PHotonique ELectronique et Ingénierie QuantiqueS

Selection of the ground state of the kagome-lattice $XXZ$ antiferromagnet by quantum fluctuations is investigated by combining nonlinear spin-wave and real-space perturbation theories. The two methods unanimously favor $\mathbf{q}=0$ over $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ magnetic order in a wide range of the anisotropy parameter $0\ensuremath{\le}\mathrm{\ensuremath{\Delta}}\ensuremath{\lesssim}0.72$. Both approaches are also in accord on the magnitude of the quantum order-by-disorder effect generated by topologically nontrivial, looplike spin-flip processes. A tentative $S\text{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}$ phase diagram of the model is proposed.

We study the finite-temperature transition in a model $XY$ antiferromagnet on a pyrochlore lattice, which describes the pyrochlore material ${\mathrm{Er}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. The ordered magnetic structure selected by thermal fluctuations is sixfold degenerate. Nevertheless, our classical Monte Carlo simulations show that the critical behavior corresponds to the three-dimensional $XY$ universality class. We determine an additional critical exponent ${\ensuremath{\nu}}_{6}=0.75>\ensuremath{\nu}$ characteristic of a dangerously irrelevant scaling variable. Persistent thermal fluctuations in the ordered phase are revealed in Monte Carlo simulations by the peculiar coexistence of Bragg peaks and diffuse magnetic scattering, the feature also observed in neutron diffraction experiments.

We observe a disappearance of the 1/3 magnetization plateau and a striking change of the magnetic configuration under a moderate doping of the model triangular antiferromagnet RbFe(MoO_{4})_{2}. The reason is an effective lifting of degeneracy of mean-field ground states by a random potential of impurities, which compensates, in the low-temperature limit, the fluctuation contribution to free energy. These results provide a direct experimental confirmation of the fluctuation origin of the ground state in a real frustrated system. The change of the ground state to a least collinear configuration reveals an effective positive biquadratic exchange provided by the structural disorder. On heating, doped samples regain the structure of a pure compound, thus allowing for an investigation of the remarkable competition between thermal and structural disorder.

The tensor cross interpolation (TCI) algorithm is a rank-revealing algorithm for decomposing low-rank, high-dimensional tensors into tensor trains/matrix product states (MPS). TCI learns a compact MPS representation of the entire object from a tiny training data set. Once obtained, the large existing MPS toolbox provides exponentially fast algorithms for performing a large set of operations. We discuss several improvements and variants of TCI. In particular, we show that replacing the cross interpolation by the partially rank-revealing LU decomposition yields a more stable and more flexible algorithm than the original algorithm. We also present two open source libraries, xfac in Python/C++ and TensorCrossInterpolation.jl in Julia, that implement these improved algorithms, and illustrate them on several applications. These include sign-problem-free integration in large dimension, the “superhigh-resolution” quantics representation of functions, the solution of partial differential equations, the superfast Fourier transform, the computation of partition functions, and the construction of matrix product operators.

Abstract GeSn alloys are promising materials for CMOS-compatible mid-infrared lasers manufacturing. Indeed, Sn alloying and tensile strain can transform them into direct bandgap semiconductors. This growing laser technology however suffers from a number of limitations, such as poor optical confinement, lack of strain, thermal, and defects management, all of which are poorly discussed in the literature. Herein, a specific GeSn-on-insulator (GeSnOI) stack using stressor layers as dielectric optical claddings is demonstrated to be suitable for a monolithically integration of planar Group-IV semiconductor lasers on a versatile photonic platform for the near- and mid-infrared spectral range. Microdisk-shape resonators on mesa structures were fabricated from GeSnOI, after bonding a Ge 0.9 Sn 0.1 alloy layer grown on a Ge strain-relaxed-buffer, itself on a Si(001) substrate. The GeSnOI microdisk mesas exhibited significantly improved optical gain as compared to that of conventional suspended microdisk resonators formed from the as-grown layer. We further show enhanced vertical out-coupling of the disk whispering gallery mode in-plane radiation, with up to 30% vertical out-coupling efficiency. As a result, the GeSnOI approach can be a valuable asset in the development of silicon-based mid-infrared photonics that combine integrated sources in a photonic platform with complex lightwave engineering.

We investigate field-induced transformations in the dynamical response of the XXZ model on the triangular lattice that are associated with the anharmonic magnon coupling and decay phenomena. Detailed theoretical predictions are made for ${\mathrm{Ba}}_{3}{\mathrm{CoSb}}_{2}{\mathrm{O}}_{9}$, which provides a close realization of the spin-$\frac{1}{2}$ XXZ model. We demonstrate that dramatic modifications in the magnon spectrum must occur in low out-of-plane fields that are easily achievable for this material. The hallmark of the effect is a coexistence of the clearly distinct well-defined magnon excitations with significantly broadened ones in different regions of the $\mathbf{k}\ensuremath{-}\ensuremath{\omega}$ space. The field-induced decays are generic for this class of models and become more prominent at larger anisotropies and in higher fields.

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We study a class of continuous spin models with bond disorder including the kagome Heisenberg antiferromagnet. For weak disorder strength, we find discrete ground states whose number grows exponentially with system size. These states do not exhibit zero-energy excitations characteristic of highly frustrated magnets but instead are local minima of the energy landscape. This represents a spin liquid version of the phenomenon of jamming familiar from granular media and structural glasses. Correlations of this jammed spin liquid, which upon increasing the disorder strength gives way to a conventional spin glass, may be algebraic (Coulomb type) or exponential.

Owing to ever increasing gate fidelities and to a potential transferability to industrial CMOS technology, silicon spin qubits have become a compelling option in the strive for quantum computation. In a scalable architecture, each spin qubit will have to be finely tuned and its operating conditions accurately determined. In this prospect, spectroscopic tools compatible with a scalable device layout are of primary importance. Here we report a two-tone spectroscopy technique providing access to the spin-dependent energy-level spectrum of a hole double quantum dot defined in a split-gate silicon device. A first GHz-frequency tone drives electric-dipole spin resonance enabled by the valence-band spin-orbit coupling. A second lower-frequency tone (approximately 500 MHz) allows for dispersive readout via rf-gate reflectometry. We compare the measured dispersive response to the linear response calculated in an extended Jaynes-Cummings model and we obtain characteristic parameters such as g-factors and tunnel/spin-orbit couplings for both even and odd occupation.

We demonstrate the existence of the giant proximity magnetoresistance (PMR) effect in a graphene spin valve where spin polarization is induced by a nearby magnetic insulator. PMR calculations were performed for yttrium iron garnet (YIG), cobalt ferrite (CFO), and two europium chalcogenides EuO and EuS. We find significant PMR (up to 100%) values defined as a relative change of graphene conductance with respect to parallel and antiparallel alignment of two proximity-induced magnetic regions within graphene. Namely, for high Curie temperature (${T}_{C}$) CFO and YIG insulators, which are particularly important for applications, we obtain 22% and 77% at room temperature, respectively. For low ${T}_{C}$ chalcogenides, EuO and EuS, the PMR is 100% in both cases. Furthermore, the PMR is robust with respect to system dimensions and edge-type termination, and it even maintains significant values (around 50% for YIG) in the presence of considerable spin-orbit coupling strength. Our findings show that it is possible to induce spin-polarized currents in graphene with no direct injection through magnetic materials.

We propose a novel “hourglass”-shaped design for highly efficient generation and collection of quantum light. The design features a quantum dot in a photonic nanowire sandwiched between tapered Bragg reflectors. For a Purcell factor of 9, the design features a spontaneous emission coupling of 0.993 to the cavity mode enabled by the strong dielectric screening of radiation modes. Thanks to a highly reflecting bottom mirror, we furthermore demonstrate a collection efficiency of 0.95 to a Gaussian profile. Finally, this photonic structure features a broad operation bandwidth, as large as 11 nm.

The ability to explain and predict the behavior in magnetic field is essential for any possible application of multiferroic materials. Motivated by the complex phase diagrams of MnWO${}_{4}$ and other spiral multiferroics, the authors formulate and investigate the anisotropic next-nearest-neighbor Heisenberg (ANNNH) model, which is a generalization of the celebrated ANNNI model to three component quantum spins. Using real-space mean-field simulations allowed for an unbiased construction of the magnetic phase diagram with multiple commensurate and incommensurate states. In particular, the biaxial anisotropy can explain the puzzling nonferroic reentrant phase of MnWO${}_{4}$ at high magnetic fields. The employed numerical technique is capable of predicting the temperature and field dependence of the ordering wave vector produced by the competition between helicity and anisotropy. An important issue raised in the present study is the effect of quantum fluctuations on the devil's staircase known to exist for the ANNNI model, but which is replaced by a ``floating'' incommensurate phase for the ANNNH model.

Compositionally graded Fe–Pt thin films were prepared on stationary 100 mm Si substrates by magnetron sputtering a base target of Fe on which a piece of Pt is asymmetrically positioned. Energy dispersive X-ray analysis was used to map the variation in film composition across the substrate, as a function of the size of the Pt piece. A scanning polar Magneto-Optical-Kerr-Effect system was used to probe the influence of composition and post-deposition annealing conditions (temperature and time) on coercivity. In this way, the maximum coercivity achievable for the sputtering system used could be established in a high throughput fashion. The evolution in coercivity with composition was correlated with the formation of L10 FePt and changes in its lattice parameters, as determined by scanning X-ray diffraction. High throughput coercivity mapping was then carried out on homogeneous Fe–Pt thin films of different compositions treated to different annealing conditions. This study serves as a step towards the integration of coercive FePt films into collectively fabricated devices.

This letter presents an 8-b differential current steering digital-to-analog converter (DAC) for the cryogenic front-end of future quantum computers. Recently published characterization of CMOS technology reveals the deterioration of transistor matching properties at cryogenic temperatures. The preliminary study of the current mismatch in the FDSOI technology at 4.2 K allows proper sizing of the 255 current-source unit cells to mitigate nonlinearities and optimize the static DAC performance. Based on this study, we minimize the power consumption while maintaining the targeted nonlinearity, i.e., 7.3 μW for DNL = 0.64 LSB. With a 6.6-mV output range and 26-μV voltage step, our DAC is compatible with the foreseen requirements for line-grain biasing a Si-based qubit matrix at an expected 100-MS/s on-chip output drive.

The mineral linarite, PbCuSO_{4}(OH)_{2}, is a spin-1/2 chain with frustrating nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic exchange interactions. Our inelastic neutron scattering experiments performed above the saturation field establish that the ratio between these exchanges is such that linarite is extremely close to the quantum critical point between spin-multipolar phases and the ferromagnetic state. We show that the predicted quantum multipolar phases are fragile and actually suppressed by a tiny orthorhombic exchange anisotropy and weak interchain interactions in favor of a dipolar fan phase. Including this anisotropy in classical simulations of a nearly critical model explains the field-dependent phase sequence of the phase diagram of linarite, its strong dependence of the magnetic field direction, and the measured variations of the wave vector as well as the staggered and the uniform magnetizations in an applied field.

Using ${\mathrm{Er}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ as motivation, we investigate finite-field properties of $XY$ pyrochlore antiferromagnets. In addition to a fluctuation-induced six-fold anisotropy present in zero field, an external magnetic field induces a combination of two-, three-, and six-fold clock terms as a function of its orientation, providing for rich and controllable magnetothermodynamics. For ${\mathrm{Er}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, we predict a phase transition in a weak magnetic field $\mathbf{H}\ensuremath{\parallel}[001]$. Re-entrant transitions are also found for $\mathbf{H}\ensuremath{\parallel}[111]$. We extend these results to the whole family the $XY$ pyrochlore antiferromagnets and show that presence and number of low-field transitions for different orientations can be used for locating a given material in the parameter space of anisotropic pyrochlores. Finite-temperature classical Monte Carlo simulations serve to confirm and illustrate these analytic predictions.

Abstract. We theoretically investigate the Heisenberg antiferromagnet on a triangular lattice doped with nonmagnetic impurities. Two nontrivial effects resulting from collective impurity behavior are predicted. The first one is related to presence of uncompensated magnetic moments localized near vacancies as revealed by the low-temperature Curie tail in the magnetic susceptibility. These moments exhibit an anomalous growth with the impurity concentration, which we attribute to the clustering mechanism. In an external magnetic field, impurities lead to an even more peculiar phenomenon lifting the classical ground-state degeneracy in favor of the conical state. We analytically demonstrate that vacancies spontaneously generate a positive biquadratic exchange, which is responsible for the above degeneracy lifting. 1.

We review the ferromagnetic superconductivity observed in the uranium based compounds, namely UGe 2 , URhGe and UCoGe, where the spin-triplet state is most likely realized. An unusual upper critical field <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>H</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">c2</mml:mi> </mml:mrow> </mml:msub> </mml:math> , which is enhanced under a magnetic field in a certain field direction, is discussed in terms of spin fluctuations and of Fermi surface instabilities.

We study the low temperature static and dynamical properties of the classical bond-disordered antiferromagnetic Heisenberg model on the kagome lattice. This model has recently been shown to host a new type of spin liquid exhibiting an exponentially large number of discrete ground states. Surprisingly, despite the rigidity of the ground states, we establish the vanishing of the corresponding spin stiffness. Locally, the low-lying eigenvectors of the Hessian appear to exhibit a fractal inverse participation ratio. Its spin dynamics resembles that of Coulomb Heisenberg spin liquids but exhibits a new low-temperature dynamically arrested regime, which however gets squeezed out with increasing system size. We also probe the properties of the energy landscape underpinning this behavior and find energy barriers between distinct ground states vanishing with system size. In turn the local minima appear highly connected and the system tends to lose memory of its initial state in an accumulation of soft directions.

The kagome metal CsV[Formula: see text]Sb[Formula: see text] is an ideal platform to study the interplay between topology and electron correlation. To understand the fermiology of CsV[Formula: see text]Sb[Formula: see text], intensive quantum oscillation (QO) studies at ambient pressure have been conducted. However, due to the Fermi surface reconstruction by the complicated charge density wave (CDW) order, the QO spectrum is exceedingly complex, hindering a complete understanding of the fermiology. Here, we directly map the Fermi surface of the pristine CsV[Formula: see text]Sb[Formula: see text] by measuring Shubnikov-de Haas QOs up to 29 T under pressure, where the CDW order is completely suppressed. The QO spectrum of the pristine CsV[Formula: see text]Sb[Formula: see text] is significantly simpler than the one in the CDW phase, and the detected oscillation frequencies agree well with our density functional theory calculations. In particular, a frequency as large as 8,200 T is detected. Pressure-dependent QO studies further reveal a weak but noticeable enhancement of the quasiparticle effective masses on approaching the critical pressure where the CDW order disappears, hinting at the presence of quantum fluctuations. Our high-pressure QO results reveal the large, unreconstructed Fermi surface of CsV[Formula: see text]Sb[Formula: see text], paving the way to understanding the parent state of this intriguing metal in which the electrons can be organized into different ordered states.