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Register a new userPossible Long-Range Order with Singlet Ground State in CeRu2Al10
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We investigate the thermal and transport properties of CexLa1-xRu2Al10 to clarify the origin of the recently discovered mysterious phase below T0=27 K in CeRu2Al10 where a large magnetic entropy is released, however, the existence of an internal magnetic field is ruled out by 27Al-NQR measurement. We find that T0 decreases with decreasing x and disappears at x~0.45. T0 of CeRu2Al10 is suppressed down to 26 K under H=14.5 T along the a-axis. These results clearly indicate that the transition has a magnetic origin and is ascribed to the interaction between Ce ions. Considering the results of specific heat, magnetic susceptibility, thermal expansion, and electrical resistivity and also 27Al NQR, we propose that the transition originates from the singlet pair formation between Ce ions. Although its properties in a Ce dilute region is basically understood by the impurity Kondo effect, CeRu2Al10 shows a Kondo-semiconductor-like behavior. The phase transition at T0 may be characterized as a new type of phase transition that appears during the crossover from the dilute Kondo to the Kondo semiconductor.
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The nature of the unconventional ordered phase occurring in CeRu2Al10 below T0 = 27 K was investigated by neutron scattering. Powder diffraction patterns show clear superstructure peaks corresponding to forbidden (h + k)-odd reflections of the Cmcm space group. Inelastic neutron scattering experiments further reveal a pronounced magnetic excitation developing in the ordered phase at an energy of 8 meV.
The $4f$-electron system YbAl$_3$C$_3$ with a non-magnetic spin-dimer ground state has been studied by neutron diffraction in an applied magnetic field. A long-range magnetic order involving both ferromagnetic and antiferromagnetic components has been revealed above the critical field H$_Csim $ 6T at temperature T=0.05K. The magnetic structure indicates that the geometrical frustration of the prototype hexagonal lattice is not fully relieved in the low-temperature orthorhombic phase. The suppression of magnetic ordering by the remanent frustration is the key factor stabilizing the non-magnetic singlet ground state in zero field. Temperature dependent measurements in the applied field H=12T revealed that the long-range ordering persists up to temperatures significantly higher than the spin gap indicating that this phase is not directly related to the singlet-triplet excitation. Combining our neutron diffraction results with the previously published phase diagram, we support the existence of an intermediate disordered phase as the first excitation from the non-magnetic singlet ground state. Based on our results, we propose YbAl$_3$C$_3$ as a new material for studying the quantum phase transitions of heavy-fermion metals under the influence of geometrical frustration.
Elastic and inelastic neutron scattering measurements have been performed on powder and single-crystal samples of orthorhombic CeRu2Al10. The order forming below T0 = 27 K was identified as a long-range antiferromagnetic state with the wave vector k = (1,0,0). The magnetic spectral response in the ordered phase, measured on powder, is characterized by a spin gap and a pronounced peak at 8 meV, whose Q dependence suggests a magnetic origin. Both features are suppressed when temperature is raised to T0, and a conventional relaxational behavior is observed at 40 K. This peculiar spin dynamics is discussed in connection with recent magnetization results for the same compound.
We show that some gapped quantum many-body systems have a ground state degeneracy that is stable to long-range (e.g., power-law) perturbations, in the sense that any ground state energy splitting induced by such perturbations is exponentially small in the system size. More specifically, we consider an Ising symmetry-breaking Hamiltonian with several exactly degenerate ground states and an energy gap, and we then perturb the system with Ising symmetric long-range interactions. For these models we prove (1) the stability of the gap, and (2) that the residual splitting of the low-energy states below the gap is exponentially small in the system size. Our proof relies on a convergent polymer expansion that is adapted to handle the long-range interactions in our model. We also discuss applications of our result to several models of physical interest, including the Kitaev p-wave wire model perturbed by power-law density-density interactions with an exponent greater than 1.
The quantum phases of one-dimensional spin $s= 1/2$ chains are discussed for models with two parameters, frustrating exchange $g = J_2 > 0$ between second neighbors and normalized nonfrustrating power-law exchange with exponent $alpha$ and distance dependence $r^{-alpha}$. The ground state (GS) at $g = 0$ has long-range order (LRO) for $alpha < 2$, long-range spin fluctuations for $alpha > 2$. The models conserve total spin $S = S_A + S_B$, have singlet GS for any $g$, $alpha ge 0$ and decouple at $1/g = 0$ to linear Heisenberg antiferromagnets on sublattices $A$ and $B$ of odd and even-numbered sites. Exact diagonalization of finite chains gives the sublattice spin $ < S^2_A >$, the magnetic gap $E_m$ to the lowest triplet state and the excitation $E_{sigma}$ to the lowest singlet with opposite inversion symmetry to the GS. An analytical model that conserves sublattice spin has a first order quantum transition at $g_c = 1/4{rm ln2}$ from a GS with perfect LRO to a decoupled phase with $S_A = S_B = 0$ for $g ge 4/pi^2$ and no correlation between spins in different sublattices. The model with $alpha = 1$ has a first order transition to a decoupled phase that closely resembles the analytical model. The bond order wave (BOW) phase and continuous quantum phase transitions of finite models with $alpha ge 2$ are discussed in terms of GS degeneracy where $E_{sigma}(g) = 0$, excited state degeneracy where $E_{sigma}(g) = E_m(g)$, and $ < S^2_A >$. The decoupled phase at large frustration has nondegenerate GS for any exponent $alpha$ and excited states related to sublattice excitations.
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