We suggest research and development efforts in the exploration of a combined certification approach , which could balance the costs and benefits of different certification systems . Because certification can be expensive, multiple certifications may be cost prohibitive, especially for smallholder farmers , but discounts or incentives could be put in place in order to minimize the costs of multiple certifications. Alternatively, government agencies could subsidize or provide loans for the initial costs of certification and transition, or these expenses could be paid after the first years’ profit are earned. In this way, the certification system could be revised to be more inclusive of small landholders. It is also essential that certification studies incorporate an analysis of the time, labor, and economic costs involved. In future work, the support needed from financial, institutional, and community agencies in order to successfully transition non-certified farmers to organic, fair trade, or biodiversity- or livelihood-friendly coffees should be explicitly investigated.Since 1989, the role of national governments directly influencing global coffee markets and prices paid to producers has decreased , and, in these years, in many regions, rural poverty rates have increased together with accelerating rates of environmental destruction . We suggest that national governments of coffee-producing regions need to play a more active role in providing basic services to their populaces and in protecting ecosystem services. Payments for ecosystem services provide one avenue for compensation from the beneficiaries directly to the landholders and have been implemented in a number of nations, including Costa Rica, Mexico, and China . Rewards for ecosystem services should not be used to directly regulate land management, raspberry cultivation pot but they could provide valuable incentives, especially with the incorporation of management extension services .

The difficulties of quantifying payments for ecosystem services or integrating them with the practices of potential stakeholders or government agencies create real challenges . Therefore, successful programs require stakeholder involvement and development of sustainable farmer livelihoods . Local, regional, and even national cooperatives with administrative capacity and accountability to their membership can leverage international development funding to improve coffee yields and quality, increase production from the diversified shade canopy, and support a wide array of social development projects . Incentives and infrastructure directed toward farmers who use sustainable practices and preserve biodiversity could encourage producers to be good stewards of the land while making a living.Our findings show that, although global coffee acreage has decreased since 1990, cultivation has grown dramatically in Asia and has been accompanied by declining levels of diverse-shade coffee, which threatens the availability and flow of ecosystem services across the globe. Although there have been several gains in the growth of sustainable certifications, research also suggests that livelihoods remain vulnerable and that poverty and hunger are persistent in many farming communities. Research in coffee systems has allowed for an improved understanding of habitat management and biodiversity, a closer examination of the relationships between biodiversity and ecosystem services, and a greater understanding of tropical spatial ecology and connectivity. Coffee has also emerged as an important test case for assessing the effects of different certification programs, evaluating the links between local and global economies, and examining the arena for participatory and interdisciplinary research. However, diversified efforts are needed to develop effective solutions for sustainable livelihoods, and it is essential that all members in the coffee value chain become active stakeholders in these efforts.

From local to global scales, it is clear that farmers, cooperatives, government agencies, and consumers all influence coffee land management and rural livelihoods. We have documented that, in many of the landscapes that generate important ecosystem services, the benefits are not necessarily harvested in terms of income, incentives, and opportunities. In order for coffee landscapes to be sustainable for humans and their ecosystems, we need to better incorporate human well-being and livelihoods into global concepts of sustainability,  encourage the diversification of coffee farms to promote greater resilience to changes in global markets and climates, and improve the valuation and reward for ecosystem services through certification and other systems in order to compensate farmers for the innumerable benefits that shaded landscapes provide. Building synergistic and cooperative relationships among farmers, certifiers, global agencies, researchers, and consumers can provide greater transparency and creative solutions for promoting ecological processes and well-being across global coffee systems.One can achieve dramatic changes in charge density using this technique, but that comes with a heavy cost- the crystal is no longer uniform, as every dopant contributes to disorder, and at high doping levels the band structure itself can be modified by the dopant atoms. More important than all of this for the purposes of experimental physics, however, is that under most circumstances the dopant concentration within a crystal can only be modified through laborious chemical treatments of a particular sample. Materials scientists working under these constraints who wish to explore electron density as an independent variable must either find ingenious material-specific techniques for modifying the dopant concentration in situ , or else they must make a separate sample for each data point they would like to present in their experiment. This is an incredibly labor-intensive process, and it also comes with another significant downside: comparing the properties of two different samples with different doping densities exposes results to systematic differences in sample geometry and imperfections in protocol repeatability, and it is difficult to deconvolute these from the effects of changes in electron density.

For these reasons electron density has generally been an awkward and labor intensive independent variable to manipulate. I have found that there are a few ideas that occur naturally to newcomers and outsiders to the field that insiders know enough to immediately discount, and I’d like to discuss one of those ideas here. Chemical doping to manipulate electron density is an ingenious and important technique, but suppose we tried something much sillier- suppose we simply forcefully deposit electrons onto a crystal using some mechanical or electrical process. Would this not achieve our goal? In fact this does indeed work, we have machines that can do this- van de Graff generators can deposit charge onto a piece of metal mechanically, and a variety of other machines can mimic this behavior electronically. So why aren’t condensed matter physicists going around gluing interesting crystals to van de Graff generators so that we can controllably charge them up and measure their responses to changes in charge density? There are a few reasons, but the most important one is that there is a fundamental issue with manipulating charge density this way in three dimensions: this process does not produce a uniform distribution of electron density within the crystal we’d like to study. In three dimensional systems subjected to this treatment, as illustrated in Fig. 1.1B, excess charge accumulates on the surfaces of the crystal, and although we can force additional electrons into acrystal this way we do not ultimately get a system with a modified but still uniform electron density for us to study. This is not the case for two dimensional systems. Those readers with any exposure to introductory physics have likely encountered parallel plate capacitors; these are highly idealized systems composed of a pair of infinitely thin conducting sheets separated by a small insulating space of consistent thickness. When a voltage is applied to one of these sheets with the other connected to a reservoir of mobile electrons, a uniform charge density per unit area appears on both sheets . Of course, low round pots in real metallic capacitors the charge density per unit volume is often still not microscopically uniform because the sheets are not actually infinitely thin, so electrons can redistribute themselves in the out-of-plane direction. To achieve true uniformity one of the plates of the capacitor must be atomically thin, so that electrons simply cannot redistribute themselves in the out-of-plane direction in response to the local electric field. An efficient technique for preparing atomically thin pieces of crystalline graphite was discovered in 2004 by Dr. Andre Geim and Dr. Konstantin Novoselov, an achievement for which they shared the Nobel prize in physics in 2010. The technique involves encapsulating a crystal within a piece of scotch tape and repeatedly ripping the tape apart; it works because the out-of-plane bonds in graphite are much weaker than the in-plane bonds. Graphite represents something of an extreme example of this condition, but it is satisfied to varying extents by a large class of other materials, and as a result the technique was rapidly generalized to produce a variety of other two-dimensional crystals. By constructing a capacitor with one gate replaced with one of these two dimensional crystals, as shown in Fig. 1.1D, researchers can easily access electron density as an independent variable in a condensed matter system. These systems also facilitate an additional degree of control, with no real analogue in three dimensional systems. By placing capacitor plates on both sides of the two dimensional crystal and applying opposite voltages to the opposing gates, researchers can apply out-of-plane electric fields to these systems . A semiclassical model- in which electrons within the system redistribute themselves in the out-of-plane direction to screen this electric field- does not apply; instead, the wave functions hosted by the two dimensional crystal are themselves deformed in response to the applied electric field .

This changes the electronic band structure of the crystal directly, without affecting the electron density. So to summarize, when a two dimensional crystal is encapsulated with gates to produce a three-layer capacitor, researchers can tune both the electron density and the band structure of the crystal at their pleasure. In the first case, this represents a degree of control that would require the creation of many separate samples to replicate in a three dimensional system. There is a temptation to focus on the exotic phenomena that these techniques for manipulating the electronic structure of two dimensional crystals have allowed us to discover, and there will be plenty of time for that. I’d first like to take a moment to impress upon the reader the remarkable degree of control and extent of theoretical understanding these technologies have allowed us to achieve over those condensed matter systems that are known not to host any new physics. I’ve included several figures from a publication produced by Andrea’s lab with which I was completely uninvolved. It contains precise calculations of the compressibility of a particular allotrope of trilayer graphene as a function of electron density and out-of-plane electric field based on the band structure of the system . It also contains a measurement of compressibility as a function of electron density and out-of-plane electric field, performed using the techniques discussed above . The details of the physics discussed in that publication aren’t important for my point here; the observation I’d like to focus on is the fact that, for this particular condensed matter system, quantitatively accurate agreement between the predictions of our models and the real behavior of the system has been achieved. I remember sitting in a group meeting early in my time working with Andrea’s lab, long before I understood much about Chern magnets or any of the other ideas that would come to define my graduate research work, and marvelling at that fact. Experimental condensed matter physics necessarily involves the study of systems with an enormous number of degrees of freedom and countless opportunities for disorder and complexity to contaminate results. Too often work in this field feels uncomfortably close to gluing wires to rocks and then arguing about how to interpret the results, with no real hope of achieving full understanding, or closure, or even agreement about the conclusions we can extract from our experiments. Within the field of exfoliated heterostructures, it is now clear that we really can hope to pursue true quantitative accuracy in calculations of the properties of condensed matter systems. Rich datasets like these, with a variety of impactful independent variables, produce extremely strong limits on theories. They allow us to be precise in our comparisons of theory to experiment, and as a result they have allowed us to bring models based on band structure theory to new heights of predictive power. But most importantly, under these conditions we can easily identify deviations from our expectations with interesting new phenomena- in particular, situations in which electronic interactions produce even subtle deviations from the predictions of single particle band.