We present here a detailed investigation into the magnetic ordering of a full Heusler alloy ${\mathrm{Ga}}_2\mathrm{MnCo}$ using dc and ac magnetization measurements, neutron diffraction, and neutron depolarization experiments. The crystal structure at room temperature was first confirmed to be $L{2}_1$ using the highly intense synchrotron x-ray diffraction technique. Temperature-dependent magnetization reveals that ${\mathrm{Ga}}_2\mathrm{MnCo}$ enters a ferromagnetic (FM) state at $T_C=154$ K, characterized by a sharp increase in magnetization and a plateaulike region hereafter. As the temperature is decreased further, a sharp drop in magnetization is observed at $T_f=50$ K, hinting toward an antiferromagnetic (AFM) phase change. Neutron diffraction (ND) recorded over the range of temperature from 6 to 300 K provides combined information regarding crystal as well as magnetic structure. Accordingly, an increase in the intensity of the ND pattern is seen at 150 K, signaling the onset of long-range FM order. However, there is no sign of the appearance of superlattice reflections corresponding to the AFM phase in the patterns recorded below 50 K. An unusual discontinuity in the unit-cell volume is seen around $T_f$, indicating a coupling of this second transition with the contraction of the lattice. Attempts to unravel this interesting magnetic behavior using ac susceptibility measurements led to the existence of glassy magnetism below $T_f$. Systematic analysis of the susceptibility results along with neutron depolarization measurement identifies the low-temperature phase as a reentrant cluster glass.

The magnetic properties of a pressure induced noncentrosymmetric heavy-fermion superconductor ${\mathrm{CeIrGe}}_3$ have been investigated by muon spin relaxation ($\mu \mathrm{SR}$), powder neutron diffraction (ND), and inelastic neutron scattering (INS) techniques at ambient pressure. For completeness we have also measured the ac magnetic susceptibility ${\chi }_{\mathrm{ac}}\left(T\right)$, dc magnetic susceptibility $\chi \left(T\right)$, dc isothermal magnetization $M\left(H\right)$, and heat capacity $C_{\mathrm{p}}(T,H)$ down to 2 K. ${\mathrm{CeIrGe}}_3$ is known to exhibit pressure induced superconductivity ($T_{\mathrm{c}}\approx 1.5$ K) at a pressure of 20 GPa and antiferromagnetic ordering at 8.7 K, 4.7 K, and 0.7 K at ambient pressure. Our ${\chi }_{\mathrm{ac}}\left(T\right)$ and $\chi \left(T\right)$ data show an additional anomaly near 6.2 K which is also captured in $C_{\mathrm{p}}\left(T\right)$ data. From ${\chi }_{\mathrm{ac}}\left(T\right), \chi \left(T\right)$, and $C_{\mathrm{p}}\left(T\right)$ measurements we infer three antiferromagnetic transitions above 2 K at $T_{\mathrm{N}1}=8.5$ K, $T_{\mathrm{N}2}=6.0$ K, and $T_{\mathrm{N}3}=4.6$ K. Our $\mu \mathrm{SR}$ study also confirms the presence of three transitions through the observation of one frequency for $T_{\mathrm{N}2}<T\le T_{\mathrm{N}1}$, two frequencies for $T_{\mathrm{N}3}<T\le T_{\mathrm{N}2}$, and three frequencies for $T\le T_{\mathrm{N}3}$ in the oscillatory asymmetry. The ND data reveal an incommensurate nature of the magnetic ordering at $T=7$ K with the propagation vector k = (0,0,0.688(3)), and a commensurate magnetic structure at $T=1.5$ K with the propagation vector locked to the value k = (0, 0, 2/3) and magnetic moments oriented along the $c$ axis. The commensurate structure couples a macroscopic ferromagnetic component, resulting in a strong dependence of the lock-in transition temperature on external magnetic field. The INS data show two well defined crystal electric field (CEF) excitations arising from the CEF-split Kramers doublet ground state of ${\mathrm{Ce}}^{3+}$. The CEF energy levels scheme and wave functions have been determined. The ND and INS results together suggest that the anisotropic magnetic exchange are playing an important role in the magnetism of ${\mathrm{CeIrGe}}_3$.

The superconducting gap structure of a charge density wave (CDW) superconductor ${\mathrm{LaPt}}_2{\mathrm{Si}}_2(T_c=1.6$ K) having a quasi-two-dimensional crystal structure has been investigated using muon spin rotation/relaxation $\left(\mu \mathrm{SR}\right)$ measurements in transverse field (TF), zero field (ZF), and longitudinal field (LF) geometries. Rigorous analysis of TF-$\mu \mathrm{SR}$ spectra in the superconducting state corroborates that the temperature dependence of the effective penetration depth, ${\lambda }_L$, derived from muon spin depolarization, fits to a two gap $s$ wave model (i.e., $s+s$ wave) suggesting that the Fermi surface contains two gaps of different magnitude rather than an isotropic gap expected for a conventional $s$ wave superconductor. On the other hand, ZF $\mu \mathrm{SR}$ data do not show any significant change in muon spin relaxation rate above and below the superconducting transition temperature indicating the fact that time-reversal symmetry is preserved in the superconducting state of this material.

The realization of quantum functionalities with polaritons in an all-semiconductor platform requires the control of the energy and spatial overlap of the wave functions of single polaritons trapped in potentials with precisely controlled shape and size. In this study we reach the confinement of microcavity polaritons in traps with an effective potential width down to 1 µm, produced by patterning the active region of the (Al,Ga)As microcavity between two molecular beam epitaxy growth runs. We correlate spectroscopic and structural data to show that the smooth surface relief of the patterned traps translates into a graded confinement potential characterized by lateral interfaces with a finite lateral width. We show that the structuring method is suitable for the fabrication of arrays of proximal traps, supporting hybridization between adjacent lattice sites.

The phase of quantum magneto-oscillations is often associated with the Berry phase and is widely used to argue in favor of topological nontriviality of the system (Berry phase $2\pi n+\pi )$. Nevertheless, the experimentally determined value may deviate from $2\pi n+\pi$ arbitrarily, therefore more care should be made analyzing the phase of magneto-oscillations to distinguish trivial systems from nontrivial. In this paper we suggest two simple mechanisms dramatically affecting the experimentally observed value of the phase in three-dimensional topological insulators: (i) magnetic field dependence of the chemical potential, and (ii) possible nonuniformity of the system. These mechanisms are not limited to topological insulators and can be extended to other topologically trivial and nontrivial systems.

We report a joint experimental and theoretical study of the interference properties of a single-photon source based on a In(Ga)As quantum dot embedded in a quasiplanar GaAs microcavity. Using resonant laser excitation with a pulse separation of 2 ns, we find near-perfect interference of the emitted photons, and a corresponding indistinguishability of $\mathcal{I}=\left(99.6{}_{-1.4}^{+0.4}\right)\%$. For larger pulse separations, quasiresonant excitation conditions, increasing pump power, or with increasing temperature, the interference contrast is progressively and notably reduced. We present a systematic study of the relevant dephasing mechanisms and explain our results in the framework of a microscopic model of our system. For strictly resonant excitation, we show that photon indistinguishability is independent of pump power, but strongly influenced by virtual phonon-assisted processes which are not evident in excitonic Rabi oscillations.

We identify a one-to-one correspondence between the charge localized around a dislocation characterized by a generic Burgers vector and the Berry phase associated with the electronic Bloch waves of two-dimensional crystalline insulators. Using this correspondence, we reveal a link between dislocation charges and the topological invariants of inversion and rotation symmetry-protected insulating phases both in the absence and in the presence of time-reversal symmetry. Our findings demonstrate that dislocation charges can be used as generic probes of crystalline topologies.

We study the magnetotransport of high-mobility electrons in monolayer and bilayer ${\mathrm{MoSe}}_2$, which show Shubnikov–de Haas (SdH) oscillations and quantum Hall states in high magnetic fields. An electron effective mass of $0.8m_e$ is extracted from the SdH oscillations' temperature dependence; $m_e$ is the bare electron mass. At a fixed electron density the longitudinal resistance shows minima at filling factors (FFs) that are either predominantly odd, or predominantly even, with a parity that changes as the density is tuned. The SdH oscillations are insensitive to an in-plane magnetic field, consistent with an out-of-plane spin orientation of electrons at the $K$ point. We attribute the FF parity transitions to an interaction enhancement of the Zeeman energy as the density is reduced, resulting in an increased Zeeman-to-cyclotron energy ratio.

The possibility to grow a graphene layer using the chemical-vapor-deposition technique over a ${\mathrm{Ni}}_2\mathrm{C}/\mathrm{Ni}\left(111\right)$ substrate has been identified experimentally, with the advantage of having a lower processing temperature ($T<500{}^{\circ }\text{C}$), compared to standard growth over a $\mathrm{Ni}\left(111\right)$ surface. To understand the role of the metal carbide/metal catalyst, we first perform a static study of the ${\mathrm{Ni}}_2\mathrm{C}/\mathrm{Ni}\left(111\right)$ structure and of the binding and removal of a carbon atom at the surface, using both a tight-binding (TB) energetic model and ab initio calculations. Grand-canonical Monte Carlo TB simulations then allow us (i) to determine the thermodynamic conditions to grow graphene and (ii) to separate key reaction steps in the growth mechanism explaining how the ${\mathrm{Ni}}_2\mathrm{C}/\mathrm{Ni}\left(111\right)$ substrate catalyzes graphene formation at low temperature.

Neutron polarization analysis has been carried out in order to clarify the magnetic structures of multiple order parameter $f$-electron system ${\mathrm{NdB}}_4$. We confirmed the noncollinear “all-in all-out” structure (${\mathrm{\Gamma }}_4$) of the in-plane moment, which is in good agreement with our previous neutron powder diffraction study. We found that the magnetic moment along the $c$-axis $m_c$ showed diagonally antiferromagnetic structure (${\mathrm{\Gamma }}_{10}$), inconsistent with previously reported “vortex” structure (${\mathrm{\Gamma }}_2$). The microscopic mixture of these two structures with ${\vec{q}}_0=(0,0,0)$ appears in phase II and remains stable in phases III and IV, where an incommensurate modulation coexists. The unusual magnetic ordering is phenomenologically understood via Landau theory with the primary order parameter ${\mathrm{\Gamma }}_4$ coupled with higher-order secondary order parameter ${\mathrm{\Gamma }}_{10}$. The magnetic moments were estimated to be $1.8\pm 0.2$ and $0.2\pm 0.05{\mu }_{\mathrm{B}}$ at $T=7.5\mathrm{K}$ for ${\mathrm{\Gamma }}_4$ and ${\mathrm{\Gamma }}_{10}$, respectively. We also found a long-period incommensurate modulation of the ${\vec{q}}_1=(0,0,1/2$) antiferromagnetic structure of $m_c$ with the propagation ${\vec{q}}_{s1}=(0.14,0.14,0.1)$ and ${\vec{q}}_{s2}=(0.2,0,0.1)$ in phase III and IV, respectively. The amplitude of sinusoidal modulation was about $m_c=1.0\pm 0.2{\mu }_{\mathrm{B}}$ at $T=1.5$ K. The local ($0,0,1/2$) structure consists of in-plane ferromagnetic and out-of-plane antiferromagnetic coupling of $m_c$, opposite to the coexisting ${\mathrm{\Gamma }}_{10}$. The $m_c$ of ${\mathrm{\Gamma }}_{10}$ is significantly enhanced up to $0.6{\mu }_{\mathrm{B}}$ at $T=1.5$ K, which is accompanied by the incommensurate modulations. The Landau phenomenological approach indicates that the higher-order magnetic and/or multipole interactions based on the pseudoquartet $f$-electron state play important roles.

The magnetic ordering of ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_3$ perovskite has been studied by neutron powder diffraction and ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy down to 2 K. From symmetry analysis, a chiral helical model and a collinear model are proposed to describe the magnetic structure. Both are commensurate, with propagation vector $\mathbf{k}=(0,0,1)$ in $R\overline{3}c$ space group. In the former model, the magnetic moments of Fe adopt the magnetic space group $P{3}_{2}21$ and have helical and antiferromagnetic ordering propagating along the $c$ axis. The model allows only a single Fe site, with a magnetic moment of 3.46(2)${\mu }_{\mathrm{B}}$ at 2 K. In the latter model, the magnetic moments of iron ions adopt the magnetic space group $C2/c$ or $C{2}^{\prime }/c^{\prime }$ and are aligned collinearly. The model allows the presence of two inequivalent Fe sites with magnetic moments of amplitude 3.26(3)${\mu }_{\mathrm{B}}$ and 3.67(2)${\mu }_{\mathrm{B}}$, respectively. The neutron-diffraction pattern is equally well fitted by either model. The Mössbauer spectroscopy study suggests a single charge state ${\mathrm{Fe}}^{3.66+}$ above the magnetic transition and a charge disproportionation into ${\mathrm{Fe}}^{(3.66-\zeta )+}$ and ${\mathrm{Fe}}^{(3.66+2\zeta )+}$ below the magnetic transition. The compatibility of the magnetic structure models with the Mössbauer spectroscopy results is discussed.

Dynamic cluster quantum Monte Carlo calculations for a doped two-dimensional extended Hubbard model are used to study the stability and dynamics of $d$-wave pairing when a nearest-neighbor Coulomb repulsion $V$ is present in addition to the on-site Coulomb repulsion $U$. We find that $d$-wave pairing and the superconducting transition temperature $T_c$ are only weakly suppressed as long as $V$ does not exceed $U/2$. This stability is traced to the strongly retarded nature of pairing that allows the $d$-wave pairs to minimize the repulsive effect of $V$. When $V$ approaches $U/2$, large momentum charge fluctuations are found to become important and to give rise to a more rapid suppression of $d$-wave pairing and $T_c$ than for smaller $V$.

When a metal undergoes continuous quantum phase transition, the correlation length diverges at the critical point and the quantum fluctuation of order parameter behaves as a gapless bosonic mode. Generically, the coupling of this boson to fermions induces a variety of unusual quantum critical phenomena, such as non-Fermi liquid behavior and various emergent symmetries. Here, we perform a renormalization group analysis of the semimetal-superconductor quantum criticality in a three-dimensional anisotropic Weyl semimetal. Surprisingly, distinct from previously studied quantum critical systems, the anomalous dimension of anisotropic Weyl fermions flows to zero very quickly with decreasing energy, and the quasiparticle residue takes a nonzero value. These results indicate that the quantum fluctuation of superconducting order parameter is irrelevant at low energies, and a simple mean-field calculation suffices to capture the essential physics of the superconducting transition. We thus obtain a phase transition that exhibits trivial quantum criticality, which is unique comparing to other invariably nontrivial quantum critical systems. Our theoretical prediction can be experimentally verified by measuring the fermion spectral function and specific heat.

We embed a hyperbolic metamaterial based on stacked layer pairs of epitaxially grown ZnO/ZnO:Ga in a monolithic optical microcavity, and we investigate the arising unique resonant effects experimentally and theoretically. Unlike traditional metals, the semiconductor-based approach allows us to utilize all three permittivity regions of the hyperbolic metamaterial in the near-infrared spectral range. This configuration gives rise to modes of identical orders appearing at different frequencies, a zeroth-order resonance in an all-positive permittivity region, and a continuum of high-order modes. In addition, an unusual lower cutoff frequency is introduced to the resonator mode spectrum. The observed effects expand the possibilities for customization of optical resonators; in particular, the zeroth-order and high-order modes hold strong potential for the realization of deeply subwavelength cavity sizes.

We report a room-temperature optical reflectivity study performed on [112]-oriented ${\mathrm{Cd}}_3{\mathrm{As}}_2$ single crystals over a broad energy range under external pressure up to 10 GPa. The abrupt drop in the band dispersion parameter ($z$ parameter) and the interruption of the gradual redshift of the band gap at $\sim 4\mathrm{GPa}$ confirm the structural phase transition from a tetragonal to a monoclinic phase in this material. The pressure-induced increase in the overall optical conductivity at low energies and the continuous redshift of the high-energy bands indicate that the system evolves towards a topologically trivial metallic state, although a complete closing of the band gap could not be observed in the studied pressure range. Furthermore, a detailed investigation of the low-pressure regime suggests the possible existence of an intermediate state between 2 and 4 GPa, that might be a precursor of the structural phase transition or due to the lifted degeneracy of the Dirac nodes. Several optical parameters show yet another anomaly at 8 GPa where low-temperature superconductivity was found in an earlier study.

We propose a nonperturbative method to calculate the butterfly diagram of energy spectra for materials immersed in a magnetic field. We apply the proposed method to a crystalline silicon immersed in a magnetic field. It is shown that the conventional Hofstadter butterfly diagram is of low accuracy not only in the high magnetic field region of the diagram but also even in the experimentally available magnetic field region. This means that the present butterfly diagram is regarded as a replacement for the Hofstadter butterfly diagram. We also show that the correction to the Hofstadter buttery diagram would be observed under the ultrahigh magnetic field that is available in experiments.

Scattering of phonons at boundaries of a crystal (grains, surfaces, or solid/solid interfaces) is characterized by the phonon wavelength, the angle of incidence, and the interface roughness, as historically evaluated using a specularity parameter $p$ formulated by Ziman [Electrons and Phonons (Clarendon Press, Oxford, 1960)]. This parameter was initially defined to determine the probability of a phonon specularly reflecting or diffusely scattering from the rough surface of a material. The validity of Ziman's theory as extended to solid/solid interfaces has not been previously validated. To better understand the interfacial scattering of phonons and to test the validity of Ziman's theory, we precisely measured the in-plane thermal conductivity of a series of Si films in silicon-on-insulator (SOI) wafers by time-domain thermoreflectance (TDTR) for a Si film thickness range of 1–10 μm and a temperature range of 100–300 K. The $\mathrm{Si}/{\mathrm{SiO}}_2$ interface roughness was determined to be $0.11\pm 0.04\mathrm{nm}$ using transmission electron microscopy (TEM). Furthermore, we compared our in-plane thermal conductivity measurements to theoretical calculations that combine first-principles phonon transport with Ziman's theory. Calculations using Ziman's specularity parameter significantly overestimate values from the TDTR measurements. We attribute this discrepancy to phonon transmission through the solid/solid interface into the substrate, which is not accounted for by Ziman's theory for surfaces. The phonons that are specularly transmitted into an amorphous layer will be sufficiently randomized by the time they come back to the crystalline Si layer, the effect of which is practically equivalent to a diffuse reflection at the interface. We derive a simple expression for the specularity parameter at solid/amorphous interfaces and achieve good agreement between calculations and measurement values.

We propose a mechanism to drive singlet-triplet spin transitions electrically in a wide class of graphene nanostructures that present pairs of in-gap zero modes, localized at opposite sublattices. Examples are rectangular nanographenes with short zigzag edges, armchair ribbon heterojunctions with topological in-gap states, and graphene islands with $s{p}^3$ functionalization. The interplay between the hybridization of zero modes and the Coulomb repulsion leads to symmetric exchange interaction that favors a singlet ground state. Application of an off-plane electric field to the graphene nanostructure generates an additional Rashba spin-orbit coupling, which results in antisymmetric exchange interaction that mixes $S=0$ and $S=1$ manifolds. We show that modulation in time of either the off-plane electric field or the applied magnetic field permits performing electrically driven spin resonance in a system with very long spin-relaxation times.