Theoretically, coupling of phonons with quasiparticles in the Landau levels (LLs) of graphene was predicted to generate a new sequence of discrete states in the density of states, i.e., phonon peaks. However, it was believed that these phonon peaks are extremely weak due to weak electron-phonon coupling in graphene. Here, we report the experimental observation of giant phonon peaks in a graphene monolayer induced by enhanced electron-phonon coupling. We demonstrate that emission or absorption of phonons of quasiparticles in the LLs of graphene generates the phonon peaks, which even exhibits fine structures corresponding to bound states of an electron and of a phonon. We also show that it is possible to tune the relative strength of the phonon peaks at nanoscale by controlling interactions between graphene and the supporting substrate. The local strain in graphene may play a vital role in enhancing the electron-phonon coupling, which results in the observed giant phonon peaks. Our experiment indicates that it is possible to explore interesting emergent phenomena, such as superconductivity, induced by electron-phonon coupling in graphene.

Several materials studied intensively in their bulk forms several decades ago have re-emerged in recent years in their thin film and monolayer manifestations. Tellurium, like black phosphorus, is one such elemental two-dimensional material with promising semiconducting properties for electronic and optoelectronic applications. To study fundamental carrier properties such as hot carrier relaxation and recombination, we performed ultrafast femtosecond pump-probe spectroscopy on thin flakes of solution-grown tellurium. To access the low band gap of tellurium, we used infrared, near-band-gap and below-band-gap probes to monitor the relaxation processes. Sweeping the probing wavelengths across the band gap helps to shed light on the anisotropic band structure of the material. We find that relaxation in flakes of 60–160 nm thickness is on the order of 100 s of picoseconds. Thinner flakes (10–20 nm), on the other hand, exhibit fast relaxation times of sub-20 ps. Radiative recombination is identified as the relaxation mechanism in thick flakes, whereas midgap trap states arising from surface defects and impurities are responsible for the fast relaxation in thin flakes. A diffusion-recombination model accounting for the surface defect and radiative recombinations explains the experimental data well.

We develop an alternative description to solve the problem of the ground-state energy of the Lieb-Liniger model that describes one-dimensional bosons with contact repulsion. For this integrable model we express the Lieb integral equation in the representation of Chebyshev polynomials. The latter form is convenient to efficiently obtain very precise numerical results in the singular limit of weak interaction. Such highly precise data enable us to use the integer relation algorithm to discover the analytical form of the coefficients in the expansion of the ground-state energy for small values of the interaction parameter. We obtained the first nine terms of the expansion using quite moderate numerical efforts. The detailed knowledge of behavior of the ground-state energy on the interaction immediately leads to exact perturbative results for the excitation spectrum.

X-ray absorption and resonant inelastic x-ray scattering measurements are carried out on lithium nitrate ${\mathrm{LiNO}}_3$. The nitrogen $\sigma$ orbitals exhibit a large lifetime effect. Experimentally, this is manifest as an apparent weakening of the x-ray emission signal from these states, but a closer examination shows that instead it is due to extreme broadening. This echoes previous studies on ammonium nitrate, which, despite large differences in the cation and space group, showed a similar effect associated with the nitrate. Using first-principles GW self-energy and Bethe-Salpeter equation calculations we show that this effect is due in part to short quasihole lifetimes for the orbitals constituting the NO $\sigma$ bonds.

The chiral anomaly is a characteristic phenomenon of Weyl fermions, which has condensed matter realizations in Weyl semimetals. Efforts to observe smoking gun signatures of the chiral anomaly in Weyl semimetals have mostly focused on a negative longitudinal magnetoresistance in electronic transport. Unfortunately, disentangling the chiral anomaly contribution in transport or optical measurements has proven nontrivial. Recent works have proposed an alternative approach of probing pseudoscalar phonon dynamics for signatures of the chiral anomaly in non-mirror-symmetric crystals. Here we show that such phonon signatures can be extended to scalar phonon modes and mirror-symmetric crystals, broadening the pool of candidate materials. We show that the presence of the background magnetic field can break mirror symmetry strongly enough to yield observable signatures of the chiral anomaly. Specifically for mirror-symmetric Weyl semimetals such as TaAs and NbAs, including the Zeeman interaction at $\left|\mathbf{B}\right|\approx 10$ T, we predict that an IR reflectivity peak will develop with an ${\mathbf{E}}_{\text{IR}}·\mathbf{B}$ dependence.

Dynamical quantum phase transitions (DQPTs) are manifested by time-domain nonanalytic behaviors of many-body systems. Introducing a quench is so far understood as a typical scenario to induce DQPTs. In this work, we discover a type of DQPTs, termed Floquet DQPTs, as intrinsic features of time-periodic systems. Floquet DQPTs occur within each period of continuous driving, without the need for any quenches. In particular, in a harmonically driven spin chain model, we find analytically the existence of Floquet DQPTs in and only in a parameter regime hosting a certain nontrivial Floquet topological phase. The Floquet DQPTs are further characterized by a dynamical topological invariant defined as the winding number of the Pancharatnam geometric phase versus quasimomentum. These findings are experimentally demonstrated with a single spin in diamond. This work thus opens a door for future studies of DQPTs in connection with topological matter.

We study a single-level quantum dot strongly coupled to a superconducting lead and tunnel coupled to a normal electrode which can exchange energy with a single-mode resonator. We show that such a system implements a single atom laser. We employ both a semiclassical treatment and a quantum master-equation approach to characterize the properties of this laser. In particular, we find that this system can be operated with efficiency approaching unity; that is, a single photon is emitted into the cavity for every Cooper pair participating in the charge current. We find also that lasing in the proposed setup is clearly identifiable in the transport properties: in the lasing state, the electrical current through the quantum dot is pinned to the maximum value achievable in this hybrid nanostructure, and hence, the onset of lasing can be detected simply by a current measurement.

We explore optical activity in thin films and bulk of type-I and type-II Weyl semimetals (WSMs), and demonstrate the existence of a giant Kerr effect in both. In time reversal symmetry broken WSM thin films, the polarization rotation is caused by the optical Hall conductivity including the anomalous Hall term. The Kerr angle is found to be $\propto Q/\omega$, with $Q$ and $\omega$ being the Weyl node separation and the optical frequency, respectively. In contrast, the optical activity in the bulk WSM is dominated by axion electrodynamics, which persists even in the Pauli-blocked regime of no optical transitions. In bulk WSMs, $Q$ acts analogously to the magnetization in magnetic materials, leading to a large polar Kerr effect (linear in $Q$), when light is incident on the WSM surface without Fermi arc states, and the Voigt effect (quadratic in $Q$), when light is incident on surface with Fermi arc states.

A number of quantum Hall isospin ferromagnetic (QHIFM) states have been predicted in the “relativistic” zero Landau level (LL) of the graphene monolayer. However, identification of these high-field broken-symmetry states has mostly relied on macroscopic transport techniques, which lack spatial resolution. Here, we demonstrate a direct approach by imaging the QHIFM states at atomic scale with a scanning tunneling microscope. At half filling of the zero LL ($\nu =0$), the system is in a spin unpolarized state and we observe a linear magnetic-field-scaling of valley splitting. The wave functions of the QHIFM states at $\nu =0$ are directly imaged at the atomic scale and we observe an interaction-driven density wave featuring a Kekulé distortion, which is responsible for the large gap in high magnetic fields. Moreover, our experiment demonstrates that both the valley and spin splittings depend on the filling factor. For example, the spin splitting in the zero LL is dramatically enhanced (in excess of about 200% at maximum) when the Fermi level lies inside spin-polarized states (at $\nu =1$ or −1), accounting for strong many-body effects.

Considering a BDI symmetric one-dimensional Su-Schrieffer-Heeger model, we explore the fate of the bulk topological invariant, namely, the winding number under a generic time-dependent perturbation; the effective Hamiltonian, which generates the temporal evolution of the initial (ground) state of the completely symmetric initial Hamiltonian, may have the same or different symmetries. To exemplify, we consider the following protocols, namely (i) a perfectly periodic protocol, (ii) sudden changes in the parameters of the initial Hamiltonian. We establish that the topological invariant may change in some cases when the effective Hamiltonian (or the Floquet Hamiltonian in the periodic situation when observed stroboscopically) does not respect all BDI symmetries; this is manifested in the associated particle (polarization) or heat current in the bulk. Our results establish a strong connection between the time evolution of the winding number (thus, the associated transport of currents) and the symmetry of the Hamiltonian generating the time evolution, which has been illustrated considering an exhaustive set of possibilities. We also briefly dwell on the situation where the driving is not perfectly periodic.

We examine the magnetic-field-induced criticality of phase boundary near saturation field $H_{\mathrm{c}}$ in the spin-$\frac{1}{2}$ ferromagnetic (FM)-leg ladder 3-Cl-4-F-V [=3-(3-chloro-4-fluorophenyl)-1,5-diphenylverdazyl], the predominant interactions of which arise from FM chains (strong-leg type). Critical temperatures were precisely determined through DC magnetization, specific heat, and magnetocaloric effect measurements. The criticality of 3-Cl-4-F-V is characterized by a linear phase boundary with respect to $H_{\mathrm{c}}-H$ near $H=H_{\mathrm{c}}$. This behavior is similar to that of another strong-leg-type FM-leg ladder. The universal critical behavior in these strong-leg-type FM-leg ladders is expected to demonstrate the theoretically predicted quasi-one-dimensional Bose–Einstein condensation.

We carry out a first-principles investigation on magnetism, electronic structure, magneto-optical effects, and topological property of newly grown cubic double perovskite ${\mathrm{Ba}}_2{\mathrm{NiOsO}}_6$ and its (111) $\left({\mathrm{Ba}}_2{\mathrm{NiOsO}}_6\right)/{\left({\mathrm{BaTiO}}_3\right)}_{10}$ superlattice, based on the density functional theory with the generalized gradient approximation (GGA) plus onsite Couloumb interactions. Interestingly, we find that both structures are rare ferromagnetic (FM) semiconductors with estimated Curie temperatures of $\sim 150$ and 70 K, respectively. The calculated near-neighbor exchange coupling parameters reveal that the ferromagnetism is driven by exotic FM coupling between Ni and Os atoms, which is due to the FM superexchange interaction caused by the abnormally strong hybridization between the half-filled Ni $e_g$ and unoccupied Os $e_g$ orbitals. The strong spin-orbit coupling (SOC) on the Os atom is found to not only open the semiconducting gap but also produce a large antiparallel orbital magnetic moment on the Os atom, thus reducing the total magnetization from $4.0{\mu }_B/\mathrm{f}.\mathrm{u}.$, expected from the ${\mathrm{Ni}}^{2+}3d^8$ ($t_{2g}^6{e}_g^2$; $S=1$) and ${\mathrm{Os}}^{6+}5d^2(t_{2g}^2{e}_g^0$; $S=1$) ions. Remarkably, we also find that because of the enhanced exchange interaction on the Os atoms caused by the Ni $3d$-Os $5d$ hybridization and the strong SOC of the Os atoms, the magneto-optical (MO) effects are large in these two structures. For example, the Kerr and Faraday rotations in bulk ${\mathrm{Ba}}_2{\mathrm{NiOsO}}_6$ can be reach $6^{\circ }$ and $250\mathrm{deg}/\mu \mathrm{m}$, respectively, which are larger than that of best-known MO materials. These interesting findings thus suggest that because of their FM semiconductivity and excellent MO properties, both structures would be promising materials for not only semiconductor-based spintronics but also magneto-optical devices. Finally, our calculated anomalous Hall conductivity shows that the band gap just below the Fermi level in the superlattice is topologically nontrivial with the gap Chern number of 2. This indicates that the (111) ${\mathrm{Ba}}_2{\mathrm{NiOsO}}_6$ and related $5d$ double-perovskite monolayers may provide an interesting material platform for exploring magnetic topological phases and phase transitions.

Dilute dipolar Ising magnets remain a notoriously hard problem to tackle both analytically and numerically because of long-ranged interactions between spins as well as rare region effects. We study a new type of anisotropic dilute dipolar Ising system in three dimensions [A. Sen and R. Moessner, Phys. Rev. Lett. 114, 247207 (2015)] that arises as an effective description of randomly diluted classical spin ice, a prototypical spin liquid in the disorder-free limit, with a small fraction $x$ of nonmagnetic impurities. The Metropolis algorithm within a parallel thermal tempering scheme fails to achieve equilibration for this problem already for small system sizes. Motivated by previous work [J. C. Andresen, H. G. Katzgraber, V. Oganesyan and M. Schechter, Phys. Rev. X 4, 041016 (2014)] on uniaxial random dipoles, we present an improved cluster Monte Carlo algorithm that is tailor made for removing the equilibration bottlenecks created by clusters of effectively frozen spins. By performing large-scale simulations down to $x=1/128$ and using finite-size scaling, we show the existence of a finite-temperature spin glass transition and give strong evidence that the universality of the critical point is independent of $x$ when it is small. In this $x\ll 1$ limit, we also provide a first estimate of both the thermal exponent, $\nu =1.27\left(8\right)$, and the anomalous exponent, $\eta =0.228\left(35\right)$.

The pinning phenomena of topological skyrmions in magnetic materials with defects are of vital importance for the precise positioning and the manipulation of skyrmions in experiment. With the Thiele equation of the particle-based model, we investigate the dynamic depinning phase transition of skyrmions, induced by quenched disorder. The phase transition from the pinned glass to the moving liquid is of second order, while the critical driving force and both the static and dynamic exponents are accurately determined for different strengths of the Magnus term and the pinning force based on the dynamic scaling behavior far from stationary. The results show that the skyrmions exhibit very different collective transport properties at the depinning phase transition due to the Magnus force which induces the skyrmion Hall effect compared to the overdamped magnetic systems. Furthermore, the critical behaviors of skyrmions are anisotropic in directions perpendicular and parallel to the driving force, providing an understanding of the force-dependent Hall angle around the phase transition in experiment. Our nonstationary dynamic approach is very efficient in tackling the dynamic phase transitions.

We investigate the effect that density wave states have on the Hofstadter butterfly. We first review the problem of the $d$-density wave on a square lattice and then numerically solve the $d$-density wave problem when an external magnetic field is introduced. As the $d$-density wave condensation strength is tuned the spectrum evolves through three topologically distinct butterflies, and a relativistic quantum Hall effect is observed. The chiral $p+ip$-density wave state demonstrates drastically different Hofstadter physics—inducing a destruction of the gaps in the butterfly which causes electrons' cyclotron orbits to not obey any type of Landau quantization, and the creation of a large gap in the spectrum with Hall conductance ${\sigma }_{xy}=0$. To investigate the quantum phases in the system we perform a multifractal analysis of the single particle wave functions. We find that tuning the $d$-density wave strength at a generic value of magnetic flux controls a metal-metal transition at charge neutrality where the wave-function multifractality occurs at energy level crossings. In the $p+ip$ case we observe another metal-metal transition occurring at an energy level crossing separated by a strongly multifractal quasi-insulating island state occurring at charge neutrality and strip dimerization of the lattice.

It is generally believed that the lattice thermal conductivity in crystalline solids is mainly carried by phonons, whereas diffusons are regarded as the most important vibrational entity in amorphous solids. Contrary to this belief, here we show that the behavior of lattice thermal conductivity (LTC) of simple crystalline $\mathrm{Tl}X\mathrm{T}{\mathrm{e}}_2\left(X=\mathrm{Ga},\mathrm{In}\right)$ compounds cannot be characterized either by phonons or diffusons alone. The calculated LTC based on first-principles density functional theory and the semiclassical Boltzmann transport equation (BTE) is only half of the experimental value. We show that the relatively weak bonding between Tl atoms and rigid $X\mathrm{T}{\mathrm{e}}_2$ chains gives rise to strong vibrational anharmonicity, which suppresses the mean free path of a large portion of phonon modes to even below the Ioffe-Regel limit and thus invalidates the precondition of the semiclassical BTE. We further show that the ultralow LTC of $\mathrm{Tl}X\mathrm{T}{\mathrm{e}}_2\left(X=\mathrm{Ga},\mathrm{In}\right)$ can be rationalized by using the two-channel transport model. This finding would enhance the understanding of the underlying heat transport behavior in such materials.

A quantized version of the magnetoelectric effect, known as the topological magnetoelectric effect, can exist in a time-reversal invariant topological insulator with all its surface states gapped out by magnetism. This topological phase, called the axion insulator phase, has been theoretically proposed but still lacks conclusive experimental evidence due to the small signal of topological magnetoelectric effect. In this work, we propose that the dynamical in-plane magnetization in an axion insulator can generate a pseudoelectric field, which acts on the surface state of topological insulator films and leads to the nonzero response current. Strikingly, we find that the current at magnetic resonance (either ferromagnetic or antiferromagnetic) is larger than that of topological magnetoelectric effect by several orders of magnitude and thereby serves as evidence to confirm the axion insulator phase in the candidate materials.

One path to high-temperature cuprate superconductors is doping a Mott insulator. In this paper, we study this system from the view point of higher-form symmetries. On the introduction of slave bosons, the $t-J$ model at a finite hole doping can be written in the form of U(1) gauge theories. After a duality transformation, they can be written as a generalized $BF$ theory, in which the higher-form symmetries are more manifest. We identify the emergent continuous and discrete higher-form symmetries in both $s$-wave and $d$-wave superconducting phases, expected to be realized from a doped Mott insulator. The existence of a topological order is tested by examining if there is a spontaneously broken discrete one-form symmetry. We claim that a spontaneous breaking of discrete one-form symmetry may extend to a phase that has massless Dirac fermions in the limit of large number of flavors. We discuss the possibility of a topological phase transition inside the superconducting dome of high-$T_c$ cuprates.

Regular surface undulations, called cross-hatch patterns, appearing at the free surface of lattice-mismatched heteroepitaxial films are a key signature of plastic relaxation. Here we show that the dynamics of cross-hatch formation is accurately described by a continuum model based on strain-mediated surface diffusion, provided that a realistic distribution of dislocations is considered. We demonstrate quantitative agreement between our time-dependent simulations and dedicated atomic force microscopy experiments on ${\mathrm{Si}}_{0.92}{\mathrm{Ge}}_{0.08}$ films grown on Si(001) at various thicknesses, finally shedding light on the origin and on the dynamical behavior of a widely investigated pattern, first observed more than half a century ago.

The structural changes of sintered powder samples of perovskite-slab layered polycrystalline ${\mathrm{Sr}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$ (SNO) ferroelectric compound subjected to high pressures are here investigated. The samples were prepared using a solid-state reaction in the presence (SNOE) or absence (SNO) of an applied electric field. Density functional theory (DFT) calculations including hydrostatic pressure indicate that SNO's ferroelectricity remains up to 25 GPa in the structure of space group $Cmc{2}_1$ derived from the condensation of one zone-center soft phonon. The predicted DFT theoretical structural changes are discussed and compared with the results of the experimental Raman spectra as a function of pressure. The pressure-dependent spectra were recorded from atmospheric pressure up to 11.5 and 13.5 GPa for SNOE and SNO, respectively. Both samples underwent a pressure induced phase transition from an incommensurate to a commensurate state at room temperature, SNO at $P_{i-c}=6.5\pm 0.2$ GPa, and SNOE at $P_{i-c}=6.9\pm 0.3$ GPa. The DFT calculations enable the identification of the change in phase to the orthorhombic structure with the space group $Cmc{2}_1$. The experimental values for $P_{i-c}$ are in reasonably good agreement with the theoretical predicted value of ∼7.3 GPa. After the critical pressures, the number of observable phonons decreases, that is, when the compound adopts a higher symmetry structure, several phonons vanish abruptly in both the SNO and SNOE samples, as expected. The Raman spectra for both samples show hysteresis effects, that is, after the pressure is removed, a few extra lines remain visible, as well as many relative intensity changes and broadenings for some phonon bands. The bulk moduli of the $\mathrm{S}{\mathrm{r}}_2\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_7$ before and after $P_{i-c}$ are calculated resulting in 117.0 and 147.8 GPa, respectively. The Grüneisen parameters of the phonons observed are finally calculated and discussed.