arXiv subjects

H. M. Rønnow

Publications and source records attributed to H. M. Rønnow.

88 records · Page 5Linked to original sources

arXiv2012

Field-induced quantum soliton lattice in a frustrated two-leg spin-1/2 ladder

The field-induced quantum phase transitions (QPT) of the spin ladder material Bi(Cu(1-x)Znx)2PO6 have been investigated via 31P nuclear magnetic resonance (NMR) on single-crystal samples with x = 0 and x = 0.01. Measurements at temperatures between 0.25 K and 20 K in magnetic fields up to 31 T served to establish the nature of the various phases. In BiCu2PO6, an incommensurate (IC) magnetic order develops above a critical field mu0*H(c1) ~ 21 T; the field and temperature dependences of the NMR lines and the resulting model for the spin structure are discussed. Supported by results of Density-Matrix Renormalization Group (DMRG) calculations it is argued that the observed field-induced IC order involves the formation of a magnetic-soliton lattice. An additional QPT is predicted to occur at H > H(c1). For x = 0.01, this IC order is found to be stable against site disorder, although with a renormalized critical field.

cond-mat.str-el↗

arXiv2012

Electric-field control of the skyrmion lattice in Cu2OSeO3

Small-angle neutron scattering has been employed to study the influence of applied electric (E-) fields on the skyrmion lattice in the chiral lattice magnetoelectric Cu2OSeO3. In an experimental geometry with the E-field parallel to the [111] axis, and the magnetic field parallel to the [1-10] axis, we demonstrate that the effect of applying an E-field is to controllably rotate the skyrmion lattice around the magnetic field axis. Our results are an important first demonstration for a microscopic coupling between applied E-fields and the skyrmions in an insulator, and show that the general emergent properties of skyrmions may be tailored according to the properties of the host system.

cond-mat.mtrl-sci↗

arXiv2012

The J_{eff}=1/2 insulator Sr3Ir2O7 studied by means of angle-resolved photoemission spectroscopy

The low-energy electronic structure of the J_{eff}=1/2 spin-orbit insulator Sr3Ir2O7 has been studied by means of angle-resolved photoemission spectroscopy. A comparison of the results for bilayer Sr3Ir2O7 with available literature data for the related single-layer compound Sr2IrO4 reveals qualitative similarities and similar J_{eff}=1/2 bandwidths for the two materials, but also pronounced differences in the distribution of the spectral weight. In particuar, photoemission from the J_{eff}=1/2 states appears to be suppressed. Yet, it is found that the Sr3Ir2O7 data are in overall better agreement with band-structure calculations than the data for Sr2IrO4.

cond-mat.str-el↗

arXiv2012

Spin-Orbital Separation in the quasi 1D Mott-insulator Sr2CuO3

As an elementary particle the electron carries spin \hbar/2 and charge e. When binding to the atomic nucleus it also acquires an angular momentum quantum number corresponding to the quantized atomic orbital it occupies (e.g., s, p or d). Even if electrons in solids form bands and delocalize from the nuclei, in Mott insulators they retain their three fundamental quantum numbers: spin, charge and orbital[1]. The hallmark of one-dimensional (1D) physics is a breaking up of the elementary electron into its separate degrees of freedom[2]. The separation of the electron into independent quasi-particles that carry either spin (spinons) or charge (holons) was first observed fifteen years ago[3]. Using Resonant Inelastic X-ray Scattering on the 1D Mott-insulator Sr2CuO3 we now observe also the orbital degree of freedom separating. We resolve an orbiton liberating itself from spinons and propagating through the lattice as a distinct quasi-particle with a substantial dispersion of ~0.2 eV.

cond-mat.str-el↗

arXiv2011

Strong coupling of Sm and Fe magnetism in SmFeAsO as revealed by magnetic x-ray scattering

The magnetic structures adopted by the Fe and Sm sublattices in SmFeAsO have been investigated using element specific x-ray resonant and non-resonant magnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are aligned along the c and a directions, respectively according to the same magnetic representation $Γ_{5}$ and the same propagation vector (1, 0, 0.5). Below 5 K, magnetic order of both sublattices change to a different magnetic structure and the Sm moments reorder in a magnetic unit cell equal to the chemical unit cell. Modeling of the temperature dependence for the Sm sublattice as well as a change in the magnetic structure below 5 K provide a clear evidence of a surprisingly strong coupling between the two sublattices, and indicate the need to include anisotropic exchange interactions in models of SmFeAsO and related compounds.

cond-mat.str-el↗

arXiv2011

Spatial Inhomogeneity in RFeAsO1-xFx (R =Pr, Nd) Determined from Rare Earth Crystal Field Excitations

We report inelastic neutron scattering measurements of crystal field transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with fluorine produces additional crystal field excitations, providing evidence that there are two distinct charge environments around the rare earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have non-magnetic and magnetic ground states, respectively, indicating that the enhancement of Tc compared to LaFeAsO1-xFx is not due to rare earth magnetism.

cond-mat.supr-con↗

arXiv2010

Measurement of magnetic excitations in the two-dimensional antiferromagnetic Sr2CuO2Cl2 insulator using resonant x-ray scattering:Evidence for extended interactions

Using high-resolution resonant inelastic x-ray scattering (RIXS), we performed a momentum-resolved study of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr_2CuCl_2O_2. We identify both a single-spin-wave feature and a multi-magnon continuum, and show that the X-ray polarization can be used to distinguish these two contributions in the cross-section. The spin-waves display a large (70 meV) dispersion between the zone-boundary points ($π$,0) and ($π$/2,$π$/2). Employing an extended $t$-$t'$-$t"$-$U$ one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions. We conclude that sizeable extended magnetic interactions are present in \scoc{} and probably important in all undoped cuprates.

cond-mat.str-el↗

arXiv2010

The Two-Dimensional Square-Lattice S=1/2 Antiferromagnet Cu(pz)$_2$(ClO$_4$)$_2$

We present an experimental study of the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)$_2$(ClO$_4$)$_2$ (pz denotes pyrazine - $C_4H_4N_2$) using specific heat measurements, neutron diffraction and cold-neutron spectroscopy. The magnetic field dependence of the magnetic ordering temperature was determined from specific heat measurements for fields perpendicular and parallel to the square-lattice planes, showing identical field-temperature phase diagrams. This suggest that spin anisotropies in Cu(pz)$_2$(ClO$_4$)$_2$ are small. The ordered antiferromagnetic structure is a collinear arrangement with the magnetic moments along either the crystallographic b- or c-axis. The estimated ordered magnetic moment at zero field is m_0=0.47(5)mu_B and thus much smaller than the available single-ion magnetic moment. This is evidence for strong quantum fluctuations in the ordered magnetic phase of Cu(pz)$_2$(ClO$_4$)$_2$. Magnetic fields applied perpendicular to the square-lattice planes lead to an increase of the antiferromagnetically ordered moment to m_0=0.93(5)mu_B at mu_0H=13.5T - evidence that magnetic fields quench quantum fluctuations. Neutron spectroscopy reveals the presence of a gapped spin excitations at the antiferromagnetic zone center, and it can be explained with a slightly anisotropic nearest neighbor exchange coupling described by J_1^{xy}=1.563(13)meV and J_1^z=0.9979(2)J_1^{xy}.

cond-mat.str-el↗

arXiv2010

Evidence for core-hole-mediated inelastic x-ray scattering from metallic Fe$_{1.087}$Te

We present a detailed analysis of resonant inelastic scattering (RIXS) from Fe$_{1.087}$Te with unprecedented energy resolution. In contrast to the sharp peaks typically seen in insulating systems at the transition metal $L_3$ edge, we observe spectra which show different characteristic features. For low energy transfer, we experimentally observe theoretically predicted many-body effects of resonant Raman scattering from a non-interacting gas of fermions. Furthermore, we find that limitations to this many-body electron-only theory are realized at high Raman shift, where an exponential lineshape reveals an energy scale not present in these considerations. This regime, identified as emission, requires considerations of lattice degrees of freedom to understand the lineshape. We argue that both observations are intrinsic general features of many-body physics of metals.

cond-mat.str-el↗

arXiv2010

Occurrence of superconductivity when the metal-insulator transition is inhibited in $1T$-TaS${_2}$

When a Mott metal-insulator transition is inhibited by a small amount of disorder in the layered dichalcogenide 1T-TaS$_2$, an inhomogeneous superconducting state arises below T=2.1 K, and coexists with a nearly-commensurate charge-density-wave. By angle-resolved photoelectron spectroscopy (ARPES) we show that it emerges from a bad metal state with strongly damped quasiparticles. Superconductivity is almost entirely suppressed by an external magnetic field of 0.1 T.

cond-mat.str-el↗

arXiv2010

Inelastic X-ray Scattering Study of superconducting SmFeAs(O$_{1-x}$F$_{y}$) Single Crystals: Evidence for Strong Momentum-Dependent Doping-Induced Renormalizations of Optical Phonons

We report inelastic x-ray scattering experiments on the lattice dynamics in SmFeAsO and superconducting SmFeAsO$_{0.6}$F$_{0.35}$ single crystals. Particular attention was paid to the dispersions along the [100] direction of three optical modes close to 23 meV, polarized out of the FeAs planes. Remarkably, two of these modes are strongly renormalized upon fluorine doping. These results provide significant insight into the energy and momentum dependence of the coupling of the lattice to the electron system and underline the importance of spin-phonon coupling in the superconducting iron-pnictides.

cond-mat.str-el↗

arXiv2009

Anomalous spin-waves and the commensurate-incommensurate magnetic phase transition in LiNiPO4

Detailed spin-wave spectra of magneto-electric LiNiPO4 have been measured by neutron scattering at low temperatures in the commensurate (C) antiferromagnetic (AF) phase with ordering temperature 20.8 K. An anomalous low-energy mode is observed at the modulation vector of the incommensurate (IC) AF phase appearing above the 20.8 K. A linear spin-wave model based on Heisenberg exchange couplings and single ion anisotropies accounts for all the observed spin-wave dispersions and intensities. Along the b axis an unusually strong next-nearest-neighbor AF coupling competes with the dominant nearest-neighbor AF exchange interaction and causes the IC structure.

cond-mat.mtrl-sci↗

arXiv2009

Collective spin excitations in a quantum spin ladder probed by high-resolution Resonant Inelastic X-ray Scattering

We investigate magnetic excitations in the spin-ladder compound Sr$_{14}$Cu$_{24}$O$_{41}$ using high-resolution Cu $L_3$-edge Resonant Inelastic X-ray Scattering (RIXS). Our findings demonstrate that RIXS couples to collective spin excitations from a quantum spin-liquid ground state. In contrast to Inelastic Neutron Scattering (INS), the RIXS cross section changes only moderately over the entire Brillouin Zone (BZ), revealing a high sensitivity also at small momentum transfers. The two-triplon energy gap is found to be $100\pm 30$ meV. Our results are supported by calculations within an effective Hubbard model for a finite-size cluster.

cond-mat.str-el↗

arXiv2009

Field-induced magnetic phases and electric polarization in LiNiPO4

Neutron diffraction is used to probe the (H,T) phase diagram of magneto-electric (ME) LiNiPO4 for magnetic fields along the c-axis. At zero field the Ni spins order in two antiferromagnetic phases. One has commensurate (C) structures and general ordering vectors (0,0,0), the other one is incommensurate (IC) with ordering vector (0,q,0). At low temperatures the C order collapses above 12 Tesla and adopts an IC structure with modulation vector parallel to (0,q,0). We show that C order is required for the ME effect and establish how electric polarization results from a field-induced reduction of the total magneto-elastic energy.

cond-mat.mtrl-sci↗

arXiv2008

Direct Observation of Magnon Fractionalization in the Quantum Spin Ladder

We measure by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field in the quantum spin-ladder material (C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the Luttinger-liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua.

cond-mat.str-el↗

arXiv2008

Field-Controlled Magnetic Order in the Quantum Spin-Ladder System (Hpip)2CuBr4

Neutron diffraction is used to investigate the field-induced, antiferromagnetically ordered state in the two-leg spin-ladder material (Hpip)2CuBr4. This "classical" phase, a consequence of weak interladder coupling, is nevertheless highly unconventional: its properties are influenced strongly by the spin Luttinger-liquid state of the ladder subunits. We determine directly the order parameter (transverse magnetization), the ordering temperature, the spin structure, and the critical exponents around the transition. We introduce a minimal, microscopic model for the interladder coupling and calculate the quantum fluctuation corrections to the mean-field interaction.

cond-mat.str-el↗