Collective Magnetic Excitations in the Spin LadderSr14Cu24O41Measured Using High-Resolution Resonant Inelastic X-Ray Scattering

Abstract

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 two-triplon collective excitations. In contrast to inelastic neutron scattering, the RIXS cross section changes only moderately over the entire Brillouin zone, revealing high sensitivity also at small momentum transfers, allowing determination of the two-triplon energy gap as 100 AE 30 meV. Our results are backed by calculations within an effective Hubbard model for a finite-size cluster, and confirm that optical selection rules are obeyed for excitations from this spherically symmetric quantum spin-liquid ground state. DOI: 10.1103/PhysRevLett.103.047401 PACS numbers: 78.70.En, 71.10.Pm, 75.25.+z, 75.30.Ds Collective excitations in strongly correlated electron materials remain a pivotal challenge in contemporary solid state physics. It is widely debated whether magnetic excitations provide the pairing interaction in the hightemperature and unconventional superconductors [1,2]. From that perspective quantum spin systems attract considerable interest. While most such materials, e.g., the cuprate superconductors, exhibit enormous complexity, the two-leg spin ladder is easier to tract theoretically [3][4][5][6]. It consists of two parallel chains (legs) with a transverse (rung) exchange coupling. This system features a singlet ground state and dispersive triplet excitations (triplons), that both have quantum mechanical origin without any classical counterpart. To date, mainly two techniques have been established as momentum-and energy-resolved probes of the dispersion of collective excitations: angleresolved photoelectron spectroscopy and inelastic neutron scattering (INS) for charge and spin degrees of freedom, respectively [7,8]. Because of the latest instrumental improvements [9,10], the energy scale of magnetic exchange is becoming readily accessible for resonant inelastic x-ray scattering (RIXS) [11][12][13][14], which is promising to give information on both, spin and charge degrees of freedom, and in addition is an element-specific technique. Furthermore, RIXS requires only small sample volumes (<0:1 mm 3 ). Recent RIXS studies were performed on long-range ordered magnets with spin-wave excitations [15][16][17].In this Letter, we report a study of the two-leg quantum spin ladder Sr 14 Cu 24 O 41 [18,19] by means of momentumresolved high-resolution RIXS at the Cu L 3 edge. Given that Cu L 3 scattering experiments have been already shown to contain valuable information about the charge degrees of freedom [20], an outstanding question we would like to address here is: how can RIXS provide information on magnetic excitations from a quantum ground state. In the ladder system of Sr 14 Cu 24 O 41 no symmetry breaking occurs-neither in spin-nor in real-space-in contrast to, e.g., a magnetically ordered state, where both symmetries are broken and the direction of the ordered moments dictates the quantization axi...

Persistent high-energy spin excitations in iron-pnictide superconductors

“…S3). This observation is representative for single-magnon excitations as revealed with Cu L 3 RIXS for many cuprates 11,[16][17][18][19] . Unlike the parent cuprates, which are long-range ordered AF Mott insulators, the Fe-pnictide parent compounds are AF ordered spin-density wave semi-metals with compensating electron-and hole-like Fermi-surface pockets involving several Fe 3d orbitals.…”

“…To measure spin excitations in parent BFA and superconducting BKFA Fepnictide samples, we employ RIXS, which has recently been established as a powerful probe of the dispersion of magnetic excitations in a wide energy-momentum window. Many studies on undoped parent cuprates have demonstrated the sensitivity of RIXS to single-magnon excitations 11,[16][17][18][19][20][21] and collective orbital excitations 17 . Doped superconducting cuprates exhibit intense paramagnons, damped spin excitations over much of the BZ with dispersions and spectral weights closely similar to those of magnons in undoped AF ordered parent systems 11,19 .…”

Collective Nature of Spin Excitations in Superconducting Cuprates Probed by Resonant Inelastic X-Ray Scattering

“…Recent examples of RIXS studies of spin excitations include cuprates [6][7][8][9][10][11][12][13][14][15][16][17], iron-based superconductors [18] or iridates [19], where the intrinsic energy scale of the spin dynamics exceeds 100 meV. Initial RIXS data on the dispersion of magnons in the antiferromagnetic "parent compounds" of the cuprate high-temperature superconductors are fully consistent with prior INS data [6,12].…”

Using RIXS to Uncover Elementary Charge and Spin Excitations

“…Although RIXS by nature has the ability to measure elementary excitations for numerous materials, it has shown an especially strong ability for characterizing elementary excitations in transition-metal oxides which manifest strong correlations [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. In particular, RIXS has been shown to be quite useful for the study of high-temperature superconducting materials: As magnetic fluctuations were proposed as candidates for the superconducting pairing glue, obtaining their dispersion relation can be crucial for unraveling the mystery of high T c [22].…”