Session Z61: Cooperative Phenomena II

Protection of spin excitations in Yb quantum magnets

Fractionalized excitations present evidence of entangled states in quantum magnets such as 1D Heisenberg S=1/2 systems (quantum spin chains). Conventionally, systems with larger angular momenta (S≥3/2) are closer to the classical limit and quantum entanglement is less important. Rare earth systems can break this rule by exhibiting quantum behaviors by  acting as S_eff=½ systems due to the selection of ground-state Kramers doublets by crystal electric fields and by virtue of strong spin-orbit coupling. Furthermore, the 4f electronic subshells responsible for magnetism in the rare-earths experience protection from coherence-destroying fluctuations by the outer 5s shells and by spin-preserving angular momentum conservation laws.

 

Here we present neutron scattering measurements on quantum magnets YbAlO3 and Yb3O5O12 to evaluate how the protection of 4f electrons might help preserve information in a spin-system despite its coupling to a heat-bath at finite temperature. At low temperatures, we evaluate quantum entanglement witnesses such as quantum Fisher information to establish lower bounds on multipartite entanglement in the system. We then show that the dispersive quasiparticle band-structure can be preserved to temperatures twenty-times greater than the strength of nearest-neighbor couplings in the system, despite an increased thermal population of phonons and crystal-field modes. Analytical calculations and finite-temperature DMRG confirm that the behavior of the spin-system approaches the infinite-temperature limit in the absence of these couplings, allowing us to extract a time-scale of quasiparticle decoherence due to the heat bath.   

First-principles calculations of the magnetic properties of a Co2 molecule

We present density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations of a Co₂ magnetic molecule in order to understand results of electron paramagnetic resonance (EPR) measurements. In the Co₂ molecule, one Co atom is six-coordinated (octahedral Co), and the other is four-coordinated (tetrahedral Co). Our CASSCF calculations yield easy-plane magnetic anisotropy for both Co atoms in agreement with the measurements. The calculated ZFS parameter D for the octahedral Co is about eight times larger than the D parameter for the tetrahedral Co. DFT calculations falsely yield easy-axis anisotropy for the octahedral Co. The effective g-factors calculated by CASSCF and the pseudo-spin Hamiltonian approach are close to experimental results. We also calculated the EPR absorption spectrum using quasi-degenerate perturbation theory (QDPT) on top of CASSCF. The QDPT calculations reproduced the polar angle dependence of the EPR signal. However, the azimuthal angle dependence of the EPR signal remains a puzzle. The results show that extra precautions should be taken when using DFT for magnetic molecules.   

Anisotropy-induced spin parity effect in an antiferromagnetic spin chain

Spin parity effects — physics which alters between integer and half-odd integer spin cases — often manifest themselves in quantum spin systems, perhaps the most prominent example being the Haldane conjecture for one-dimensional antiferromagnets. In this study we discuss a new and very general mechanism by which spin parity effects can emerge, through the detailed study of an antiferromagnetic spin chain that possesses a spin-ion anisotropy and is subjected to a transverse magnetic field. Using numerical exact diagonalization, we examine the manner in which the ground state undergoes a series of level crossings upon sweeping the magnetic field, between states with crystal momenta 0 and π. We find that such level crossings disappear in integer-spin systems when the anisotropy becomes large, whereas they are robustly present in the case of half-odd integer spins. These results are attributed to the quantitative difference in the nature of the excitation gap in the thermodynamic limit for the two cases, which can be understood in terms of Tomonaga–Luttinger liquid notions, augmented with perturbation theory, as well as with field theory. We will point out how the anisotropy-induced spin parity effect we discovered can be extended to higher-dimensional spin systems.   

Quantum geometry induced nonlinear transport in altermagnets

Quantum geometry plays a pivotal role in the second-order response of PT-symmetric antiferromagnets. Here we study the nonlinear response of 2D altermagnets protected by C_n T- symmetry and show that their leading nonlinear response is third-order. Furthermore, we show that the contributions from the quantum metric and Berry curvature enter separately: the longitudinal response for all planar altermagnets only has a contribution from the quantum metric quadrupole (QMQ), while transverse responses in general have contributions from both the Berry curvature quadrupole (BCQ) and QMQ. We show that for the well-known example of d-wave altermagnets the Hall response is dominated by the BCQ. Both longitudinal and transverse responses are strongly dependent on the crystalline anisotropy. While altermagnets are strictly defined in the limit of vanishing SOC, real altermagnets exhibit weak SOC, which is essential to observe this response. Specifically, SOC gaps the spin-group protected nodal line, generating a response peak that is sharpest when SOC is weak.

Two Dirac nodes also contribute a divergence to the nonlinear response, whose scaling changes as a function of SOC. Finally, we apply our results to thin films of the 3D altermagnet RuO₂. Our work uncovers distinct features of altermagnets in nonlinear transport, providing experimental signatures as well as a guide to disentangling the different components of their quantum geometry.   

Electronic, magnetic, and structural properties of CoVMnSb: ab initio study

We present results of a computational study of electronic, magnetic, and structural properties of CoVMnSb, a quaternary Heusler alloy. Our calculations indicate that this compound may crystallize in two energetically close structural phases: inverted and regular cubic. The inverted cubic phase is the ground state, with ferromagnetic alignment, and around 80% spin polarization. Despite having a relatively large band gap in the minority-spin channel close to the Fermi level, this phase does not undergo a half-metallic transition under pressure. At the same time, the regular cubic phase is half-metallic, and retains its perfect spin polarization under a wide range of mechanical strain. Transition to regular cubic phase may be attained by applying uniform pressure (but not biaxial strain). In practice, this pressure may be realized by atomic substitution of non-magnetic atom (Sb) with another non-magnetic atom (Si) of smaller radius. Our calculations indicate that 25% substitution of Sb with Si results in half-metallic regular cubic phase being the ground state. In addition, CoVMnSb_0.5Si_0.5 retains its half-metallic properties under a considerable range of mechanical pressure, thus making it attractive for potential spintronic applications. We hope that the presented results will stimulate experimental efforts to synthesize this material.   

Complex magnetic interactions and large inverse magnetocaloric effect in TbSi and TbSi0.6Ge0.4

We aim to investigate correlations between physical behaviors and crystal structure of TbSi and TbSi_0.6Ge_0.4 compounds through comprehensive analysis of magnetization, specific heat, and temperature-dependent x-ray diffraction (XRD) measurements. Despite their distinct crystal structures (FeB-type Pnma for TbSi and CrB-type Cmcm for TbSi_0.6Ge_0.4), both samples exhibit similar temperature and magnetic field-dependent magnetization behaviors. Each sample displays two antiferromagnetic (AFM) transitions, consistent with previous studies [1, 2], as well as metamagnetic behavior. Moreover, both samples exhibit a significant inverse magnetocaloric effect, with magnetic entropy change (∆S_M­) of 9.6 J/Kg-K for TbSi and 11.6 J/Kg-K for TbSi_0.6Ge_0.4 at µ₀∆H=7T. Remarkably, the temperature-dependent ∆S_M­ for the TbSi_0.6Ge_0.4 sample reveals the appearance of two additional minima at higher magnetic fields, indicating the emergence of field-induced ferromagnetic (FM) coupling, which is further supported by the evolution of the additional peaks in the specific heat curves at elevated magnetic fields. Additionally, our temperature-dependent XRD measurements unveil anisotropic thermal expansion of the unit cell in both samples, while combined XRD and magnetization data highlight the critical role of the 3^rd and 4^th nearest neighbor Tb-Tb interactions in governing low- and high-temperature AFM coupling in both samples.   

Spin Dynamics And Quadrupole Fluctuations In The Rare-Earth Vanadates YbVO4 And TmVO4

Nematic fluctuations in a number of correlated quantum materials are intricately linked to their physical properties and quantum phase transitions. Here, we propose that a group of rare-earth vanadates (RVO₄) can be used to capture multipole-lattice dynamics associated with the localized nematic order of the 4f degree of freedom. As an illustrative example, we present an experimental study of the dynamical susceptibilities of YbVO₄ and TmVO₄, with Kramers’ and non-Kramers’ doublet crystal field ground states, respectively. We will discuss the thermal modeling of the dynamic susceptibility, as well as the implications of the observed slow dynamics in the context of a potential “nematic glass” phase.   

Site symmetry analysis and magnonic dispersion of Er2O3 including the dipolar interaction

Rare-earth magnets may provide useful magnetic properties for quantum technologies, including quantum transduction and quantum memories. Erbium(III) oxide (Er2O3) may be able to host magnons because of the localized f shell electrons. Out of the 32 Erbiums in Er2O3 non primitive unit cells, 24 have C2 symmetry and 8 have C3i symmetry. At zero temperature, Er2O3 is in non collinear antiferromagnetic state and the site symmetry plays an important role in exchange interaction in this state. Here we analyze the site symmetry of the crystal and show how C3i sites do not provide exchange coupling that influences the magnon dispersion. Then the Holstein-Primakoff representation and paraunitary diagonalization are employed to quantize the Hamiltonian. Er2O3. The effect of the dipolar interaction in the magnons is presented and the energy gap between the dipolar magnons and exchange magnons is calculated.   

Possible Devil’s Staircase magnetization in Manganese Bismuthides

Manganese bismuthides represent a class of intriguing intermetallic compounds with interesting magnetic and electronic properties. Here, we present the successful growth of single crystals of manganese bismuthides using the flux method and provide a comprehensive analysis of their magnetic and electronic transport properties. Our research has unveiled a remarkable magnetic phenomenon known as the "devil's staircase" within these compounds that is tunable by composition and hence provides a promising platform to study unusual magnetism in materials.   

Linear and non-linear THz spectroscopy of CoNb2O6

Understanding both the linear and non-linear spectral responses of magnetic insulators to pulses of THz light is a powerful window into the nature of their low-energy excitations. Here we theoretically study the response in the context of the domain wall bound states of the one-dimensional Ising quantum magnet CoNb₂O₆, using density matrix renormalization group calculations.   

Spin Seebeck measurements in a frustrated magnetic system

Frustrated magnetic systems and quantum spin liquid candidates can have complex magnetic phase diagrams as a function of temperature and field, due to the competition between different ordered and disordered states. Phase boundaries are typically detected through detailed measurements of inelastic neutron scattering, magnetization or magnetic susceptibility, and thermodynamic quantities like specific heat. Local spin Seebeck effect measurements are an electrically driven, electrically detected method that is sensitive to the low energy spin-carrying excitations in magnetic systems, and thus is another approach for transitions between different magnetic phases. We present initial measurements of the spin Seebeck effect in devices patterned on the surface of single crystals of BaCo2(AsO4)2 as a function of field and temperature. We discuss the signals observed in the context of the rich magnetic phase diagram for this system below 10 K.   

Unusual Magnetic Field and Temperature Dependence of the Metamagnetic Transition in UCu0.6Bi2

Uranium-based compounds are an active source of interest due to the wide variety of exotic states that they can host. UCu_0.6Bi₂ orders antiferromagnetically below T_N ≈ 54 K. For T < T_N, there is a first-order metamagnetic transition under the application of magnetic field. The temperature and field-orientation dependence of the transition have been mapped using magnetization and MHz susceptibility measurements in fields of up to 60 T and for temperatures down to 0.65 K. Unusually large hysteresis is observed at the transition under certain conditions; the possible origin of this and other features will be discussed.