Uncommon Magnetic Ordering in the Quantum Magnet Yb_{3}Ga_{5}O_{12}.

The antiferromagnetic structure of Yb_{3}Ga_{5}O_{12} is identified by neutron diffraction experiments below the previously known transition at T_{λ}=54  mK. The magnetic propagation vector is found to be k=(1/2,1/2,0), an unusual wave vector in the garnet structure. The associated complex magnetic structure highlights the role of exchange interactions in a nearly isotropic system dominated by dipolar interactions and finds echoes with exotic structures theoretically proposed.

Magneto-Optical Sensing of the Pressure Driven Magnetic Ground States in Bulk CrSBr.

Competition between exchange interactions and magnetocrystalline anisotropy may bring new magnetic states that are of great current interest. An applied hydrostatic pressure can further be used to tune their balance. In this work, we investigate the magnetization process of a biaxial antiferromagnet in an external magnetic field applied along the easy axis.

Where is the Quantum Spin Nematic?

We provide strong evidence of the spin-nematic state in a paradigmatic ferro-antiferromagnetic J_{1}-J_{2} model using analytical and density-matrix renormalization group methods. In zero field, the attraction of spin-flip pairs leads to a first-order transition and no nematic state, while pair repulsion at larger J_{2} stabilizes the nematic phase in a narrow region near the pair-condensation field. A devil's staircase of multipair condensates is conjectured for weak pair attraction.

High-Angular Momentum Excitations in Collinear Antiferromagnet FePS.

We report on magneto-optical studies of the quasi-two-dimensional van der Waals antiferromagnet FePS. Our measurements reveal an excitation that closely resembles the antiferromagnetic resonance mode typical of easy-axis antiferromagnets; nevertheless, it displays an unusual, four-times larger Zeeman splitting in an applied magnetic field. We identify this excitation with an || = 4 multipolar magnon─a single-ion 4-magnon bound state─that corresponds to a full reversal of a single magnetic moment of the Fe ion.

Spin texture induced by non-magnetic doping and spin dynamics in 2D triangular lattice antiferromagnet h-Y(Mn,Al)O.

Novel effects induced by nonmagnetic impurities in frustrated magnets and quantum spin liquid represent a highly nontrivial and interesting problem. A theoretical proposal of extended modulated spin structures induced by doping of such magnets, distinct from the well-known skyrmions has attracted significant interest. Here, we demonstrate that nonmagnetic impurities can produce such extended spin structures in h-YMnO, a triangular antiferromagnet with noncollinear magnetic order.

Field-induced States and Excitations in the Quasicritical Spin-1/2 Chain Linarite.

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.

Order by Quenched Disorder in the Model Triangular Antiferromagnet RbFe(MoO_{4})_{2}.

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.

Jammed Spin Liquid in the Bond-Disordered Kagome Antiferromagnet.

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.

Quantum selection of order in an XXZ antiferromagnet on a Kagome lattice.

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 q=0 over sqrt[3]×sqrt[3] magnetic order in a wide range of the anisotropy parameter 0≤Δ≲0.72.

Direct determination of exchange parameters in Cs2CuBr4 and Cs2CuCl4: high-field electron-spin-resonance studies.

Spin-1/2 Heisenberg antiferromagnets Cs2CuCl4 and Cs2CuBr4 with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters.

Triangular antiferromagnet with nonmagnetic impurities.

The effect of nonmagnetic impurities on the phase diagram of the classical Heisenberg antiferromagnet on a triangular lattice is investigated. We present analytical arguments confirmed by numerical calculations that at zero temperature vacancies stabilize a conical state providing an example of "order by quenched disorder" effect. Competition between thermal fluctuations and the site disorder leads to a complicated H-T phase diagram, which is deduced from the classical Monte Carlo simulations for a representative vacancy concentration.

Roton-phonon interactions in superfluid 4He.

High-resolution neutron resonance spin-echo measurements of superfluid 4He show that the roton energy does not have the same temperature dependence as the inverse lifetime. Diagrammatic analysis attributes this to the interaction of rotons with thermally excited phonons via both four- and three-particle processes, the latter being allowed by the broken gauge symmetry of the Bose-condensate. The distinct temperature dependence of the roton energy at low temperatures suggests that the net roton-phonon interaction is repulsive.

Quantum order by disorder and accidental soft mode in Er2Ti2O7.

Motivated by recent neutron scattering experiments, we derive and study an effective "pseudodipolar" spin-1/2 model for the XY pyrochlore antiferromagnet Er(2)Ti(2)O(7). While a bond-dependent in-plane exchange anisotropy removes any continuous symmetry, it does lead to a one-parameter 'accidental' classical degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the noncoplanar spin structure observed experimentally-a rare experimental instance of quantum order by disorder.

Lifetime of gapped excitations in a collinear quantum antiferromagnet.

We demonstrate that local modulations of magnetic couplings have a profound effect on the temperature dependence of the relaxation rate of optical magnons in a wide class of antiferromagnets in which gapped excitations coexist with acoustic spin waves. In a two-dimensional collinear antiferromagnet with an easy-plane anisotropy, the disorder-induced relaxation rate of the gapped mode, Γ(imp)≈Γ(0)+A(TlnT)2, greatly exceeds the magnon-magnon damping, Γ(m-m)≈BT5, negligible at low temperatures. We measure the lifetime of gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using a high-resolution neutron-resonance spin-echo technique and find experimental data in close accord with the theoretical prediction.

Magnetic phase diagrams of classical triangular and kagome antiferromagnets.

We investigate the effect of geometrical frustration on the H-T phase diagrams of the classical Heisenberg antiferromagnets on triangular and kagome lattices. The phase diagrams for the two models are obtained from large-scale Monte Carlo simulations. For the kagome antiferromagnet, thermal fluctuations are unable to lift degeneracy completely and stabilize translationally disordered multipolar phases.

Magnon decay in noncollinear quantum antiferromagnets.

Instability of the excitation spectrum of an ordered noncollinear Heisenberg antiferromagnet with respect to spontaneous two-magnon decays is investigated. We use a spin-1/2 antiferromagnet on a triangular lattice as an example and examine the characteristic long- and short-wavelength features of its zero-temperature spectrum within the 1/S approximation. The kinematic conditions are shown to be crucial for the existence of decays and for overall properties of the spectrum.

Field-induced transitions in a kagomé antiferromagnet.

The thermal order by disorder effect in magnetic field is studied for a classical Heisenberg antiferromagnet on the Kagomé lattice. Using analytical arguments we predict a unique H- T phase diagram for this strongly frustrated magnet: states with a coplanar and a uniaxial triatic order parameter, respectively, at low and high magnetic fields and an incompressible collinear spin-liquid state at one-third of the saturation field. We also present the Monte Carlo data which confirm the existence of these phases.

Interband proximity effect and nodes of superconducting gap in Sr2RuO4.

The power-law temperature dependences of the specific heat, the nuclear relaxation rate, and the thermal conductivity suggest the presence of line nodes in the superconducting gap of Sr2RuO4. These recent experimental observations contradict the scenario of a nodeless (k(x)+ik(y))-type superconducting order parameter. We propose that interaction of superconducting order parameters on different sheets of the Fermi surface is a key to understanding the above discrepancy.

Field induced ordering in highly frustrated antiferromagnets.

We predict that an external field can induce a spin ordering in highly frustrated classical Heisenberg magnets. We find analytically stabilization of collinear states by thermal fluctuations at a one-third of the saturation field for kagome and garnet lattices and at a half of the saturation field for pyrochlore and frustrated square lattices. This effect is studied numerically for the frustrated square-lattice antiferromagnet by Monte Carlo simulations for classical spins and by exact diagonalization for S = 1/2.