Presence Of Antiferromagnetic Order Research Articles

Antiferromagnetic topological insulator with selectively gapped Dirac cones

Antiferromagnetic (AF) topological materials offer a fertile ground to explore a variety of quantum phenomena such as axion magnetoelectric dynamics and chiral Majorana fermions. To realize such intriguing states, it is essential to establish a direct link between electronic states and topology in the AF phase, whereas this has been challenging because of the lack of a suitable materials platform. Here we report the experimental realization of the AF topological-insulator phase in NdBi. By using micro-focused angle-resolved photoemission spectroscopy, we discovered contrasting surface electronic states for two types of AF domains; the surface having the out-of-plane component in the AF-ordering vector displays Dirac-cone states with a gigantic energy gap, whereas the surface parallel to the AF-ordering vector hosts gapless Dirac states despite the time-reversal-symmetry breaking. The present results establish an essential role of combined symmetry to protect massless Dirac fermions under the presence of AF order and widen opportunities to realize exotic phenomena utilizing AF topological materials.

Role of disorder in electronic and magnetic properties of Ag3LiIr2O6

The nature of magnetism in the intercalated honeycomb iridate Ag$_3$LiIr$_2$O$_6$ has been a subject of recent intensive debate, where the absence or presence of antiferromagnetic order has been reported to be related to possible structural disorder effects and, an enhanced Ir-O hybridization and itinerancy with respect to the parent Li$_2$IrO$_3$ has been suggested as the origin of distinct x-ray spectroscopy features. In the present work we investigate the microscopic nature of the electronic and magnetic properties of Ag$_3$LiIr$_2$O$_6$ via a combination of density functional theory combined with exact diagonalization of ab initio-derived models for various experimental and theoretical structures. We evaluate two possible scenarios, the itinerant quasimolecular framework (QMO) on the one hand, and the localized relativistic $j_{\rm eff} = 1/2$ and $j_{\rm eff} = 3/2$ picture on the other hand, and find that the latter description is still viable for this system. We further calculate resonant inelastic x-ray scattering spectra and show that agreement with experimental observations can be obtained if the presence of Ag vacancies leading to changes in Ir filling and structural disorder is assumed. Finally, we show that the experimentally observed antiferromagnetic spiral magnetic order is reproduced by our ab-initio derived magnetic models.

Majorana bound states in a superconducting Rashba nanowire in the presence of antiferromagnetic order

Theoretical studies have shown that Majorana bound states can be induced at the ends of a one dimensional wire, a phenomenon possible due to the interplay between s-wave superconductivity, spin-orbit coupling, and an external magnetic field. These states have been observed in superconductor-semiconductor hybrid nanostructures in the presence of a Zeeman field, and in the limit of a low density of particles. In this paper, we demonstrate and discuss the possibility of the emergence of Majorana bound states in a superconducting Rashba nanowire deposited on an antiferromagnetically ordered surface. We calculate the relevant topological invariant in several complementary ways. Studying the topological phase diagram reveals two branches of the non trivial topological phase -- a main branch, which is typical for Rashba nanowires, and an additional branch emerging due to the antiferromagnetic order. In the case of the additional topological branch, Majorana bound states can also exist close to half-filling, obviating the need for either doping or gating the nanowire to reach the low density regime. Moreover, we show the emergence of the Majorana bound states in the absence of the external magnetic field, which is possible due to the antiferromagnetic order. We also discuss the properties of the bound states in the context of real space localization and the spectral function of the system. This allows one to perceive the band inversion within the spin and sublattice subspaces in the additional branch, contrary to the main branch, where the only band inversion reported in previous studies exists in the spin subspace. Finally, we demonstrate how these topological phases can be confirmed experimentally in transport measurements.

Accelerating the laser-induced demagnetization of a ferromagnetic film by antiferromagnetic order in an adjacent layer

We study the ultrafast demagnetization of Ni/NiMn and Co/NiMn ferromagnetic/antiferromagnetic bilayer systems after excitation by a laser pulse. We probe the ferromagnetic order of Ni and Co using magnetic circular dichroism in time-resolved pump--probe resonant X-ray reflectivity. Tuning the sample temperature across the antiferromagnetic ordering temperature of the NiMn layer allows to investigate effects induced by the magnetic order of the latter. The presence of antiferromagnetic order in NiMn speeds up the demagnetization of the ferromagnetic layer, which is attributed to bidirectional laser-induced superdiffusive spin currents between the ferromagnetic and the antiferromagnetic layer.

Exchange Bias in Magnetic Topological Insulator Superlattices.

Magnetic doping and proximity coupling can open a band gap in a topological insulator (TI) and give rise to dissipationless quantum conduction phenomena. Here, by combining these two approaches, we demonstrate a novel TI superlattice structure that is alternately doped with transition and rare earth elements. An unexpected exchange bias effect is unambiguously confirmed in the superlattice with a large exchange bias field using magneto-transport and magneto-optical techniques. Further, the Curie temperature of the Cr-doped layers in the superlattice is found to increase by 60 K compared to a Cr-doped single-layer film. This result is supported by density-functional-theory calculations, which indicate the presence of antiferromagnetic ordering in Dy:Bi2Te3 induced by proximity coupling to Cr:Sb2Te3 at the interface. This work provides a new pathway to realizing the quantum anomalous Hall effect at elevated temperatures and axion insulator state at zero magnetic field by interface engineering in TI heterostructures.

Doping-induced states near the Mott transition in the presence of antiferromagnetic order

A central issue of the Mott transition is how electronic states change in the Mott transition. Here, by taking into account spin fluctuation around the mean-field state using the random-phase approximation, electronic states exhibiting the spin-wave dispersion relation shifted by the Fermi momenta are shown to emerge in the Mott gap on doping an antiferromagnetically ordered Mott insulator. This characteristic contrasts with that of the mean-field approximation where doping does not induce electronic states in the gap. The results imply that this characteristic is general and fundamental to the Mott transition even in the presence of antiferromagnetic order.

Magnetic and dielectric study of Fe-doped CdSe nanoparticles

Nanoparticles of cadmium selenide (CdSe) and Fe (5% and 10%) doped CdSe have been synthesized by soft chemical route and found to have cubic structure. The magnetic field dependent magnetization measurement of the doped samples indicates the presence of anti-ferromagnetic order. The temperature dependent magnetization (M-T) measurement under zero field cooled and field cooled conditions has also ruled out the presence of ferromagnetic component in the samples at room temperature as well as low temperature. In order to estimate the anti-ferromagnetic coupling among the doped Fe atoms, an M-T measurement at 500 Oe has been carried out, and the Curie-Weiss temperature θ of the samples has been estimated from the inverse of susceptibility versus temperature plots. The dielectric relaxation peaks are observed in the spectra of imaginary part of dielectric constant. The temperature dependent relaxation time is found to obey the Arrhenius law having activation energy ~ 0.4 eV for Fe doped samples. The frequency dependent conductivity spectra are found to obey the power law.

Quantum phases of the Shastry-Sutherland Kondo lattice: implications for the global phase diagram of heavy-fermion metals.

Considerable recent theoretical and experimental effort has been devoted to the study of quantum criticality and novel phases of antiferromagnetic heavy-fermion metals. In particular, quantum phase transitions have been discovered in heavy-fermion compounds with geometrical frustration. These developments have motivated us to study the competition between the Ruderman-Kittel-Kasuya-Yosida and Kondo interactions on the Shastry-Sutherland lattice. We determine the zero-temperature phase diagram as a function of magnetic frustration and Kondo coupling within a slave-fermion approach. Pertinent phases include the valence bond solid and heavy Fermi liquid. In the presence of antiferromagnetic order, our zero-temperature phase diagram is remarkably similar to the global phase diagram proposed earlier based on general grounds. We discuss the implications of our results for the experiments on Yb2Pt2Pb and related compounds.

ANDERSON LOCALIZATION VS. MOTT–HUBBARD METAL–INSULATOR TRANSITION IN DISORDERED, INTERACTING LATTICE FERMION SYSTEMS

We review recent progress in our theoretical understanding of strongly correlated fermion systems in the presence of disorder. Results were obtained by the application of a powerful nonperturbative approach, the dynamical mean-field theory (DMFT), to interacting disordered lattice fermions. In particular, we demonstrate that DMFT combined with geometric averaging over disorder can capture Anderson localization and Mott insulating phases on the level of one-particle correlation functions. Results are presented for the ground state phase diagram of the Anderson–Hubbard model at half-filling, both in the paramagnetic phase and in the presence of antiferromagnetic order. We find a new antiferromagnetic metal which is stabilized by disorder. Possible realizations of these quantum phases with ultracold fermions in optical lattices are discussed.

Fermi Surface and Antiferromagnetism in the Kondo Lattice: An Asymptotically Exact Solution ind>1Dimensions

Interest in the heavy fermion metals has motivated us to examine the quantum phases and their Fermi surfaces within the Kondo lattice model. We demonstrate that the model is soluble asymptotically exactly in any dimension d>1, when the Kondo coupling is small compared with the RKKY interaction and in the presence of antiferromagnetic ordering. We show that the Kondo coupling is exactly marginal in the renormalization group sense, establishing the stability of an ordered phase with a small Fermi surface AFS. Our results have implications for the global phase diagram of the heavy fermion metals, suggesting a Lifshitz transition inside the antiferromagnetic region and providing a new perspective for a Kondo-destroying antiferromagnetic quantum critical point.

Magnetic properties of single crystals of a new cobaltite TbBaCo4O7+x

The temperature and field dependences of the magnetization of a single crystal of a new class of layered cobaltites, TbBaCo4O7+x , with a structure containing a Kagome lattice and a triangular lattice were measured. The measurements were performed on a SQUID magnetometer at temperatures in the range 2–300 K and in magnetic fields of up to 55 kOe for two field orientations. The anisotropy of the magnetization was studied, and the presence of antiferromagnetic ordering in fields H < H c and a weak magnetic-field-induced (H > H c ) ferromagnetic component in the low-temperature range was demonstrated. The magnetic characteristics of the initial TbBaCo4O7+x single crystal and the single crystal annealed in an O2 atmosphere were compared.

Exchange Bias and Spin-Glass-Like Ordering in ε-Fe3N–CrN Nanocomposites

ε-Fe3N–CrN nanocomposite system is synthesized by wet chemical technique. Exchange bias coupling exists at the interface of the ferromagnetic (FM) ε-Fe3N and antiferromagnetic (AF) CrN phases and the FM ε-Fe3N and AF Fe–O–N/Cr–O–N surface layers. However, the exchange bias is not found to be dependent on the CrN concentration and the coupling of the spins at the surface Fe–O–N/Cr–O–N–ε-Fe3N has a dominating influence. The maximum shift in hysteresis loop is observed to be 78 Oe and the low magnitude of exchange bias is attributed to the roughness of AF–FM interface that induces spin-disorder and random exchange anisotropy. The relaxation dynamics study indicates the presence of antiferromagnetic ordering in this system. Below 65 K, the broad peak in zero-field cooled (ZFC) magnetization curves (TE\\congTf) indicates the presence of unidirectional anisotropy and spin-glass-like ordering. The spin-glass-like ordering, which arises from the freezing of the clustered randomly oriented spins, is confirmed from the frequency dependence in AC susceptibility measurements and dynamic scaling analysis.

Competition between Superconductivity and Antiferromagnetism in the Extended Hubbard Model

We investigate a mean-field theory of the extended Hubbard model for the coexistence of antiferromagnetism (AF) and superconductivity (SC) in high-temperature homogeneous superconductive materials. We calculate the ordering parameter and the critical temperature as a function of filling. It is concluded that the superconducting state is destroyed when the antiferromagnetic state develops. For weak Coulomb correlation we show that the superconducting d-wave ordering parameter, which by itself has a maximum in the limit of a half-filled Hubbard band, goes to zero in the presence of antiferromagnetic ordering. Our phase diagram of antiferromagnetic and d-wave superconductivity critical temperatures as a function of filling resembles the experimental evidence.

Neutron diffraction studies of the binary uranium stannides USn 2 and U 3Sn 7

Neutron powder diffraction experiments have been performed on the two structurally related compounds U 3Sn 7 and USn 2. The crystal structures of these two compounds were refined for the first time, confirming isotypism with Ce 3Sn 7 and ZrGa 2 structure types, respectively. The antiferromagnetic ordering suggested by susceptibility measurement on USn 2 was confirmed and the magnetic structure was solved. It corresponds to a collinear antiferromagnetic structure with a propagation vector k=(0, 0, 1/2). The magnetic moment derived from Rietveld refinement of the neutron powder diffraction pattern at 1.5 K is 1.56 μ B/U. The presence of antiferromagnetic ordering in the U 3Sn 7 compound was confirmed, but the magnetic structure model could not be determined due to the very low intensity of the magnetic Bragg peaks.

Weak ferromagnetism and spin-glass-like behavior in the rare-earth cuprates R2CuO4 (R=Tb, Dy, Ho, Er, Tm, and Y).

dc magnetization, ac magnetic susceptibility, and microwave magnetoabsorption measurements on rare-earth cuprates, ${\mathit{R}}_{2}$${\mathrm{CuO}}_{4}$ (R=Tb, Dy, Ho, Er, Tm, and Y), synthesized under high pressure, indicate the presence of antiferromagnetic order with a weak-ferromagnetic component below a characteristic temperature ${\mathit{T}}_{0}$. From the maximum in the amplitude of the microwave absorption, we have determined ${\mathit{T}}_{0}$=295(3) K for R=Tb. The values of ${\mathit{T}}_{0}$ decrease monotonically along the series down to ${\mathit{T}}_{0}$=260(3) K for R=Tm. For R=Y we have measured ${\mathit{T}}_{0}$=265(3). The in-phase component of the ac magnetic susceptibility \ensuremath{\chi}'(T) presents a peak whose shape and intensity are dependent on both the amplitude and the frequency of excitation of the magnetic field. The simultaneous observation of a nonzero and also frequency-dependent out-of-phase term, \ensuremath{\chi}''(T), suggests the presence of relaxation phenomena. The dc magnetization of ${\mathrm{Tb}}_{2}$${\mathrm{CuO}}_{4}$, measured in magnetic fields up to 50 kOe, shows: (i) a linear and reversible part associated with the polarization of the rare-earth ions by the applied magnetic field, (ii) a contribution ${\mathit{M}}_{\mathrm{Cu}}$ related to the appearance of weak ferromagnetism and spin-glass-like features in the Cu lattice, and (iii) an effective internal field ${\mathit{H}}_{\mathit{i}}$ acting on the Tb ions, which is linearly dependent on ${\mathit{M}}_{\mathrm{Cu}}$.

On fluctuations and antiferromagnetism in high-temperature superconductors

Fluctuations in anisotropic high-temperature superconductors are studied in the presence of antiferromagnetic order. Their effects on the coherence lengths and the critical fields are studied using perturbation theory.

LOW-TEMPERATURE THERMOELECTRIC POWER OF HEAVILY DOPED n-TYPE GERMANIUM

A large, temperature-dependent component in the low-temperature thermoelectric power of heavily doped n-type germanium has been observed in the temperature region of liquid helium. All of the samples (6.8–62 × 1017 per cm3) exhibit a negative magnetoresistance in the low-temperature region which saturates at reasonably low magnetic-field values. On the basis of an assumed magnetic scattering, the magnetoresistance and magnetic susceptibility data have been analyzed in order to estimate the magnitude and concentration of the localized magnetic moment of the spin-scattering center, and also to estimate the exchange integral giving rise to the coupling between the mobile carriers and the localized impurity electrons. The resistivity behavior, in both a finite and zero magnetic field, appears to be best described by assuming an antiferromagnetic transition in the region of the thermoelectric anomaly. Attempts are made to interpret the observed temperature dependence of the thermopower. However, as yet, there is no quantitative theory to predict the thermoelectric behavior of a degenerate semiconductor in the presence of antiferromagnetic ordering.

Electron Paramagnetic Resonance and Antiferromagnetism in LaCrO3

THE presence of antiferromagnetic ordering in LaCrO3 has been verified before by both neutron diffraction1 and static magnetic susceptibility2 techniques. In the former case, a Neel temperature of approximately 320° K. is reported, while the susceptibility results indicate Tn ≍ 295° K. Above Tn, LaCrO3 is paramagnetic and hence amenable to investigation by standard electron paramagnetic resonance techniques. We have carried out the experimental resonance investigations over the temperature-range 200–450° K.