We study a tight-binding model of an antiferromagnet on a square lattice with Rashba spin-orbit coupling and disorder. By exact diagonalization of a finite system connected to reservoirs within the Landauer-Büttiker formalism, we compute the transverse spin Hall current in response to a longitudinal voltage difference. Surprisingly, the spin Hall conductance can be considerably larger in antiferromagnets than in normal metals. We compare our results to the Berry-phase-induced spin Hall effect governed by the intrinsic contribution in the Kubo formula. The Berry-phase-induced intrinsic spin Hall conductivity in bulk systems shows the opposite behavior. The intrinsic spin Hall effect is drastically reduced when the exchange couplings become large.

Spin pumping is a recently established means for generating a pure spin current, whereby spins are pumped from a magnet into the adjacent target material under the ferromagnetic resonance condition. We theoretically investigate the spin pumping from an insulating ferromagnet into spin glass materials. Combining a dynamic theory of spin glasses with the linear-response formulation of the spin pumping, we calculate temperature dependence of the spin pumping near the spin glass transition. The analysis predicts that a characteristic peak appears in the spin pumping signal, reflecting that the spin fluctuations slow down upon the onset of spin freezing.

We report the inverse Laplace transform analysis of the ${}^{139}\mathrm{La}$ nuclear spin-lattice relaxation rate $1/T_1$ (${\mathrm{ILTT}}_1$ analysis) in charge-ordered ${\mathrm{La}}_{1.885}{\mathrm{Sr}}_{0.115}{\mathrm{CuO}}_4$ ($T_{\mathrm{charge}}\simeq 80$ K, $T_c\simeq T_{\mathrm{spin}}^{\mathrm{neutron}}=30$ K), and shed light on its magnetic inhomogeneity. We deduce the probability density function $P(1/T_1)$ of the distributed $1/T_1$ (i.e., the histogram of distributed $1/T_1$) by taking the inverse Laplace transform of the experimentally observed nuclear magnetization recovery curve $M\left(t\right)$. We demonstrate that spin freezing sets in in some domains precisely below the onset of charge order at $T_{\mathrm{charge}}$, but their volume fraction grows only gradually toward $T_c$. Nearly a half of the sample volume exhibits properties expected for canonical high-$T_c$ cuprates without charge order even near $T_c$. Our findings explain why charge order does not suppress $T_c$ of ${\mathrm{La}}_{1.885}{\mathrm{Sr}}_{0.115}{\mathrm{CuO}}_4$ as significantly as in ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$.

The dynamical cluster approximation (DCA) is a quantum cluster extension to the single-site dynamical mean-field theory that incorporates spatially nonlocal dynamic correlations systematically and nonperturbatively. The ${\mathrm{DCA}}^{+}$ algorithm addresses the cluster shape dependence of the DCA and improves the convergence with cluster size by introducing a lattice self-energy with continuous momentum dependence. However, we show that the ${\mathrm{DCA}}^{+}$ algorithm is plagued by a fundamental problem when its self-consistency equations are formulated using the bare Green's function of the cluster. This problem is most severe in the strongly correlated regime at low doping, where the ${\mathrm{DCA}}^{+}$ self-energy becomes overly metallic and local, and persists to cluster sizes where the standard DCA has long converged. In view of the failure of the ${\mathrm{DCA}}^{+}$ algorithm, we propose to complement DCA simulations with a post-interpolation procedure for single-particle and two-particle correlation functions to preserve continuous momentum dependence and the associated benefits in the DCA. We demonstrate the effectiveness of this practical approach with results for the half-filled and hole-doped two-dimensional Hubbard model.

We investigate charge fluctuations in the two-dimensional Hubbard model as a function of doping, interaction strength, next-nearest-neighbor hopping, and temperature within the eight-site dynamical cluster approximation. In the regime of intermediate interaction strengths, we find that $d$ density wave fluctuations, which had previously been postulated using analytical arguments, are present and strong but cannot be interpreted as the cause of the pseudogap in the model, due to their evolution with doping and interaction strength. For all parameters away from half filling, the charge fluctuations investigated, including $d$ density wave fluctuations, are weaker than $d$-wave superconducting fluctuations.

The electronic and magnetic properties of two isoelectronic layered transition-metal fluorides (${\mathrm{AgF}}_2$ and ${\mathrm{CuF}}_2$) are considered in the context of high-temperature superconducting cuprates. The properties of ${\mathrm{AgF}}_2$ are found to be cupratelike comprising a layered spin-$\frac{1}{2}$ system with strong $p-d$ hybridization, and a large two-dimensional antiferromagnetic superexchange interaction, comparable to cuprates. Contrary to its Ag-counterpart, ${\mathrm{CuF}}_2$ shows a small degree of $p-d$ hybridization and a superexchange interaction one order of magnitude smaller than in cuprates. Within the Zaanen-Sawatzky-Allen model ${\mathrm{AgF}}_2$ and ${\mathrm{CuF}}_2$ could be classified as a charge-transfer and Mott-Hubbard insulator, respectively. As a consequence, this work further demonstrates the promise ${\mathrm{AgF}}_2$ holds as a parent compound to a new class of Ag-based superconducting materials, whereas ${\mathrm{CuF}}_2$ is not promising as a cuprate analog.

The (one-dimensional) Su-Schrieffer-Heeger Hamiltonian, augmented by spin-orbit coupling and longer-range hopping, is studied at half filling for an even number of sites. The ground-state phase diagram depends sensitively on the symmetry of the model. Charge-conjugation (particle-hole) symmetry is conserved if hopping is only allowed between the two sublattices of even and odd sites. In this case, we find a variety of topologically nontrivial phases, characterized by different numbers of edge states (or, equivalently, different quantized Zak phases). The transitions between these phases are clearly signalled by the entanglement entropy. Charge-conjugation symmetry is broken if hopping within the sublattices is admitted. We study specifically next-nearest-neighbor hopping with amplitudes $t_a$ and $t_b$ for the $A$ and $B$ sublattices, respectively. For $t_a=t_b$, parity is conserved, and also the quantized Zak phases remain unchanged in the gapped regions of the phase diagram. However, metallic patches appear due to the overlap between conduction and valence bands in some regions of parameter space. The case of alternating next-nearest-neighbor hopping, $t_a=-t_b$, is also remarkable, as it breaks both charge-conjugation $C$ and parity $P$ but conserves the product $CP$. Both the Zak phase and the entanglement spectrum still provide relevant information, in particular about the broken parity. Thus the Zak phase for small values of $t_a$ measures the disparity between bond strengths on $A$ and $B$ sublattices, in close analogy to the proportionality between the Zak phase and the polarization in the case of the related Aubry-André model.

Majorana bound states are interesting candidates for applications in topological quantum computation. Low-energy models allowing one to grasp their properties are hence conceptually important. The usual scenario in these models is that two relevant gapped phases, separated by a gapless point, exist. In one of the phases, topological boundary states are absent, while the other one supports Majorana bound states. We show that a customary model violates this paradigm. The phase that should not host Majorana fermions supports a fractional soliton exponentially localized at only one end. By varying the parameters of the model, we describe analytically the transition between the fractional soliton and two Majorana fermions. Moreover, we provide a possible physical implementation of the model. We further characterize the symmetry of the superconducting pairing, showing that the odd-frequency component is intimately related to the spatial profile of the Majorana wave functions.

Due to their band-structure and optical properties, InAs/InP quantum dots (QDs) constitute a promising system for single-photon generation at the third telecom window of silica fibers and for applications in quantum communication networks. However, obtaining the necessary low in-plane density of emitters remains a challenge. Such structures are also still less explored than their InAs/GaAs counterparts regarding optical properties of confined carriers. Here, we report on the growth via metal-organic vapor phase epitaxy and investigation of low-density InAs/InP QD-like structures, emitting in the range of $1.2–1.7\mu \mathrm{m}$, which includes the S, C, and L bands of the third optical window. We observe multiple photoluminescence (PL) peaks originating from flat QDs with the height of a few material monolayers. Temperature-dependent PL reveals a redistribution of carriers between families of QDs. Via time-resolved PL, we obtain radiative lifetimes nearly independent of emission energy in contrast to previous reports on InAs/InP QDs, which we attribute to strongly height-dependent electron-hole correlations. Additionally, we observe neutral and charged exciton emission from spatially isolated emitters. Using the eight-band $\mathit{k}·\mathit{p}$ model and configuration-interaction method, we successfully reproduce the energies of emission lines, the dispersion of exciton lifetimes, the carrier activation energies, as well as the biexciton binding energy, which allows for a detailed and comprehensive analysis of the underlying physics.

We study the electronic structure of heterostructures formed by a graphene nanoribbon (GNR) and a transition metal dichalcogenides (TMD) monolayer using first principles. We consider both semiconducting TMDs and metallic TMDs, and different stacking configurations. We find that when the TMD is semiconducting the effects on the band structure of the GNRs are small. In particular the spin splitting induced by proximity on the GNRs bands is only of the order of few meV irrespective of the stacking configuration. When the TMD is metallic, such as ${\mathrm{NbSe}}_2$, we find that the spin splitting induced in the GNRs can be very large and strongly dependent on the stacking configuration. For optimal stacking configurations the proximity-induced spin splitting is of the order of 20 meV for armchair graphene nanoribbons and as high as 40 meV for zigzag graphene nanoribbons. This results are encouraging for the prospects of using GNR-TMD heterostructures to realize quasi-one-dimensional topological superconducting states supporting Majorana modes.