Spin Density Wave Ground State Research Articles

Field-induced quantum critical point in the itinerant antiferromagnet Ti3Cu4

New phases of matter emerge at the edge of magnetic instabilities, which can occur in materials with moments that are localized, itinerant or intermediate between these extremes. In local moment systems, such as heavy fermions, the magnetism can be tuned towards a zero-temperature transition at a quantum critical point (QCP) via pressure, chemical doping, and, rarely, magnetic field. By contrast, in itinerant moment systems, QCPs are more rare, and they are induced by pressure or doping; there are no known examples of field induced transitions. This means that no universal behaviour has been established across the whole itinerant-to-local moment range—a substantial gap in our knowledge of quantum criticality. Here we report an itinerant antiferromagnet, Ti3Cu4, that can be tuned to a QCP by a small magnetic field. We see signatures of quantum criticality and the associated non-Fermi liquid behaviour in thermodynamic and transport measurements, while band structure calculations point to an orbital-selective, spin density wave ground state, a consequence of the square net structural motif in Ti3Cu4. Ti3Cu4 thus provides a platform for the comparison and generalisation of quantum critical behaviour across the whole spectrum of magnetism.

Antiferromagnetism and spin density waves in three-dimensional Dirac metals

We perform mean-field study of possible magnetic instabilities in Dirac semimetals. We find that Dirac electrons naturally host antiferromagnetic or spin density wave ground states, though their specific configurations may vary depending on specific model, as well as chemical potential and temperature. We also discuss paramagnetic susceptibility of Dirac semimetals. In the cases, when Dirac electrons do not have orbital momentum, the magnetic properties may be $\mu$ and $T$ independent.

Fermi surface electron–hole instability of the (TMTSF)2PF6 Bechgaard salt revealed by the first-principles Lindhard response function

We report the first-principles DFT calculation of the electron–hole Lindhard response function of the (TMTSF)2PF6 Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2kF spin density wave (SDW) instability it has the shape of a broad triangular plateau as a result of the multiple nesting associated with the warped quasi-one-dimensional Fermi surface. The evolution of the 2kF broad maximum as well as the effect of pressure and deuteration is calculated and analyzed. The thermal dependence of the electron–hole coherence length deduced from these calculations compares very well with the experimental thermal evolution of the 2kF bond order wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron–hole Lindhard response of (TMTSF)2PF6 should favor a substantial mixing of q-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2kF SDW ground state, the mechanism of superconductivity and the magneto-transport of this paradigmatic quasi-one-dimensional material. The first-principles DFT Lindhard response provides a very accurate and unbiased approach to the low-temperature instabilities of (TMTSF)2PF6 which can take into account in a simple way 3D effects and subtle structural variations, thus providing a very valuable tool in understanding the remarkable physics of molecular conductors.

Magnetotransport studies of optimally doped Sr(Fe1−xCox)2As2

We report magnetotransport measurements and their scaling analysis for the optimally electron-doped Sr(Fe0.88CoAs2 system. We observe that both the Kohler’s and modified Kohler’s scalings are violated. Interestingly, the Hall angle displays a quadratic temperature dependence (cot T2) similar to many cuprates and heavy fermion systems. The fact that this T2 dependence is seen in spite of the violation of modified Kohler’s scaling suggests that the Hall angle and magnetoresistance are not governed by the same scattering mechanism. We also observe a linear magnetoresistance in this system, which does not harbor a spin density wave ground state. Implications of our observations are discussed in the context of existing models for the magnetotransport of these strongly correlated electron systems.

Ultrafast melting of spin density wave order in BaFe2As2 observed by time- and angle-resolved photoemission spectroscopy with extreme-ultraviolet higher harmonic generation

Transient single-particle spectral function of BaFe$_{2}$As$_{2}$, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin-density-wave (SDW) ordered state within $\sim$ 1 ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above $E_{F}$ at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From the comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are $\sim$ 0.60 ps both around the zone center and the zone corner.

Relaxation time spectrum of low-energy excitations in one- and two-dimensional materials with charge or spin density waves

The long-time thermal relaxation of (TMTTF)$_2$Br, Sr$_{14}$Cu$_{24}$O$_{41}$ and Sr$_2$Ca$_{12}$Cu$_{24}$O$_{41}$ single crystals at temperatures below 1 K and magnetic field up to 10 T is investigated. The data allow us to determine the relaxation time spectrum of the low energy excitations caused by the charge-density wave (CDW) or spin-density wave (SDW). The relaxation time is mainly determined by a thermal activated process for all investigated materials. The maximum relaxation time increases with increasing magnetic field. The distribution of barrier heights corresponds to one or two Gaussian functions. The doping of Sr$_{14-x}$Ca$_{x}$Cu$_{24}$O$_{41}$ with Ca leads to a drastic shift of the relaxation time spectrum to longer time. The maximum relaxation time changes from 50 s (x = 0) to 3000 s (x = 12) at 0.1 K and 10 T. The observed thermal relaxation at x=12 clearly indicates the formation of the SDW ground state at low temperatures.

Field-induced spin density wave and spiral phases in a layered antiferromagnet

We determine the low-field ordered magnetic phases of the $S=1$ dimerized antiferromagnet ${\mathrm{Ba}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{8}$ using single-crystal neutron diffraction. We find that for magnetic fields between ${\ensuremath{\mu}}_{0}H=8.80\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ and $10.56\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ applied along the $[1\overline{1}0]$ direction the system exhibits spin density wave order with incommensurate wave vectors of type $(\ensuremath{\eta},\ensuremath{\eta},\ensuremath{\epsilon})$. For ${\ensuremath{\mu}}_{0}Hg10.56\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the $[hh0]$ direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order.

An itinerant antiferromagnetic metal without magnetic constituents.

The origin of magnetism in metals has been traditionally discussed in two diametrically opposite limits: itinerant and local moments. Surprisingly, there are very few known examples of materials that are close to the itinerant limit, and their properties are not universally understood. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn2 and Sc3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet with a spin density wave ground state, its 3d electron character has been deemed crucial to it being magnetic. Here, we report evidence for an itinerant antiferromagnetic metal with no magnetic constituents: TiAu. Antiferromagnetic order occurs below a Néel temperature of 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. This itinerant antiferromagnet challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing insights into the effects of spin fluctuations in itinerant–electron systems.

Properties of Hartree-Fock solutions of the three-dimensional electron gas

In a previous letter, L. Baguet et al., (Phys. Rev. Lett. {\bf 111}, 166402 (2013)), we presented the ground state phase diagram of the homogeneous electron gas in three dimensions within the Hartree-Fock approximation yielding incommensurate crystal states at high density. Here, we analyze the properties of these solutions. In particular, at high density we find universal behavior of the incommensurate crystal strongly supporting the existence of a spin density wave ground state.

Electrically controllable magnetic order in the bilayer Hubbard model on honeycomb lattice: A determinant quantum Monte Carlo study

In this paper, we use the determinant quantum Monte Carlo method to study the effect of the electric field on the magnetic order in a bilayer Hubbard model on a honeycomb lattice, in which only the direct interlayer hopping energy is included. Our results qualitatively support the layered antiferromagnetic, spin-density wave ground state found in the mean-field theory at the charge neutrality point. The obtained magnetic moments, however, are much smaller than what are estimated in the mean-field theory. As the electric field increases, the magnetic order parameter rapidly decreases.

P-orbital nanomagnetism in an organic chain magnet

A long-standing challenge in spintronics is the development of a stable, processable and tunable organic magnetic semiconductor. We reveal, through first-principles calculations, that a $p$-electron organic molecular magnet, lithium phthalocyanine (LiPc), can display surprisingly strong antiferromagnetic coupling. The strong coupling, far exceeding that observed in the widely studied transition-metal phthalocyanines, is found to be due to the delocalized spin orbital of the ligand which facilitates intermolecular interactions. The enhanced hopping between the $\ensuremath{\pi}$-conjugated orbitals is also responsible for the wide bandwidth required for high spin mobility. The interactions are a strong function of the intermolecular arrangement and increase when approaching a face-on geometry resulting in a crossover to an itinerant spin density wave ground state, which we propose as an explanation for the unusual spin susceptibility in the related $x$-LiPc phase [M. Brinkmann et al., J. Mater. Chem. 8, 675 (1998)]. This strong coupling, in conjunction with the structural flexibility of the metal phthalocyanine organic semiconductors, suggests a promising route for the fabrication of transition-metal-free, room-temperature, chain magnets for spintronic applications.

Anomalous magnetoresistance and hidden spin canting in (DIETSe)2MCl4(M=Fe, Ga)

The quasi-one-dimensional (Q1D) molecular conductors (DIETSe)${}_{2}$$M$Cl${}_{4}$ [$M=\text{Fe}$, Ga] undergo a spin density wave (SDW) transition below 12 K. The SDW ground state is suppressed by applying high pressure, recovering the Q1D Fermi surface which is confirmed by the appearance of Lebed resonance in the angle-dependent magnetoresistance (MR). Above the critical pressure of SDW, MR shows kink structures at high magnetic fields, reminiscent of field-induced spin density wave (FISDW) transition in both salts. The $\ensuremath{\pi}$-$d$ hybrid (DIETSe)${}_{2}$FeCl${}_{4}$ also exhibits an antiferromagnetic (AF) transition of $d$-electron spins at 2.5 K, below which the spin-flop-induced positive large MR are observed. The change in the interlayer MR reaches 130$%$ at 10.5 kbar. The resistance anomalies associated with spin flop are also observed in the angle-dependent MR at low magnetic fields below 5 T, associated with clear hysteresis. A polar plot of these anomalies reveals the presence of hidden spin canting. Two magnetic easy axes of $d$-electron spins are found to be tilted $\ifmmode\pm\else\textpm\fi{}$16 degrees from the $b$ axis towards the $c$ axis. The interplay between the SDW instability of Q1D $\ensuremath{\pi}$ electrons and the local moments of AF $d$-electron spins is considered as the origin of the anomalous transport behaviors.

Ordinary and intrinsic anomalous Hall effects in Nb1−yFe2+y

The Hall effect of the dilution series Nb${}_{1\ensuremath{-}y}$Fe${}_{2+y}$ has been studied on selected samples spanning ferromagnetic (FM) and spin-density-wave (SDW) ground states, including a sample in the vicinity of the quantum critical point. Ordinary and anomalous contributions are observed, with positive ordinary Hall effect for all single crystals dominating at high temperatures. Consistent analysis of the anomalous contribution is possible for iron-rich Nb${}_{0.985}$Fe${}_{2.015}$ with ferromagnetic ground state and a SDW state at elevated temperatures and stoichiometric NbFe${}_{2}$ with a SDW ground state. The anomalous Hall coefficient is consistent with an intrinsic (Berry-phase) contribution, which is constant below the ordering temperature ${T}_{N}$ and gradually vanishes above ${T}_{N}$. The ordinary Hall coefficient is positive at all temperatures with a moderate temperature dependence. For stoichiometric NbFe${}_{2}$ and niobium-rich Nb${}_{1.01}$Fe${}_{1.99}$---both having a SDW ground state---an additional contribution to the Hall resistivity impedes a complete analysis and indicates the need for more sophisticated models of the anomalous Hall effect in itinerant antiferromagnets.

Orbital Selective Fermi Surface Shifts and Mechanism of HighTcSuperconductivity in CorrelatedAFeAs(A=Li, Na)

Based on the dynamical mean field theory and angle resolved photoemission spectroscopy, we have investigated the mechanism of high T(c) superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the measured angle resolved photoemission spectroscopy. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect within the dynamical mean field theory method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that a fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.

Magnetic properties of spin-diluted iron pnictides fromμSR and NMR in LaFe1−xRuxAsO

The effect of isoelectronic substitutions on the microscopic properties of LaFe1-xRuxAsO, for 0< x< 0.8, has been investigated by means of muSR and 139La NMR. It was found that Ru substitution causes a progressive reduction of the N\`eel temperature (T_N) and of the magnetic order parameter without leading to the onset of superconductivity. The temperature dependence of 139La nuclear spin-lattice relaxation rate 1/T_1 can be suitably described within a two-band model. One band giving rise to the spin density wave ground-state, while the other one is characterized by weakly correlated electrons. Fe for Ru substitution yields to a progressive decrease of the density of states at the Fermi level close to the one derived from band structure calculations. The reduction of T_N with doping follows the predictions of the J_1-J_2 model on a square lattice, which appears to be an effective framework to describe the magnetic properties of the spin density wave ground-state.

Anisotropy of Upper Critical Field in a One-Dimensional Organic System, (TMTTF)2PF6 under High Pressure

We have measured the temperature-dependent resistivity of (TMTTF) 2 PF 6 up to 7 GPa using a turnbuckle-type diamond anvil cell (DAC) and at magnetic fields of up to 5 T. Unlike previous reports on organic compounds, a zero resistance was observed in the superconducting state even at high pressures. Superconductivity was observed in the range of P = 4.18 to 6.03 GPa and showed a peak T c of 2.25 K at 4.58 GPa. The temperature dependence of the upper critical magnetic field H c2 ( T ) was determined via resistivity at P = 4.58 GPa, for the intrachain ( a ), interchain ( b '), and interlayer ( c * ) configurations, and H c2 ( T ) displays a positive curvature without saturation. This could be related to the wide pressure range of the coexistence between the spin density wave (SDW) and superconductivity (SC) ground state, and may be caused by an FFLO state, for a magnetic field along the a - and b '-axes with T ≥0.5 K for P = 4.48 GPa. This feature is suppressed with increasing pressure when the orbital pair breaking mechanism becomes dominant. The Ginzburg–Landau coherence lengths for three different axes obtained from this work show that (TMTTF) 2 PF 6 is an anisotropic three-dimensional superconductor.

Different response of the crystal structure to isoelectronic doping inBaFe2(As1−xPx)2and(Ba1−xSrx)Fe2As2

Superconductivity up to 30 K in charge neutrally doped ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ has been ascribed to chemical pressure caused by the shrinking unit cell. But the latter induces no superconductivity in $({\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}){\text{Fe}}_{2}{\text{As}}_{2}$ in spite of the same volume range. We show that the spin-density-wave (SDW) state of ${\text{BaFe}}_{2}{\text{As}}_{2}$ becomes suppressed in ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ by a subtle reorganization of the crystal structure, where arsenic and phosphorus are located at different coordinates ${z}_{\text{As}}$ and ${z}_{\text{P}}$. High-resolution x-ray diffraction experiments with ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ single crystals reveal almost unchanged Fe-P bonds, but a contraction of the Fe-As bonds, which remain nearly unchanged in $({\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}){\text{Fe}}_{2}{\text{As}}_{2}$. Since the Fe-As bond length is a gauge for the magnetic moment, our results show why the SDW is suppressed by P doping, but not by Sr doping. Only the Fe-P interaction increases the width of the iron $3d$ bands, which destabilizes the magnetic SDW ground state. The simultaneous contraction of the Fe-As bonds is rather a consequence of the vanishing magnetism. Ordered structure models of ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ obtained by density-functional theory calculations agree perfectly with the single-crystal x-ray structure determinations. The contraction of the Fe-As bonds saturates at doping levels above $x\ensuremath{\approx}0.3$, which corrects the unreasonable linear decrease in the so-called pnictide height.

Diffraction line-shapes, Fermi surface nesting, and quantum criticality in antiferromagnetic chromium at high pressure (invited)

We explore the behavior of the nested bandstructure of chromium as a function of temperature and pressure to the point where magnetism disappears. X-ray diffraction measurements of the charge order parameter suggest that the nesting condition is maintained at high pressure, where the spin density wave ground state is destabilized by a continuous quantum phase transition. By comparing diffraction line-shapes measured throughout the temperature-pressure phase diagram we are able to identify and describe three regimes: thermal near-critical, weak coupling ground state, and quantum critical.

Strong electronic correlations in LixM(Pc,Nc) organic conductors near half filling

EPR and NMR measurements in LixM(Pc,Nc) (M=Zn,Mg; Pc=phthalocyanine, Nc=naphthalocyanine) are presented. The changes of the electronic properties induced by the substitution of the metal M and by the expansion of the organic core are discussed for the half filling of the conduction band, i.e. for x ≊ 2. The comparison with Li2ZnPc, which behaves as a Fermi liquid, suggests that the substitution of Zn with Mg does not lead to substantial effects. On the contrary in Li2.2MgNc, where a more two-dimensional packing of the molecules is attained, a stronger tendency to localization and a low temperature phase transition towards a spin density wave ground state are observed.

Effects of cobalt doping and three-dimensionality inBaFe2As2

We investigate the dual roles of a cobalt impurity in the Ba-122 ferropnictide superconductor in the state with coexisting collinear spin-density wave (SDW) order as a dopant and as a scattering center, using first-principles electronic-structure methods. The Co atom is found to dope the FeAs plane where it is located with a single delocalized electron as expected, but also induces a strong perturbation of the SDW ground state of the system. This in turn induces a stripelike modulation of the density of states in nearby planes which may be observable in scanning tunnel microscope (STM) experiments. The defect is found to have an intermediate strength nonmagnetic scattering potential with a range of roughly $1\text{ }\text{\AA{}}$, and the Co gives rise to a smaller but longer range magnetic scattering potential. The impurity potential in both channels is highly anisotropic, reflecting the broken symmetry of the SDW ground state. We give values for the effective Co potentials for each $d$ orbital on the impurity and nearby sites. The calculation also shows a clear local resonance comprised of Co states about 200 meV above the Fermi level, in quantitative agreement with a recent report from STM. Finally, we discuss the issue of the effective dimensionality of the 122 materials, and show that the hybridization of the out-of-phase As atoms leads to a higher density of states between the FeAs planes relative to the 1111 counterparts.