A quantum magnetic analogue to the critical point of water.

At the liquid-gas phase transition in water, the density has a discontinuity at atmospheric pressure; however, the line of these first-order transitions defined by increasing the applied pressure terminates at the critical point1, a concept ubiquitous in statistical thermodynamics2. In correlated quantum materials, it was predicted3 and then confirmed experimentally4,5 that a critical point terminates the line of Mott metal-insulator transitions, which are also first-order with a discontinuous charge carrier density. In quantum spin systems, continuous quantum phase transitions6 have been controlled by pressure7,8, applied magnetic field9,10 and disorder11, but discontinuous quantum phase transitions have received less attention. The geometrically frustrated quantum antiferromagnet SrCu2(BO3)2 constitutes a near-exact realization of the paradigmatic Shastry-Sutherland model12-14 and displays exotic phenomena including magnetization plateaus15, low-lying bound-state excitations16, anomalous thermodynamics17 and discontinuous quantum phase transitions18,19. Here we control both the pressure and the magnetic field applied to SrCu2(BO3)2 to provide evidence of critical-point physics in a pure spin system. We use high-precision specific-heat measurements to demonstrate that, as in water, the pressure-temperature phase diagram has a first-order transition line that separates phases with different local magnetic energy densities, and that terminates at an Ising critical point. We provide a quantitative explanation of our data using recently developed finite-temperature tensor-network methods17,20-22. These results further our understanding of first-order quantum phase transitions in quantum magnetism, with potential applications in materials where anisotropic spin interactions produce the topological properties23,24 that are useful for spintronic applications.

Thermal Evolution of Dirac Magnons in the Honeycomb Ferromagnet CrBr_{3}.

CrBr_{3} is an excellent realization of the two-dimensional honeycomb ferromagnet, which offers a bosonic equivalent of graphene with Dirac magnons and topological character. We perform inelastic neutron scattering measurements using state-of-the-art instrumentation to update 50-year-old data, thereby enabling a definitive comparison both with recent experimental claims of a significant gap at the Dirac point and with theoretical predictions for thermal magnon renormalization. We demonstrate that CrBr_{3} has next-neighbor J_{2} and J_{3} interactions approximately 5% of J_{1}, an ideal Dirac magnon dispersion at the K point, and the associated signature of isospin winding. The magnon lifetime and the thermal band renormalization show the universal T^{2} evolution expected from an interacting spin-wave treatment, but the measured dispersion lacks the predicted van Hove features, pointing to the need for more sophisticated theoretical analysis.

Thermal Control of Spin Excitations in the Coupled Ising-Chain Material RbCoCl_{3}.

We have used neutron spectroscopy to investigate the spin dynamics of the quantum (S=1/2) antiferromagnetic Ising chains in RbCoCl_{3}. The structure and magnetic interactions in this material conspire to produce two magnetic phase transitions at low temperatures, presenting an ideal opportunity for thermal control of the chain environment. The high-resolution spectra we measure of two-domain-wall excitations therefore characterize precisely both the continuum response of isolated chains and the "Zeeman-ladder" bound states of chains in three different effective staggered fields in one and the same material. We apply an extended Matsubara formalism to obtain a quantitative description of the entire dataset, Monte Carlo simulations to interpret the magnetic order, and finite-temperature density-matrix renormalization-group calculations to fit the spectral features of all three phases.

Multiple Magnetic Bilayers and Unconventional Criticality without Frustration in BaCuSi_{2}O_{6}.

The dimerized quantum magnet BaCuSi_{2}O_{6} was proposed as an example of "dimensional reduction" arising near the magnetic-field-induced quantum critical point (QCP) due to perfect geometrical frustration of its interbilayer interactions. We demonstrate by high-resolution neutron spectroscopy experiments that the effective intrabilayer interactions are ferromagnetic, thereby excluding frustration. We explain the apparent dimensional reduction by establishing the presence of three magnetically inequivalent bilayers, with ratios 3∶2∶1, whose differing interaction parameters create an extra field-temperature scaling regime near the QCP with a nontrivial but nonuniversal exponent. We demonstrate by detailed quantum Monte Carlo simulations that the magnetic interaction parameters we deduce can account for all the measured properties of BaCuSi_{2}O_{6}, opening the way to a quantitative understanding of nonuniversal scaling in any modulated layered system.

One Proximate Kitaev Spin Liquid in the K-J-Γ Model on the Honeycomb Lattice.

In addition to the Kitaev (K) interaction, candidate Kitaev materials also possess Heisenberg (J) and off-diagonal symmetric (Γ) couplings. We investigate the quantum (S=1/2) K-J-Γ model on the honeycomb lattice by a variational Monte Carlo method. In addition to the "generic" Kitaev spin liquid (KSL), we find that there is just one proximate KSL (PKSL) phase, while the rest of the phase diagram contains different magnetically ordered states. The PKSL is a gapless Z_{2} state with 14 Majorana cones, which in contrast to the KSL has a gapless spin response. In a magnetic field applied normal to the honeycomb plane, it realizes two of Kitaev's gapped chiral spin-liquid phases, of which one is non-Abelian with Chern number ν=5 and the other is Abelian with ν=4. These two phases could be distinguished by their thermal Hall conductance.

Dirac and Chiral Quantum Spin Liquids on the Honeycomb Lattice in a Magnetic Field.

Motivated by recent experimental observations in α-RuCl_{3}, we study the K-Γ model on the honeycomb lattice in an external magnetic field. By a slave-particle representation and variational Monte Carlo calculations, we reproduce the phase transition from zigzag magnetic order to a field-induced disordered phase. The nature of this state depends crucially on the field orientation. For particular field directions in the honeycomb plane, we find a gapless Dirac spin liquid, in agreement with recent experiments on α-RuCl_{3}. For a range of out-of-plane fields, we predict the existence of a Kalmeyer-Laughlin-type chiral spin liquid, which would show an integer-quantized thermal Hall effect.

Thermal Critical Points and Quantum Critical End Point in the Frustrated Bilayer Heisenberg Antiferromagnet.

We consider the finite-temperature phase diagram of the S=1/2 frustrated Heisenberg bilayer. Although this two-dimensional system may show magnetic order only at zero temperature, we demonstrate the presence of a line of finite-temperature critical points related to the line of first-order transitions between the dimer-singlet and -triplet regimes. We show by high-precision quantum Monte Carlo simulations, which are sign-free in the fully frustrated limit, that this critical point is in the Ising universality class. At zero temperature, the continuous transition between the ordered bilayer and the dimer-singlet state terminates on the first-order line, giving a quantum critical end point, and we use tensor-network calculations to follow the first-order discontinuities in its vicinity.

Giant Pressure Dependence and Dimensionality Switching in a Metal-Organic Quantum Antiferromagnet.

We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [CuF_{2}(H_{2}O)_{2}]_{2}pyrazine. At zero pressure, this material realizes a quasi-two-dimensional spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 on compression. Above 18 kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the quasi-two-dimensional lattice into weakly coupled chains. We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction studies with quantitative modeling using spin-polarized density functional theory.

Amplitude Mode in Three-Dimensional Dimerized Antiferromagnets.

The amplitude ("Higgs") mode is a ubiquitous collective excitation related to spontaneous breaking of a continuous symmetry. We combine quantum Monte Carlo (QMC) simulations with stochastic analytic continuation to investigate the dynamics of the amplitude mode in a three-dimensional dimerized quantum spin system. We characterize this mode by calculating the spin and dimer spectral functions on both sides of the quantum critical point, finding that both the energies and the intrinsic widths of the excitations satisfy field-theoretical scaling predictions. While the line width of the spin response is close to that observed in neutron scattering experiments on TlCuCl_{3}, the dimer response is significantly broader. Our results demonstrate that highly nontrivial dynamical properties are accessible by modern QMC and analytic continuation methods.

Gapless Spin-Liquid Ground State in the S=1/2 Kagome Antiferromagnet.

The defining problem in frustrated quantum magnetism, the ground state of the nearest-neighbor S=1/2 antiferromagnetic Heisenberg model on the kagome lattice, has defied all theoretical and numerical methods employed to date. We apply the formalism of tensor-network states, specifically the method of projected entangled simplex states, which combines infinite system size with a correct accounting for multipartite entanglement. By studying the ground-state energy, the finite magnetic order appearing at finite tensor bond dimensions, and the effects of a next-nearest-neighbor coupling, we demonstrate that the ground state is a gapless spin liquid. We discuss the comparison with other numerical studies and the physical interpretation of this result.

Anomalous Thermal Conductivity and Magnetic Torque Response in the Honeycomb Magnet α-RuCl_{3}.

We report on the unusual behavior of the in-plane thermal conductivity κ and torque τ response in the Kitaev-Heisenberg material α-RuCl_{3}. κ shows a striking enhancement with linear growth beyond H=7 T, where magnetic order disappears, while τ for both of the in-plane symmetry directions shows an anomaly at the same field. The temperature and field dependence of κ are far more complex than conventional phonon and magnon contributions, and require us to invoke the presence of unconventional spin excitations whose properties are characteristic of a field-induced spin-liquid phase related to the enigmatic physics of the Kitaev model in an applied magnetic field.

Gapless Spin Excitations in the Field-Induced Quantum Spin Liquid Phase of α-RuCl_{3}.

α-RuCl_{3} is a leading candidate material for the observation of physics related to the Kitaev quantum spin liquid (QSL). By combined susceptibility, specific-heat, and nuclear-magnetic-resonance measurements, we demonstrate that α-RuCl_{3} undergoes a quantum phase transition to a QSL in a magnetic field of 7.5 T applied in the ab plane. We show further that this high-field QSL phase has gapless spin excitations over a field range up to 16 T. This highly unconventional result, unknown in either Heisenberg or Kitaev magnets, offers insight essential to establishing the physics of α-RuCl_{3}.

Bound States and Field-Polarized Haldane Modes in a Quantum Spin Ladder.

The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.

In situ monitoring of thin alumina passive film growth by surface plasmon resonance (SPR) during an electrochemical process.

This article presents a sensing technique to characterize the growth of an alumina passive film on an aluminum micro structured layer in situ. The technique uses surface plasmon resonance (SPR) on aluminum coated gratings with spectroscopic measurements during electrochemical polarization in 0.02M Na2SO4. The structure of the sensor was first simulated and then fabricated by photolithography. The grating was then replicated by nanoimprint (NIL) in Sol-Gel before pure aluminum layer was deposited by RF magnetron sputtering to produce the samples used in this study. Coupled plasmonic and electrochemical measurements confirmed the feasibility of in situ characterization (thickness) of alumina passive film on aluminum-based gratings in neutral aqueous media. Combining both measurements with an appropriated SPR spectrum fitting lead to alumina thickness monitoring within a few nanometers' accuracy. The objectives and challenges of this study are to better characterize the alumina growth during electrochemical process combining in situ electrochemical process and SPR spectra in order to determine thin passive layer characteristics.

Simultaneous optimization of spin fluctuations and superconductivity under pressure in an iron-based superconductor.

We present a high-pressure NMR study of the overdoped iron pnictide superconductor NaFe0.94Co0.06As. The low-energy antiferromagnetic spin fluctuations in the normal state, manifest as the Curie-Weiss upturn in the spin-lattice relaxation rate 1/(75)T1T, first increase strongly with pressure but fall again at P>Popt=2.2 GPa. Neither long-ranged magnetic order nor a structural phase transition is encountered up to 2.5 GPa. The superconducting transition temperature Tc shows a pressure dependence identical to the spin fluctuations. Our observations demonstrate that magnetic correlations and superconductivity are optimized simultaneously as a function of the electronic structure, thereby supporting very strongly a magnetic origin of superconductivity.

Complete field-induced spectral response of the spin-1/2 triangular-lattice antiferromagnet CsYbSe2.

Fifty years after Anderson's resonating valence-bond proposal, the spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF) remains the ultimate platform to explore highly entangled quantum spin states in proximity to magnetic order. Yb-based delafossites are ideal candidate TLHAF materials, which allow experimental access to the full range of applied in-plane magnetic fields. We perform a systematic neutron scattering study of CsYbSe2, first proving the Heisenberg character of the interactions and quantifying the second-neighbor coupling. We then measure the complex evolution of the excitation spectrum, finding extensive continuum features near the 120°-ordered state, throughout the 1/3-magnetization plateau and beyond this up to saturation. We perform cylinder matrix-product-state (MPS) calculations to obtain an unbiased numerical benchmark for the TLHAF and spectacular agreement with the experimental spectra. The measured and calculated longitudinal spectral functions reflect the role of multi-magnon bound and scattering states. These results provide valuable insight into unconventional field-induced spin excitations in frustrated quantum materials.

Microscopic Coexistence of Superconductivity and Antiferromagnetism in Underdoped Ba(Fe(1-x)Ru(x))2As2.

We use (75)As nuclear magnetic resonance to investigate the local electronic properties of Ba(Fe(1-x)Ru(x))(2)As(2) (x = 0.23). We find two phase transitions: to antiferromagnetism at T(N) ≈ 60 K and to superconductivity at T(C) ≈ 15 K. Below T(N), our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4µ(B)/Fe. The spin-lattice relaxation rate 1/(75)T(1) is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below T(C), 1/(75)T(1) on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.

Partial order and finite-temperature phase transitions in Potts models on irregular lattices.

We evaluate the thermodynamic properties of the 4-state antiferromagnetic Potts model on the Union-Jack lattice using tensor-based numerical methods. We present strong evidence for a previously unknown, "entropy-driven," finite-temperature phase transition to a partially ordered state. From the thermodynamics of Potts models on the diced and centered diced lattices, we propose that finite-temperature transitions and partially ordered states are ubiquitous on irregular lattices.

Structures of synthetic O-antigen fragments from serotype 2a Shigella flexneri in complex with a protective monoclonal antibody.

The anti-LPS IgG mAb F22-4, raised against Shigella flexneri serotype 2a bacteria, protects against homologous, but not heterologous, challenge in an experimental animal model. We report the crystal structures of complexes formed between Fab F22-4 and two synthetic oligosaccharides, a decasaccharide and a pentadecasaccharide that were previously shown to be both immunogenic and antigenic mimics of the S. flexneri serotype 2a O-antigen. F22-4 binds to an epitope contained within two consecutive 2a serotype pentasaccharide repeat units (RU). Six sugar residues from a contiguous nine-residue segment make direct contacts with the antibody, including the nonreducing rhamnose and both branching glucosyl residues from the two RUs. The glucosyl residue, whose position of attachment to the tetrasaccharide backbone of the RU defines the serotype 2a O-antigen, is critical for recognition by F22-4. Although the complete decasaccharide is visible in the electron density maps, the last four pentadecasaccharide residues from the reducing end, which do not contact the antibody, could not be traced. Although considerable mobility in the free oligosaccharides can thus be expected, the conformational similarity between the individual RUs, both within and between the two complexes, suggests that short-range transient ordering to a helical conformation might occur in solution. Although the observed epitope includes the terminal nonreducing residue, binding to internal epitopes within the polysaccharide chain is not precluded. Our results have implications for vaccine development because they suggest that a minimum of two RUs of synthetic serotype 2a oligosaccharide is required for optimal mimicry of O-Ag epitopes.

Active metabolism of arachidonic acid by Kaposi sarcoma cells cultured from lung biopsies (KS-3); identification by HPLC and MS/MS of the predominant metabolite secreted as the 11,12-epoxy-eicosatrienoic acid.

The development of long-term culture of AIDS-KS cells has allowed us to investigate further a possible vascular origin of Kaposi sarcoma. Taking into account the relative specificity of arachidonic acid (AA) metabolism according to cell type, the AA 'cascade' was analyzed in cultured KS-3 cells established from lung biopsies and compared to human umbilical venous endothelial (H-UVE) cells and human myometrial smooth muscle (H-MSM) cells, under basal conditions and after stimulation with vasoactive agents such as histamine or thrombin. Considering strictly the 'prostaglandin' profile given by RIAs, the metabolism of AA was closer, whilst not identical, to H-UVE than to H-MSM cells. However, evaluation of all the eicosanoids released from [3H]AA labeled KS-3 cells revealed that the predominant metabolite was not prostacyclin (PGI2), as suggested from PG RIAs, but an epoxy-eicosatrienoic acid (EET), identified as the 11, 12 isomer by HPLC and MS/MS. The synthesis of this EET is probably cytochrome P-450 monooxygenase dependent. Its potential role in the development of the KS tumor cells is under investigation.