Scientific Highlights 2012

The strong-leg S=1/2 Heisenberg spin ladder system (C₇H₁₀N)₂CuBr₄ is investigated using density matrix renormalization group calculations, inelastic neutron scattering, and bulk magnetothermodynamic measurements. Measurements showed qualitative differences compared to the strong-rung case. A long- lived two-triplon bound state is confirmed to persist across most of the Brillouin zone in a zero field. In applied fields, in the Tomonaga-Luttinger spin-liquid phase, elementary excitations are attractive, rather than repulsive. In the presence of weak interladder interactions, the strong-leg system is considerably more prone to three-dimensional ordering.
 

It is widely believed that magnetic excitations become increasingly incoherent as the temperature is raised due to random collisions which limit their lifetime. This picture is based on spin-wave calculations for gapless magnets in 2 and 3 dimensions and is observed experimentally as a symmetric Lorentzian broadening in energy. Here, we investigate a three-dimensional dimer antiferromagnet and find un- expectedly that the broadening is asymmetric—indicating that far from thermal decoherence, the excitations behave collectively like a strongly correlated gas. This result suggests that a temperature activated coherent state of quasiparticles is not confined to special cases like the highly dimerized spin-1/2 chain but is found generally in dimerized antiferromagnets of all dimensionalities and perhaps gapped magnets in general.
 

We report inelastic neutron scattering measurements on Na₂IrO₃, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T_N = 15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90° Ir-O-Ir bonds required for dominant Kitaev exchange.
 

The influence of bond randomness on long-range magnetic ordering in the weakly coupled S = 1/2 antiferromagnetic spin chain materials Cu(py)₂ (Cl_1−xBr_x)₂ is studied by muon spin rotation and bulk measurements. Disorder is found to have a strong effect on the ordering temperature T_N, and an even stronger one on the saturation magnetization m₀, but considerably more so in the effectively lower-dimensional Br-rich materials. The observed behavior is attributed to random singlet ground states of individual spin chains, but remains in contradiction with chain mean-field theory [Joshi and Yang, Phys. Rev. B 67, 174403 (2003)] predictions. In this context, we discuss the possibility of a universal distribution of ordered moments in the weakly coupled random singlet chains model.
 

We report the observation of weak magnetism in superlattices of LaAlO₃/SrTiO₃ using β-detected nuclear magnetic resonance. The spin lattice relaxation rate of 8Li in superlattices with a spacer layers of 8 and 6 unit cells of LaAlO₃ exhibits a strong peak near ∼ 35 K, whereas no such peak is observed in a superlattice with spacer layer thickness of 3 unit cells. We attribute the observed temperature dependence to slowing down of weakly coupled electronic moments at the LaAlO₃/SrTiO₃ interface. These results show that the magnetism at the interface depends strongly on the thickness of the spacer layer, and that a minimal thickness of ∼ 4–6 unit cells is required for the appearance of magnetism. A simple model is used to determine that the observed relaxation is due to small fluctuating moments (∼0.002μ_B) in the two samples with a larger LaAlO₃ spacer thickness.

Magnets built of molecular rings of magnetic ions are fundamental model systems for studying the complex correlations and dynamics of quantum spins at the atomic scale. A new generation of neutron spectrometers can reveal complete four-dimensional maps of the spin correlations in spin rings.

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe₂O₅Br is surprisingly found to persist down to 53(3) mK (T/T_N ≈ 1/200), according to neutron scattering and muon spin relaxation. In this state, finite spin fluctuations at T → 0 are evidenced by muon depolarization, which is in agreement with specific-heat data indicating the presence of both gapless and gapped excitations. We thus show that the amplitude-modulated magnetic order intrinsically accommodates contradictory persistent spin dynamics and long-range order and can serve as a model structure to investigate their coexistence.

Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac susceptibility, specific heat, and muon-spin relaxation (μSR). Short-range static magnetism occurs below the freezing temperature T_g ≈ 0.7  K, which prevents the system from accessing a putative ferromagnetic quantum critical point. In the μSR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms.

BiCu₂PO₆ is a frustrated two-leg spin-ladder compound with a spin gap that can be closed with a magnetic field of approximately 20 T. This quantum phase transition and its related phase diagram as a function of magnetic field and temperature (H, T) are investigated up to 60 T by means of specific heat, magnetocaloric effect, magnetization, and magnetostriction measurements. In contrast to other gapped quantum magnets, BiCu₂PO₆ undergoes a series of unexpected first- and second-order phase transitions when an external magnetic field is applied along the crystallographic c axis. The application of a magnetic field along the b axis induces two second-order phase transitions. We propose that the anisotropy and complex phase diagram result from the interplay between strong geometrical frustration and spin-orbit interaction necessary for the description of this fascinating magnetic system.

Magnetic susceptibility, NMR, muon spin relaxation, and inelastic neutron scattering measurements show that kapellasite, Cu₃Zn(OH)₆Cl₂, a geometrically frustrated spin-1/2 kagome antiferromagnet polymorphic with herbertsmithite, is a gapless spin liquid showing unusual dynamic short-range correlations of noncoplanar cuboc2 type which persist down to 20 mK. The Hamiltonian is determined from a fit of a high-temperature series expansion to bulk susceptibility data and possesses competing exchange interactions. The magnetic specific heat calculated from these exchange couplings is in good agreement with experiment. The temperature dependence of the magnetic structure factor and the muon relaxation rate are calculated in a Schwinger-boson approach and compared to experimental results.

The exceptional nature of the rht-MOF platform, based on a singular edge-transitive net (the only net for the combination of 3- and 24-connected nodes), makes it an ideal target in crystal chemistry. The high level of control indicates an unparalleled blueprint for isoreticular functional materials (without concern for interpenetration) for targeted applications.

We present local probe results on the honeycomb lattice antiferromagnet Ba₃CuSb₂O₉. Muon spin relaxation measurements in a zero field down to 20 mK show unequivocally that there is a total absence of spin freezing in the ground state. Sb NMR measurements allow us to track the intrinsic susceptibility of the lattice, which shows a maximum at around 55 K and drops to zero in the low-temperature limit. The spin-lattice relaxation rate shows two characteristic energy scales, including a field-dependent crossover to exponential low-temperature behavior, implying gapped magnetic excitations.

The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating SrTiO₃ and LaAlO₃ have remained a contentious subject since its discovery in 2004. Opinion is divided between an intrinsic mechanism involving the build-up of an internal electric potential due to the polar discontinuity at the interface between SrTiO₃ and LaAlO₃, and extrinsic mechanisms attributed to structural imperfections. Here we show that interface conductivity is also exhibited when the LaAlO₃ layer is diluted with SrTiO₃, and that the threshold thickness required to show conductivity scales inversely with the fraction of LaAlO₃ in this solid solution, and thereby also with the layer's formal polarization. These results can be best described in terms of the intrinsic polar-catastrophe model, hence providing the most compelling evidence, to date, in favour of this mechanism.

Cuprates and other high-temperature superconductors consist of two-dimensional layers that are crucial to their properties. The dynamics of the quantum spins in these layers lie at the heart of the mystery of the cuprates. In bulk cuprates such as La2CuO4, the presence of a weak coupling between the two-dimensional layers stabilizes a three-dimensional magnetic order up to high temperatures. In a truly two-dimensional system however, thermal spin fluctuations melt long-range order at any finite temperature. Here, we measure the spin response of isolated layers of La2CuO4 that are only one-unit-cell-thick. We show that coherent magnetic excitations, magnons, known from the bulk order, persist even in a single layer of La2CuO4, with no evidence for more complex correlations such as resonating valence bond correlations. These magnons are, therefore, well described by spin-wave theory (SWT). On the other hand, we also observe a high-energy magnetic continuum in the isotropic magnetic response that is not well described by two-magnon SWT, or indeed any existing theories.

We present measurements of the magnetic properties of thin film TbPc₂ single-molecule magnets evaporated on a gold substrate and compare them to those in bulk. Zero-field muon spin relaxation measurements were used to determine the molecular spin fluctuation rate of TbPc2 as a function of temperature. At low temperature, we find that the fluctuations in films are much faster than in bulk and depend strongly on the distance between the molecules and the Au substrate. We measure a molecular spin correlation time that varies between 1.4 μs near the substrate and 6.6 μs far away from it. We attribute this behavior to differences in the packing of the magnetic cores, which change gradually on the scale of 10–20 nm away from the TbPc₂/Au interface.

We investigate the low-temperature state of the rare-earth pyrochlore Tb₂Ti₂O₇ using polarized neutron scattering. Tb₂Ti₂O₇ is often described as an antiferromagnetic spin liquid with spin correlations extending over lengths comparable to individual tetrahedra of the pyrochlore lattice. We confirm this picture at 20 K but find that at 0.05 K the data contain evidence of pinch-point scattering, suggesting that the low temperature state of Tb₂Ti₂O₇ has power-law spin correlations.

We have explored the spin liquid state in Tb₂Ti₂O₇ with vibrating-coil magnetometry down to ∼0.04  K under magnetic fields up to 5 T. We observe magnetic history dependence below T^* ∼ 0.2 K reminiscent of the classical spin ice systems Ho₂Ti₂O₇ and Dy₂Ti₂O₇. The magnetic phase diagram inferred from the magnetization is essentially isotropic, without evidence of magnetization plateaus as anticipated for so-called quantum spin ice, predicted theoretically for [111] when quantum fluctuations renormalize the interactions. Instead, the magnetization for T << T^* agrees semiquantitatively with the predictions of “all-in–all-out” (AIAO) antiferromagnetism. Taken together, this suggests that the spin liquid state in Tb₂Ti₂O₇ is akin to an incipient AIAO antiferromagnet.

Mechanochemical synthesis using CeCl₃-MBH₄ (M = Li, Na or K) mixtures are investigated and produced a new compound, LiCe(BH₄)₃Cl, which crystallizes in a cubic space group I43 m, a = 11.7204(2) Å. The structure contains isolated tetranuclear anionic clusters [Ce₄Cl₄(BH₄)₁₂]⁴⁻ with a distorted cubane Ce₄Cl₄ core, charge-balanced by Li⁺ cations. Each Ce atom coordinates three chloride ions and three borohydride groups via the η³−BH₃ faces, thus completing the coordination environment to an octahedron. Combination of synchrotron radiation powder X-ray diffraction (SR-PXD), powder neutron diffraction and density functional theory (DFT) optimization show that Li cations are disordered, occupying 2/3 of the 12d Wyckoff site. DFT calculation indicates that LiCe(BH₄)₃Cl is stabilized by higher entropy rather than lower enthalpy, in accord with the disorder in Li positions. The structural model also agrees well with the very high lithium ion conductivity measured for LiCe(BH₄)₃Cl of 1 ⋅ 10⁻⁴ Scm⁻¹ at T = 20 °C. In situ SR-PXD reveals that the decomposition products consist of LiCl, CeB₆ and CeH₂. The Sieverts measurements show that 4.7 wt % H₂ is released during heating to 500 °C. After rehydrogenation at 400 °C and p(H₂) = 100 bar for 24 h an amount of 1.8 wt % H₂ is released in the second dehydrogenation. The ¹¹B MAS NMR spectra of the central and satellite transitions for LiCe(B(D/H)₄)₃Cl reveal highly asymmetric manifolds of spinning sidebands from a single ¹¹B site, reflecting dipolar couplings of the ¹¹B nuclear spin with the paramagnetic electron spin of the Ce³⁺ ions.
 

Nowadays, all diesel motor vehicles are fitted with a particulate filter as standard, as part of the Euro5 Emission Standard. These filters prevent the harmful soot and ash particles in exhaust gases from entering the environment. However, within the automotive industry, exactly how the soot particles are deposited inside these filters has not been known. Now, using a special imaging technique - Neutron Tomography - researchers at the Paul Scherrer Institute have made the soot inside filters visible, creating a foundation from which these filters can be optimised and developed further.

We studied phase separation in the single-crystalline antiferromagnetic superconductor Rb₂Fe₄Se₅ (RFS) using a combination of scattering-type scanning near-field optical microscopy and low-energy muon spin rotation (LE-μSR). We demonstrate that the antiferromagnetic and superconducting phases segregate into nanometer-thick layers perpendicular to the iron-selenide planes, while the characteristic in-plane size of the metallic domains reaches 10 μm. By means of LE-μSR we further show that in a 40-nm thick surface layer the ordered antiferromagnetic moment is drastically reduced, while the volume fraction of the paramagnetic phase is significantly enhanced over its bulk value. Self-organization into a quasiregular heterostructure indicates an intimate connection between the modulated superconducting and antiferromagnetic phases.

The magnetoelectric (ME) coupling on spin-wave resonances in single-crystal Cu₂OSeO₃ was studied by a novel technique using electron spin resonance combined with electric field modulation. An external electric field E induces a magnetic field component μ₀Hⁱ = γ E along the applied magnetic field H with γ = 0.7(1) μT/(v/mm) at 10 K. The ME coupling strength γ is found to be temperature dependent and highly anisotropic. γ(T) nearly follows that of the spin susceptibility J^M(T) and rapidly decreases above the Curie temperature T_c. The ratio γ/J^M monotonically decreases with increasing temperature without an anomaly at T_c.

We report on superconductivity in CeFeAs_1−xP_xO and the possible coexistence with Ce ferromagnetism (FM) in a small homogeneity range around x=30% with ordering temperatures of T_SC≅T_C≅4 K. The antiferromagnetic (AFM) ordering temperature of Fe at this critical concentration is suppressed to T_N^Fe≈40 K and does not shift to lower temperatures with a further increase of the P concentration. Therefore, a quantum-critical-point scenario with T_N^Fe→0 K which is widely discussed for the iron based superconductors can be excluded for this alloy series. Surprisingly, thermal expansion and x-ray powder diffraction indicate the absence of an orthorhombic distortion despite clear evidence for short-range AFM Fe ordering from muon-spin-rotation measurements. Furthermore, we discovered the formation of a sharp electron spin resonance signal unambiguously connected with the emergence of FM ordering.

Superconductivity in the iron pnictides develops near antiferromagnetism, and the antiferromagnetic (AF) phase appears to overlap with the superconducting phase in some materials such as BaFe_2-xNi_xAs₂ (where T=Co or Ni). Here we use neutron scattering to demonstrate that genuine long-range AF order and superconductivity do not coexist in BBaFe_2-xNi_xAs₂ near optimal superconductivity. In addition, we find a first-order-like AF-to-superconductivity phase transition with no evidence for a magnetic quantum critical point. Instead, the data reveal that incommensurate short-range AF order coexists and competes with superconductivity, where the AF spin correlation length is comparable to the superconducting coherence length.

Magnetism has been predicted to occur in systems in which dipolar interactions dominate exchange. We present neutron scattering, specific heat, and magnetic susceptibility data for LiErF₄, establishing it as a model dipolar-coupled antiferromagnet with planar spin-anisotropy and a quantum phase transition in applied field H_c∥ = 4.0 ± 0.1 kilo-oersteds. We discovered non–mean-field critical scaling for the classical phase transition at the antiferromagnetic transition temperature that is consistent with the two-dimensional XY/h₄ universality class; in accord with this, the quantum phase transition at H_c exhibits three-dimensional classical behavior. The effective dimensional reduction may be a consequence of the intrinsic frustrated nature of the dipolar interaction, which strengthens the role of fluctuations.

We report β-NMR investigations of polarized ⁸Li implanted in thin Pb and Ag/Nb films. At the critical superconducting temperature, we observe a singular peak in the spin relaxation rate in small longitudinal magnetic fields, which is attributed to unexpected slow fluctuations in the superconducting order parameter. The peak is several orders of magnitude larger than the prediction based on the enhancement of the dynamic electron spin susceptibility by superconducting fluctuations. The observed peak in 1/T₁ is rapidly suppressed in a small magnetic field, indicating that it is due to remarkably slow diamagnetic fluctuations which are undetectable with conventional NMR.

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa₂Cu₃O₇ and ferromagnetic-metallic La_0.67Ca_0.33MnO₃ or ferromagnetic-insulating LaMnO_3+δ. We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La_0.67Ca_0.33MnO₃ and almost absent for ferromagnetic-insulating LaMnO_3+δ. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.

We report on muon spin rotation studies of the noncentrosymmetric heavy fermion antiferromagnet CeRhSi₃. A drastic and monotonic suppression of the internal fields, at the lowest measured temperature, was observed upon an increase of external pressure. Our data suggest that the ordered moments are gradually quenched with increasing pressure, in a manner different from the pressure dependence of the Neel temperature. At 23.6 kbar, the ordered magnetic moments are fully suppressed via a second-order phase transition, and T_N is zero. Thus, we directly observed the quantum critical point at 23.6 kbar hidden inside the superconducting phase of CeRhSi₃.

We use small-angle neutron scattering to study the superconducting vortex lattice in La_2−xSr_xCuO₄ as a function of doping and magnetic field. We show that near optimally doping the vortex lattice coordination and the superconducting coherence length ξ are controlled by a Van Hove singularity crossing the Fermi level near the Brillouin zone boundary. The vortex lattice properties change dramatically as a spin-density-wave instability is approached upon underdoping. The Bragg glass paradigm provides a good description of this regime and suggests that spin-density-wave order acts as a source of disorder on the vortex lattice.

We report on muonium (Mu) emission into vacuum following μ⁺ implantation in mesoporous thin SiO₂ films. We obtain a yield of Mu into vacuum of (38±4)% at 250 K and (20±4)% at 100 K for 5 keV μ⁺ implantation energy. From the implantation energy dependence of the Mu vacuum yield we determine the Mu diffusion constants in these films: D_Mu^250K=(1.6±0.1)x10^-4  cm²/s and D_Mu^100K=(4.2±0.5)x10^-5  cm²/s. Describing the diffusion process as quantum mechanical tunneling from pore to pore, we reproduce the measured temperature dependence ∼T^3/2 of the diffusion constant. We extract a potential barrier of (-0.3±0.1) eV which is consistent with our computed Mu work function in SiO₂ of [-0.3,-0.9]  eV. The high Mu vacuum yield, even at low temperatures, represents an important step toward next generation Mu spectroscopy experiments.

Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy. Here we demonstrate a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S=1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two- to quasi-one-dimensional while effectively maintaining the nearest-neighbor coupling strength.

We report on the superconducting properties of A_xFe_2-ySe₂ (A=Rb, K) single crystals studied with the muon spin relaxation or rotation (μSR) technique. At low temperatures, close to 90% of the sample volumes exhibit large-moment magnetic order which impedes the investigation of their superconducting properties by μSR. On the other hand, about 10% of the sample volumes remain paramagnetic and clearly show a superconducting response. The temperature dependence of the superconducting carrier density was analyzed within the framework of a single s-wave gap scenario. The zero-temperature values of the in-plane magnetic penetration depths λ_ab(0)=258(2) and 225(2) nm and the superconducting gaps Δ(θ)=7.7(2) and 6.3(2) meV have been determined for A = Rb and K, respectively. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed.

The absolute value and temperature dependence of the in-plane magnetic penetration depth λ have been measured on a single crystal of Ba(Co_0.074Fe_0.926)₂As₂ using low-energy muon-spin rotation and microwave cavity perturbation. The magnetic field profiles in the Meissner state are consistent with a local London model beyond a depth of 15 nm. We determine the gap symmetry through measurements of the temperature dependence of the superfluid density which follows a two-gap s-wave model over the entire temperature range below Tc. While the intermediate to high temperature data is well fit by an energy gap model in the BCS-like (weak-coupling) limit, a second smaller gap becomes apparent at low temperatures.

We present small angle neutron scattering studies of the vortex lattice (VL) in CeCoIn5 with magnetic fields applied parallel (H) to the antinodal [100] and nodal [110] directions. For H || [100], a single VL orientation is observed, while a 90° reorientation transition is found for H || [110]. For both field orientations and VL configurations we find a distorted hexagonal VL with an anisotropy, Γ=2.0±0.05. The VL form factor shows strong Pauli paramagnetic effects similar to what have previously been reported for H || [001]. At high fields, above which the upper critical field (H_c2) becomes a first-order transition, an increased disordering of the VL is observed.

The olivine compound Mn₂GeO₄ is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity, and a canted commensurate order leads to weak ferromagnetism. Symmetry suggests that the direct coupling between the ferromagnetism and ferroelectricity is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multicomponent magnetic structures found in magnetically frustrated materials like Mn₂GeO₄ provide a new route towards functional materials that exhibit coupled ferromagnetism and ferroelectricity.

The flux-line lattice in CaAlSi has been studied by small-angle neutron scattering. A well-defined hexagonal flux-line lattice is seen just above Hc1 in an applied field of only 54 Oe. A 30° reorientation of this vortex lattice has been observed in a very low field of 200 Oe. This reorientation transition appears to be first-order and could be explained by nonlocal effects. The magnetic field dependence of the form factor is well-described by a single penetration depth of λ=1496(1)  Å and a single coherence length of ξ=307(1)  Å at 2 K. At 1.5 K, the penetration depth anisotropy is γλ=2.7(1), with the field applied perpendicular to the c axis, and agrees with the coherence length anisotropy determined from critical field measurements.