Laboratoire National des Champs Magnétiques Intenses

Coherent backscattering of waves by a disordered scattering medium is responsible for weak localization. We have directly observed this effect for the first time using visible light and concentrated aqueous suspensions of submicron-size polystyrene spheres. The scattered intensity is found enhanced by up to 75% within a narrow cone centered at the backscattering direction. As predicted by theory, the aperture of the cone is inversely proportional to the light mean-free path; the latter was controlled by the concentration of spheres. The importance of light polarization and particle size is discussed.

We describe variations in the resistance of $\mathrm{Co}/\mathrm{Cu}$ multilayers, induced by means of a high current density $\ensuremath{\approx}{10}^{8}\mathrm{A}/{\mathrm{cm}}^{2}$ injected into the multilayer through a point contact. We propose that the observed resistance changes are due to excitations of zero-wave-number spin waves in the magnetic layers. As predicted, such current-driven excitation of a magnetic multilayer occurs for only one direction of current flow and has a current threshold which increases linearly with the applied magnetic field.

Abstract Thin films of Wo3 deposited on quartz substrates at room temperature have been shown to be amorphous in structure. The optical absorption spectra of the amorphous and crystalline films have been measured in the temperature range 110° to 500°K. The fundamental absorption edge of an amorphous film occurs at 3800 Å which on crystallization moves to 4500 Å. On the high-energy side of the absorption edge several absorption peaks are resolvable in both types of film. The frequency dependence of the absorption coefficient below 104 cm−1 is described by an expression of the form K (v, T) = K 0 exp[− (β/kT) (E 0 − hv)] and above 104 cm−1 it follows a square law dependency. The temperature coefficient of the band edges was found to be − 5.0 × 10−4 eV/°K and the estimated band gaps at 0°K were found to be 3.65 and 3.27 eV for the amorphous and crystalline films, respectively. The electrical conductivity of a thin film has been measured in the temperature range 298–573°K and the activation energy was found to be 1.04 eV. Irradiation within the fundamental absorption edge gives rise to photoconductivity. Threshold wavelengths for photoconductivity were observed at 3250 and 5500 Å for the amorphous and crystalline films, respectively. A broad colour-centre band having a maximum at 9100 Å and a shoulder at 1.6 μ has been observed on irradiating the amorphous film with wavelengths shorter than 3500 Å and also on applying an electric field of ∼ 104 V./cm. The colour centre, thus formed, shows a slight bleaching with light. However, it bleaches thermally and in presence of oxidizing atmosphere. The formation of colour-centres is associated with increased electrical conductivity of the film. No colouration is observed in fully oxidized samples of WO3. An energy level diagram has been proposed to account for the optical and electrical properties as well as the colour-centre formation in WO3 films.

The reduced effective mass (<italic>μ</italic>) and excitonic Rydberg (<italic>R</italic>*) are measured by magneto-optics for new perovskite semiconductors.

Multilayer epitaxial graphene is investigated using far infrared transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance-like absorption is observed at low temperature in magnetic fields below 50 mT, probing the nearest vicinity of the Dirac point. The carrier mobility is found to exceed 250,000 cm2/(V x s). In the limit of high temperatures, the well-defined Landau level quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unusual in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated.

We have investigated the dimensionality and origin of the magnetotransport properties of ${\mathrm{LaAlO}}_{3}$ films epitaxially grown on ${\mathrm{TiO}}_{2}$-terminated ${\mathrm{SrTiO}}_{3}\left(001\right)$ substrates. High-mobility conduction is observed at low deposition oxygen pressures (${P}_{\mathrm{O}2}&lt;{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{mbar}$) and has a three-dimensional character. However, at higher ${P}_{\mathrm{O}2}$ the conduction is dramatically suppressed and nonmetallic behavior appears. Experimental data strongly support an interpretation of these properties based on the creation of oxygen vacancies in the ${\mathrm{SrTiO}}_{3}$ substrates during the growth of the ${\mathrm{LaAlO}}_{3}$ layer. When grown on ${\mathrm{SrTiO}}_{3}$ substrates at low ${P}_{\mathrm{O}2}$, other oxides generate the same high mobility as ${\mathrm{LaAlO}}_{3}$ films. This opens interesting prospects for all-oxide electronics.

Far infrared transmission experiments are performed on ultrathin epitaxial graphite samples in a magnetic field. The observed cyclotron resonancelike and electron-positron-like transitions are in excellent agreement with the expectations of a single-particle model of Dirac fermions in graphene, with an effective velocity of $\stackrel{\texttildelow{}}{c}=1.03\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }\mathrm{m}/\mathrm{s}$.

Experimental support is found for the multiband model of the superconductivity in the recently discovered system ${\mathrm{MgB}}_{2}$ with the transition temperature ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}39\mathrm{K}$. By means of Andreev reflection, evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps ( ${\ensuremath{\Delta}}_{S}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.8\mathrm{meV}$ and ${\ensuremath{\Delta}}_{L}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}7\mathrm{meV}$) are, respectively, smaller and larger than the expected weak coupling value. Because of the temperature smearing of the spectra the two gaps are hardly distinguishable at elevated temperatures, but when a magnetic field is applied the presence of two gaps can be demonstrated close to the bulk ${T}_{c}$ in the raw data.

Implementation of modern Quantum Technologies might benefit from the remarkable quantum properties shown by molecular spin systems. In this Perspective, we highlight the role that molecular chemistry can have in the current second quantum revolution, i.e., the use of quantum physics principles to create new quantum technologies, in this specific case by means of molecular components. Herein, we briefly review the current status of the field by identifying the key advances recently made by the molecular chemistry community, such as for example the design of molecular spin qubits with long spin coherence and the realization of multiqubit architectures for quantum gates implementation. With a critical eye to the current state-of-the-art, we also highlight the main challenges needed for the further advancement of the field toward quantum technologies development.

Electrical conductors can be chiral, i.e., can exist in two forms where one is the other's mirror image. Thus far, no effect of chirality on magnetotransport has been observed. We argue that the electrical resistance of any chiral conductor should depend linearly both on the external magnetic field and the current through the conductor and on its handedness. We suggest two mechanisms to carry this effect and show experimentally on model systems that both are effective.

One of the leading issues in high-T(c) superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YB(2)Cu(3)O(6+) system. The observed magnetic order preserves translational symmetry of the lattice as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of a hidden order parameter characterizing the pseudogap phase in high-T(c) cuprates.

In R-Fe compounds, the magnetic properties are mainly determined by the Fe-Fe interatomic distances and the number of Fe nearest neighbors. Below the ordering temperatures, the R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</inf> compounds are generally ferromagnetic or ferrimagnetic colinear. The compounds with rare earth ions of small atomic radii, Ce <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</inf> , Tm <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</inf> , and Lu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</inf> , are exceptions; they are helimagnetic. At low temperatures they exhibit a transition either to a fan spin or to a ferrimagnetic colinear configuration. For all compounds, a negative thermal expansion is observed below their ordering temperatures. Negative interactions occuring mainly in the substitution zones are thus deduced. Varying strongly with distances, they lead to the observed anomalous thermal expansion, and being largest in the compounds with the shortest Fe-Fe distances, their helimagnetic structure is explained.

The presence or absence of a quantum critical point and its location in the phase diagram of high-temperature superconductors have been subjects of intense scrutiny. Clear evidence for quantum criticality, particularly in the transport properties, has proved elusive because the important low-temperature region is masked by the onset of superconductivity. We present measurements of the low-temperature in-plane resistivity of several highly doped La2-xSrxCuO4 single crystals in which the superconductivity had been stripped away by using high magnetic fields. In contrast to other quantum critical systems, the resistivity varies linearly with temperature over a wide doping range with a gradient that scales monotonically with the superconducting transition temperature. It is maximal at a critical doping level (pc) approximately 0.19 at which superconductivity is most robust. Moreover, its value at pc corresponds to the onset of quasi-particle incoherence along specific momentum directions, implying that the interaction that first promotes high-temperature superconductivity may ultimately destroy the very quasi-particle states involved in the superconducting pairing.

Single-molecule magnets display magnetic bistability of molecular origin, which may one day be exploited in magnetic data storage devices. Recently it was realised that increasing the magnetic moment of polynuclear molecules does not automatically lead to a substantial increase in magnetic bistability. Attention has thus increasingly focussed on ions with large magnetic anisotropies, especially lanthanides. In spite of large effective energy barriers towards relaxation of the magnetic moment, this has so far not led to a big increase in magnetic bistability. Here we present a comprehensive study of a mononuclear, tetrahedrally coordinated cobalt(II) single-molecule magnet, which has a very high effective energy barrier and displays pronounced magnetic bistability. The combined experimental-theoretical approach enables an in-depth understanding of the origin of these favourable properties, which are shown to arise from a strong ligand field in combination with axial distortion. Our findings allow formulation of clear design principles for improved materials.

Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest react and decompose in air, which has severely hindered their investigation and potential applications. Here we introduce a remedial approach based on cleavage, transfer, alignment, and encapsulation of air-sensitive crystals, all inside a controlled inert atmosphere. To illustrate the technology, we choose two archetypal two-dimensional crystals that are of intense scientific interest but are unstable in air: black phosphorus and niobium diselenide. Our field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, which is in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of experimentally accessible two-dimensional crystals and their heterostructures.

Abstract Suspensions of rod-like cellulose crystallites of axial ratio ≈ 20–40, prepared by acid hydrolysis of natural cellulose fibres with sulphuric acid, give stable ordered fluids that display well-formed textures and disclinations characteristic of chiral nematic liquid crystalline phases. The critical volume fraction for phase separation of salt-free suspensions is typically 0.03, with a relatively narrow biphasic region. Because of the negative diamagnetic susceptibility of cellulose, the ordered phase becomes oriented in a magnetic field with its chiral nematic axis parallel to the applied field.

The ground-state energy E and momentum distribution nk of the electrons are expanded from the atomic limit for the half-filled Hubbard model. The coefficients of expansion are represented by the spin correlation functions of the spin 1/2 Heisenberg model. Using the spin-wave theory, approximate values of coefficients are calculated for the square lattice and the simple cubic lattice. In one dimension, the theory shows a good agreement with the exact solution. An effective spin Hamiltonian for the half-filled Hubbard model with arbitrary hopping integrals is obtained up to the fifth order. It is shown that the fourth term contains four spin interactions.

We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications.

Using high-frequency EPR spectroscopy we have found that a cluster comprising eight iron(III) ions, Fe8, which is essentially flat, has a ground S = 10 state and an Ising-type anisotropy. For the first time both ac susceptibility and Mössbauer spectroscopy could be used in order to monitor the relaxation time of the magnetization, which was found to follow a thermally activated behavior, as in a superparamagnet, with τ0 = 1.9 × 10−7 s and an energy barrier of 22.2 K. The set of data allowed us to conclude that the origin of the anisotropy in nanosize molecular clusters is associated with the single ion contributions and not with the shape of the clusters.

Transport measurements are presented on a class of electrostatically defined lateral dots within a high mobility two-dimensional electron gas (2DEG). The design allows Coulomb blockade (CB) measurements to be performed on a single lateral dot containing 0, 1 to over 50 electrons. The CB measurements are enhanced by the spin polarized injection from and into 2DEG magnetic edge states. This combines the measurement of charge with the measurement of spin through spin-blockade spectroscopy. The results of Coulomb and spin-blockade spectroscopy for the first 45 electrons enable us to construct the addition spectrum of a lateral device. We also demonstrate that a lateral dot containing a single electron is an effective local probe of a 2DEG edge.