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The quest for exotic electronic orderings is driven by our ability to produce said states as cleanly as possible, in a tunable manner and preferably cheap. Organic conductors with reduced dimensionality offer all of these desirable properties as their rich phase diagrams are created by the competition between the tendency of electrons to delocalize and the pronounced interactions between charge, spin and lattice. We discuss here the organic charge-transfer salt α-(BEDT-TTF)2I3, a highly anisotropic material whose phase diagram features a number of intriguing quantum effects, most notably at ambient pressure it undergoes a metal-to-insulator transition into a charge-ordered (CO) state with CO-induced ferroelectricity. The electronic behavior in the CO phase and its exact microscopic underpinnings are however still under heated discussion. As an intriguing link to Mott-like spin-liquid systems from the same family of salts, the dielectric response and structural considerations point toward the important role of disorder. Here we show a detailed systematic study of the titular compound at ambient pressure above and below the CO phase transition, in particular of its dielectric properties both in-plane and out-of-plane, resistivity, Hall effect and magnetoresistance. In the high-temperature semimetallic phase, dominant inter-pocket scattering equalizes mobilities of the electrons and holes creating an almost temperature-independent conductivity as well as a low Hall effect. At low temperatures, within the insulating charge-ordered phase dc transport, magnetoresistance, as well as dielectric response show fingerprints of disorder whose origin is found in the anion layers tightly hydrogen-bonded to the conducting BEDT-TTF molecules.

Manganites R1−xAxMnO3, R and A being rare-earth and alkaline-earth ions respectively, show a wide variety of intriguing phenomena such as colossal magnetoresistance (CMR), multiferroicity, charge ordering, electronic phase separation, etc. Strong coupling of the spin, charge, orbital, and lattice degrees of freedom and the intense competition between the ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating phases cause very complicated phase diagrams. The exact phase depends on chemical substitution, level of doping, magnetic field, pressure, grain size, etc. La1−xCaxMnO3 is the most investigated member of the manganites family. Two phases dominate the well known phase diagram of La1−xCaxMnO3: a ferromagnetic metallic ground state for doping 0.2 < x < 0.5 and an antiferromagnetic (AFM) insulating charge order (CO) ground state for 0.5 < x < 0.9. However, the exact boundary between FM and CO/AFM phases is somehow sample dependent and it is believed that close to x = 0.5 the two dissimilar phases can coexist. In this work we show characterization of ceramic polycrystalline samples of La1-xCaxMnO3 with x = 0.5 using dc conductivity, ac susceptibility, magnetotransport and dielectric spectroscopy at low temperatures. Most importantly we find two prominent phase transitions, a high-temperature one corresponding to CO and a low-temperature transition with a FM fingerprint. Between these two features a metastable region is present both in charge as well as the spin sector. We discuss these findings in the context of phase separation and compare them with other La/Ca or Pr/Ca substitutions x.

The Mott insulator κ-(BEDT-TTF)2Ag2(CN)3 forms a highly-frustrated triangular lattice of S = 1/2 dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response, and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasidegenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound κ-(BEDT-TTF)2Cu2(CN)3.

Manganites La1-xCaxMnO3 belong to the family of manganese oxides with a general chemical formula R1-xAxMnO3, where R represents a rare earth element or Bi and A an alkali or alkaline earth metal or Pb. Replacement of trivalent R and divalent A introduces a mixed valence state Mn3+/Mn4+ and depending on the concentration x causes a presence of various phases in the phase diagrams of R1-xAxMnO3. A huge interest for this family of compounds started in 1994. when Jin et al. found a huge decrease of resistivity in external magnetic field close to Curie temperature known as colossal magnetoresistance (CMR). CMR is present in the part of the phase diagram with ferromagnetic metallic ground state which for the investigated system La1-xCaxMnO3 appears for concentrations 0.2 < x < 0.5. Here we focused on the poorly explored part of the phase diagram x > 0.5 with a paramagnetic insulating phase at high, charge order phase at intermediate and antiferromagnetic insulating phase at low temperatures. We performed magnetotransport and magnetization measurements in the temperature interval 4.2-300 K and magnetic fields up to 5 T on thin film manganites for different concentrations 0.52 ≤ x ≤ 0.75. Our results show traces of CMR and ferromagnetism and a strong coupling between magnetic and transport properties which can be interpreted in terms of spin dependent variable range hopping mechanism and a coexistence of ferromagnetic and antiferromagnetic phase.

The Mott insulator $\kappa$-(BEDT-TTF)$_2$\-Ag$_2$(CN)$_3$ forms a highly-frustrated triangular lattice of $S=1/2$ dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasi-degenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound $\kappa$-(BEDT-TTF)$_2$\-Cu$_2$(CN)$_3$.

Manganites La1-xCaxMnO3 belong to the family of manganese oxides with a general chemical formula R1- xAxMnO3, where R represents a rare earth element or Bi and A an alkali or alkaline earth metal or Pb. Replacement of trivalent R and divalent A introduces a mixed valence state Mn3+/Mn4+ and depending on the concentration x causes a presence of various phases in the phase diagrams of R1-xAxMnO3. A huge interest for this family of compounds started in 1994. when Jin et al. found a huge decrease of resistivity in external magnetic field close to Curie temperature known as colossal magnetoresistance (CMR). CMR is present in the part of the phase diagram with ferromagnetic metallic ground state which for the investigated system La1-xCaxMnO3 appears for concentrations 0.2 < x < 0.5. Here we focused on the poorly explored part of the phase diagram x > 0.5 [3] with a paramagnetic insulating phase at high, charge order phase at intermediate and antiferromagnetic insulating phase at low temperatures. We performed magnetotransport and magnetization measurements in the temperature interval 4.2-300 K and magnetic fields up to 5 T on thin film manganites for different concentrations 0.5 < x < 0.8. Our results show traces of CMR and ferromagnetism and a strong coupling between magnetic and transport properties which can be interpreted in terms of spin dependent variable range hopping mechanism and a coexistence of ferromagnetic and antiferromagnetic phase.

Novel forms of the low-temperature phases in the two-dimensional molecular solids with competing interactions between charges, spins and lattice, in particular those featuring anomalous dielectric relaxation, have been the focus of intense activity in recent years. Open issues concern the nature of collective charge excitations as well as their coupling to applied ac and dc electric fields. Dimer Mott insulator with quantum spin-liquid phase κ-(BEDT-TTF)2Cu2(CN)3 shows variable-range hopping in dc limit and dielectric response featuring relaxor-like ferroelectricity [1, 2] despite missing symmetry-breaking evidences [3]. In this presentation we report our study of the anisotropic dc and ac charge response in a novel spin-liquid candidate κ-(BEDT-TTF)2Ag2(CN)3 [4]. The obtained results indicate the presence of an anomalous dielectric response, and we discuss and compare them with κ-(BEDT-TTF)2Cu2(CN)3. The importance of anion network geometry and cation-anion coupling is emphasized. [1] M. Abdel-Jawad et al., Phys.Rev.B 82, 125119 (2010). [2] M. Pinterić et al., Phys.Rev.B 90, 195139 (2014). [3] K. Sedlmeier et al., Phys.Rev.B 86, 245103 (2012). [4] M. Pinterić et al., in preparation (2015).

Novel forms of the low-temperature phases in the two-dimensional molecular solids with competing interactions between charges, spins and lattice, in particular those featuring anomalous dielectric relaxation, have been the focus of intense activity in recent years. Open issues concern the nature of collective charge excitations as well as their coupling to applied ac and dc electric fields. The charge response is reasonably well understood by now in the charge-ordered phase with the formation of ferroelectric-like domains below the metal-to-insulator phase transition [1, 2]. Conversely, the dielectric response observed in dimer Mott insulator phases with no charge ordering is rather intriguing [3, 4, 5]. In this presentation we review our recent results of anisotropic complex conductivity (dc – MHz) in the magnetic phase of kappa-(BEDT-TTF)2Cu[N(CN)2]Cl and in the spin-liquid phase of kappa-(BEDT-TTF)2Cu2(CN)3 [6, 7]. We discuss possible explanations for the observed dynamics and compare them with the well-known fingerprints of the charge/spin density wave response to ac electric fields. [1] T.Ivek et al., Phys.Rev.B83, 165128 (2011). [2] T.Ivek et al., Phys.Rev.B86, 245125 (2012). [3] M.Abdel-Jawad et al., Phys.Rev.B82, 125119 (2010). [4] S.Iguchi et al., Phys.Rev.B87, 075107 (2013). [5] P.Lunkenheimer et al., Nat. Mater. 11, 755 (2012). [6] S.Tomic et al., J.Phys.: Condens.Matter 25, 436004 (2013). [7] M.Pinteric et al., in preparation (2014).

A variety of organics with reduced dimensionality and competing interactions between charges, spins and lattice display a multiplicity of ordering phenomena and complex phase diagrams. Novel forms of the low-temperature phases, in particular those featuring ferroelectricity in the two-dimensional molecular solids, have been the focus of intense activity in recent years. Open issues concern the nature of collective charge excitations in the charge- and spin-ordered phases as well as their coupling to applied dc and ac fields. And while some of their features resemble the well-established electrodynamics of conventional charge-density waves in 1D, others appear quite different and have not been encountered until now. In the charge-ordered phase with the formation of ferroelectric-like domains below the metal-to-insulator phase transition, the charge response seems to be reasonably well understood now. Conversely, rather intriguing is the dielectric response in dimer Mott insulator phases with either canted antiferromagnetism or spin liquid. The result that neither charge disproportionation nor charge fluctuations could be detected by infrared vibrational spectroscopy leaves the proposal of ferroelectricity fully open. In order to explain the prominent dielectric relaxation we rather suggest an alternative scenario invoking collective excitations associated to domain walls or charge defects formed in an either magnetically ordered or intrinsically disordered background.

Much experimental and theoretical attention has been attracted by organic systems with reduced dimensionality and strong Coulomb interactions, and deservedly so due to their novel broken-symmetry phases and corresponding excitations. Here we take a detailed look at the electrodynamics of one of the most prominent charge-ordered systems, the quasi-2D conductor α (BEDT-TTF)2I3. A semimetal at high temperatures, at 136 K this particular system transitions into an insulating, diamagnetic ground state. Within the insulating phase a long-range commensurate ordering appears in the BEDT-TTF molecular planes, the so-called "horizontal stripe" charge order. [1, 2, 3] We characterize the charge response of the low-temperature phase using dc resistivity, dielectric and optical spectroscopy in different crystallographic directions within the BEDT-TTF layer. [4, 5] Interestingly, two dielectric relaxation modes appear in the kHz-MHz range. The large mode features an anisotropic phason-like behavior, while the small mode presents a soliton-like characteristic. The observed type of excitations agrees with the most relevant physical picture of this charge order as a cooperative bond-charge density wave with ferroelectric-like features. [6] On the other hand, puzzling phenomena including negative differential resistance and voltage oscillations have been reported under application of high electric fields. [7] Our carefully designed electric-field-dependent measurements of conductivity anisotropy within the molecular plane qualitatively confirm these findings ; additionally, they reveal novel intriguing behaviors. [8]. [1] Y. Takano et al., J. Phys. Chem. Solids 62, 393 (2001). [2] Organic Conductors, Special Topics Section, J. Phys. Soc. Jpn. 75, No. 5, (2006). [3] T. Kakiuchi et al., J. Phys. Soc. Jpn. 76, 113702 (2007). [4] T. Ivek et al., Phys. Rev. Lett. 104, 206406 (2010) [5] T. Ivek et al., Phys.Rev.B 83, 165128, (2011). [6] R. T. Clay et al., J. Phys. Soc. Jpn. 71, 1816 (2002). [7] K.Tamura et al., J. Appl.Phys.107, 103716(1-5), (2010). [8] T. Ivek et al., Phys. Rev. B 86, 245125 (2012).

A variety of organics with reduced dimensionality and strong Coulomb interactions between charges and spins display novel forms of ordering phenomena. These broken-symmetry phases and their excitations have been in the focus of intense activity in recent years. In this presentation we overview results of complex conductivity (Hz – MHz) and electric-field dependent measurements in the bond charge density wave phase of alpha-(BEDT-TTF)2I3 and in the canted antiferromagnetic phase of kappa-(BEDT-TTF)2Cu[N(CN)2]Cl. Observed dynamics is discussed within theoretical models for charge excitations and compared with the well-known fingerprints of the charge and spin density wave response to ac and dc electric fields.

The only published Hall effect measurements of TTF-TCNQ date back to 1977. They were performed in the high temperature metallic region above the phase transitions, and the results vary in their sign and values. The aim of the present work has been to extend the previous investigations to lower temperatures and higher magnetic fields and to compare the new data with the known results.

I will present our comparative study of the magnetotransport properties of organic conductors (TMTCF)2X (C= Se, S ; X=PF6, ReO4, AsF6..). Their phase diagram spanning from more anisotropic C = S to C = Se series shows the increasing interchain coupling that leads to the crossing-over of the dimensionality. Our results of the conductivity anisotropy, Hall effect and magnetoresistance enabled us to gain information about the spin density wave ground state at low temperature, Fermi liquid or Luttinger liquid behavior in the normal phase at high temperatures, possible dimensional crossover and phase transitions. The results are discussed in the framework of the existing theoretical models. I will also present our results of the Hall effect measurements of quasi-one-dimensional cuprates Sr14-xCaxCu24O41 ( 0 ≤ x ≤ 11.5) that have been conducted in order to determine the number of carriers participating in dc transport, and its relationship with doping.

We present Hall effect measurements of two-leg ladder Sr14-xCaxCu24O41 (0 10). They are the first superconducting copper oxides (superconductivity occurs under pressure for x>10) without the CuO2 planes, characteristic for high-temperature superconductors. The question of charge dynamics and hole distribution between the ladders and chains is still open. Our Hall effect results show typical semiconducting behavior for x

I will present our comparative study of the magnetotransport properties of organic conductors (TMTCF)2X (C= Se, S). Their phase diagram spanning from more anisotropic C = S to C = Se series shows the increasing interchain coupling that leads to the crossing-over of the dimensionality. Our results of the conductivity anisotropy, Hall effect and magnetoresistance enabled us to gain information about the spin density wave ground state at low temperature, Fermi liquid or Luttinger liquid behavior in the normal phase at high temperatures, possible dimensional crossover and phase transitions. The results are discussed in the framework of the existing theoretical models. The evidence that the 1D Luttinger liquid features appear in the transport properties at high temperatures in C=S series will be addressed. I will also present our results of the Hall effect measurements of quasi-one-dimensional cuprates Sr14-xCaxCu24O41 ( 0 ≤ x ≤ 11.5) that have been conducted in order to determine the number of carriers participating in dc transport, and its relationship with doping. Namely, a fair number of investigations were dedicated to the distribution of holes in the parent system at room temperature with contradictory results ; the reason for that has not been clarified.

The charge transport in the copper-oxygen chain/ladder layers of \laysrca{; ; }; ; is investigated along two crystallographic directions in the temperature range from 50~K to 700~K and for doping levels from $y \approx 6$ (number of holes $n_h

Among many Bechgaard salts, TMTSF2NO3 exhibits very anomalous low temperature properties. Unlike conventional spin density wave (SDW), TMTSF2NO3 undergoes the SDW transition at $\T_SDW\approx 9.5$ K and the low temperature quasiparticle excitations are gapless. Also, it is known that TMTSF2NO3 does not exhibit superconductivity even under pressure, while FISDW is found in TMTSF2NO3 only for P=8.5 kbar and B>20 T. Here we shall show that both the angle dependent magnetoresistance data and the nonlinear Hall resistance of TMTSF2NO3 at ambient pressure are interpreted satisfactory in terms of unconventional spin density wave (USDW). Based on these facts, we propose a new phase diagram for Bechgaards salts., 4 pages, 4 figs, RevTeX

The quantum spin-chain and ladder material Sr14Cu24O41 presents an interesting opportunity to study collective charge dynamics in the charge ordered state inside a spin-gapped en¬ vi¬ ron¬ ment (for a most recent review see [1]). We have characterized the charge response of sin¬ gle crystals along the crystallographic directions parallel to ladder legs and rungs by means of dc resistivity and low frequency dielectric spectroscopy. The ground state in the ladder planes can be identified as a charge density wave insulator, but of a non-standard nature. Resonant X ray scattering [2] shows that the charge order develops in the ladder planes with a 2D long-range order. Our results demonstrate that the CDW sets in below an insulator-to-in¬ su¬ la¬ tor phase transition and possesses an anisotropic phason-like dispersion [1, 3]. In varying dc fields only negligible nonlinear effects emerging from the background noise were observed, at va¬ ri¬ an¬ ce with strong nonlinearities [4] above a finite threshold field reported by other authors [4, 5].

The charge response of the ladders in Sr14-xCaxCu24O41 is characterized by dc resistivity, low frequency dielectric, and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x=0, 3, and 6, in addition to the mode along the legs. For x=8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.

The most comprehensive charge response study of the intrinsically hole doped, spin-chain and spin-ladder composites is overviewed. Results of dc and electric-field-dependent resistivity, low frequency dielectric, and optical spectroscopy in all crystallographic directions are used to build phase diagrams of the underdoped materials (hole count= 6 − y per formula unit, f.u.) and of the fully doped , 6 holes per f.u. The underdoped materials are insulators with hopping transport along the chains, which behave as a onedimensional disordered system. For the fully doped materials the charge transport switches to the ladders due to transfer of holes from the chains. Two-dimensional (2D) charge-density wave (CDW) ground state is formed, i.e. besides CDWphason response along the ladders for 0 ≤ x ≤ 9, we also found it along the ladder rungs for x ≤ 6. However, CDW sliding conductivity, as observed in the standard CDW materials, is not observed in either of directions. Normal to the ladder planes no CDW response was found, and transport is presumably hopping-like. For the first time in any system, CDWresponse away from the principal direction was identified. For x > 9 both CDW in ladders and CO in chains are supressed.

The quantum spin chain and ladder system Sr14-xCaxCu24O41 provides a nice possibility to study spin and charge dynamics and their interplay in a spin-gapped environment. The compound possesses a complex layered structure of alternating Cu2O3 two-leg ladders (the subsystem responsible for the conductivity and superconductivity) and CuO2 chain sheets, which are separated by (Sr, Ca) layers. Independently of x, the average valence of copper is +2.25 so that the compound is inherently doped by six holes per formular unit. We have characterized the charge response of the ladders by dc resistivity, low frequency dielectric and optical spectroscopy on single crystals in all three crystallographic directions. For x=0 and at ambient pressure, the ground state in ladders was identified as a charge density wave insulator with a CDW gap almost three times larger than the spin gap [1]. Substituting Ca2+ for Sr2+ suppresses the CDW insulating phase fully for x>9 [2], while the spin gap remains finite when SC sets in under high pressure. The CDW is not of the standard nature since it sets in below an insulator-to-insulator phase transition and in some features resembles a charge order driven by correlations, it develops in the ladder planes with 2D long-range order, and posses an anisotropic phason-like dispersion [3]. We discuss possible candidates among exotic charge-ordered phases with strong electron-electron interactions and a most probable mechanism, which governs the CDW suppression by Ca-substitution. Finally a modified phase diagram is suggested. [1]B.Gorshunov et al., Phys.Rev.B66, 060508R (2002). [2]T.Vuletic et al., Phys.Rev.Lett. 90, 257002 (2003). [3]T.Vuletic et al., submitted to Phys.Rev.Lett.(2004).

The charge response in the spin chain and ladder compound Sr14-xCaxCu24O41 is characterized by dc resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify an insulator-to-insulator phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x=0 down to 7 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively. Whereas at high doping levels the competing CDW order fully vanishes, external pressure is needed to suppress an insulating high-temperature phase and to establish superconductivity. We propose that this is due to increased disorder and two-dimensionality induced by a subtle interplay of both doping and pressure.

We investigated the transport properties of the quasi-one-dimensional organic metal (TMTSF)2ReO4 above the anion-ordering metal-insulator transition (TAO≈180). The pronounced conductivity anisotropy, a small and smoothly temperature-dependent Hall effect; and a small, positive, and temperature-dependent magnetoresistance are analyzed within the existing Fermi-liquid and non-Fermi-liquid models. We propose that the transport properties of quasi-one-dimensional Bechgaard salts at high temperatures can be described within the Fermi-liquid description.

The presence of subphases in spin-density wave (SDW) phase of (TMTSF)2PF6 below T* ~ 4K has been suggested by several experiments but the nature of the new phase is still controversial. We have investigated the temperature dependence of the angular dependence of the magnetoresistance in the SDW phase which shows different features for temperatures above and below T*. For T > 4K the magnetoresistance can be understood in terms of the Landau quantization of the quasiparticle spectrum in a magnetic field, where the imperfect nesting plays the crucial role. We propose that below T* ~ 4K the new unconventional SDW (USDW) appears modifying dramatically the quasiparticle spectrum. Unlike conventional SDW the order parameter of USDW depends on the quasiparticle momentum. The present model describes many features of the angular dependence of magnetoresistance reasonably well. Therefore, we may conclude that the subphase in (TMTSF)2PF6 below T* ~ 4K is described as SDW plus USDW., 7 pages, 9 figures, RevTeX4; misprint corrected, references updated, a few sentences added

The presence of subphases in spin-density wave (SDW) phase of (TMTSF)_2PF_6 below T* \approx 4 K has been suggested by several experiments but the nature of the new phase is still controversial. We have investigated the temperature dependence of the angular dependence of the magnetoresistance in the SDW phase which shows different features for temperatures above and below T* \approx 4 K. For T > 4 K the magnetoresistance can be understood in terms of the Landau quantization of the quasiparticle spectrum in a magnetic field, where the imperfect nesting plays the crucial role. We propose that below T* \approx 4 K the new unconventional SDW (USDW) appears modifying dramatically the quasiparticle spectrum. Unlike conventional SDW the order parameter of USDW depends on the quasiparticle momentum $\Delta_1(\mvk)\propto \cos 2bk_y$. The present model describes many features of the angular dependence of magnetoresistance reasonably well. Therefore, we may conclude that the subphase in (TMTSF)2PF6 below T \approx 4 K is described as SDW plus USDW.

We present the b' axis transverse magnetoresistance study of the spin-density-wave (SDW) ground state in the quasi-one-dimensional conductor (TMTSF)_2PF_6 for temperatures down to 2.2 K and magnetic fields up to 5 T. The magnetoresistance is very large and highly anisotropic. We present the clear change in the activation energy for T above 4.2K and T below 4.2K, and which depends on magnetic field. The results can be well described by the quasi-particles with the energy gap given in the model with imperfect nesting.

We report the observation of the large Hall effect in the incommensurate spin-density wave (SDW) phase of (TMTSF)_2NO_3 and negative magnetoresistances in the semiconducting state above and below the commensurate SDW phase transition of (TMTTF)_2Br.

We have used electric-field-dependent measurements to probe the magnetic-field dependence of the phase-coherence length in the spin-density-wave (SDW) state of the organic conductor tetramethyltetraselenafulvalinium nitrate (TMTSF)2NO3. The threshold electric field increases if a magnetic field is applied in the plane perpendicular to the highly conducting axis. The magnitude of the increase is angle-dependent and vanishes when the field is applied along the intermediate- conductivity axis. This behavior results from a reduction of the phase-coherence length of the SDW with strongly imperfect nesting in a magnetic field.